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diff --git a/old/3772-h/files/ch23.html b/old/3772-h/files/ch23.html new file mode 100644 index 0000000..bd5f858 --- /dev/null +++ b/old/3772-h/files/ch23.html @@ -0,0 +1,1299 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> +<!-- saved from url=(0036)http://../Lyell/The Student's Elements of Geology --> +<html> +<head> +<meta name="generator" content="HTML Tidy, see www.w3.org"> +<title>The Student's Elements of Geology: Title</title> +<meta content="text/html; charset=iso-8859-1" http-equiv= +"Content-Type"> +<meta content="MSHTML 5.00.2919.6307" name="GENERATOR"> +<link rel="stylesheet" href="geology.css" type="text/css"> +</head> +<body> +<p><b>The Student’s Elements of Geology</b></p> + +<hr> +<p class="page"><a name="page 394">[ 394 ]</a></p> + +<p> </p> + +<center><b>Chapter XXIII</b><br> +<br> +THE COAL OR CARBONIFEROUS GROUP.</center> + +<p class="intro">Principal Subdivisions of the Carboniferous Group. +— Different Thickness of the sedimentary and calcareous +Members in Scotland and the South of England. — +Coal-measures. — Terrestrial Nature of the Growth of Coal. +— Erect fossil Trees. — Uniting of many Coal-seams into +one thick Bed. — Purity of the Coal explained. — +Conversion of Coal into Anthracite. — Origin of +Clay-ironstone. — Marine and brackish-water Strata in Coal. +— Fossil Insects. — Batrachian Reptiles. — +Labyrinthodont Foot-prints in Coal-measures. — Nova Scotia +Coal-measures with successive Growths of erect fossil Trees. +— Similarity of American and European Coal. — +Air-breathers of the American Coal. — Changes of Condition of +Land and Sea indicated by the Carboniferous Strata of Nova +Scotia.</p> + +<p><b>Principal Subdivisions of the Carboniferous +Group.</b>—The next group which we meet with in the +descending order is the Carboniferous, commonly called “The +Coal,” because it contains many beds of that mineral, in a +more or less pure state, interstratified with sandstones, shales, +and limestones. The coal itself, even in Great Britain and Belgium, +where it is most abundant, constitutes but an insignificant portion +of the whole mass. In South Wales, for example, the thickness of +the coal-bearing strata has been estimated at between 11,000 and +12,000 feet, while the various coal seams, about 80 in number, do +not, according to Professor Phillips, exceed in the aggregate 120 +feet.</p> + +<p>The Carboniferous formation assumes various characters in +different parts even of the British Islands. It usually comprises +two very distinct members: first, the sedimentary beds, usually +called the Coal-measures, of mixed fresh-water, terrestrial, and +marine origin, often including seams of coal; second, that named in +England the Mountain or Carboniferous Limestone, of purely marine +origin, and made up chiefly of corals, shells, and encrinites, and +resting on shales called the shales of the Mountain Limestone.</p> + +<p>In the south-western part of our island, in Somersetshire and +South Wales, the three divisions usually spoken of are:</p> + +<ol> +<li>Coal-measures: Strata of shale, sandstone, and grit, from 600 +to 12,000 feet thick, with occasional seams of coal.</li> + +<li>Millstone grit: A coarse quartzose sandstone passing into a +conglomerate, sometimes used for millstones, with beds of shale; +usually devoid of coal; occasionally above 600 feet thick.</li> + +<li>Mountain or Carboniferous Limestone: A calcareous rock +containing marine shells, corals, and encrinites; devoid of coal; +thickness variable, sometimes more than 1500 feet.</li> +</ol> + +<br> +<br> + + +<p> </p> + +<hr> +<p class="page"><a name="page 395">[ 395 ]</a></p> + +<p>If the reader will refer to the section in <a href= +"../images/fig85.jpg">Fig. 85,</a> he will see that the Upper and +Lower Coal-measures of the coal-field near Bristol are divided by a +micaceous flaggy sandstone called the Pennant Rock. The Lower +Coal-measures of the same section rest sometimes, especially in the +north part of the basin, on a base of coarse grit called the +Millstone Grit (No. 2 on the previous page).</p> + +<p>In the South Welsh coal-field Millstone Grit occurs in like +manner at the base of the productive coal. It is called by the +miners the “Farewell Rock,” as when they reach it they +have no longer any hopes of obtaining coal at a greater depth in +the same district. In the central and northern coal-fields of +England this same grit, including quartz pebbles, with some +accompanying sandstones and shales containing coal plants, acquires +a thickness of several thousand feet, lying beneath the productive +coal-measures, which are nearly 10,000 feet thick.</p> + +<p>Below the Millstone Grit is a continuation of similar sandstones +and shales called by Professor Phillips the Yoredale series, from +Yoredale, in Yorkshire, where they attain a thickness of from 800 +to 1000 feet. At several intervals bands of limestone divide this +part of the series, one of which, called the Main Limestone or +Upper Scar Limestone, composed in great part of encrinites, is 70 +feet thick. Thin seams of coal also occur in these lower Yoredale +beds in Yorkshire, showing that in the same region there were great +alternations in the state of the surface. For at successive periods +in the same area there prevailed first terrestrial conditions +favourable to the growth of pure coal, secondly, a sea of some +depth suited to the formation of Carboniferous Limestone, and, +thirdly, a supply of muddy sediment and sand, furnishing the +materials for sandstone and shale. There is no clear line of +demarkation between the Coal-measures and the Millstone Grit, nor +between the Millstone Grit and underlying Yoredale rocks.</p> + +<p>On comparing a series of vertical sections in a north-westerly +direction from Leicestershire and Warwickshire into North +Lancashire, we find, says Mr. Hull, within a distance of 120 miles +an augmentation of the sedimentary materials to the extent of +16,000 feet.</p> + +<center> +<table border="0" cellpadding="0" cellspacing="0" width="60%" +summary="Augmentation of sedimentary materials"> +<tr> +<td align="left">Leicestershire and Warwickshire</td> +<td align="right">2,600 feet</td> +</tr> + +<tr> +<td align="left">North Staffordshire</td> +<td align="right">9,000 feet</td> +</tr> + +<tr> +<td align="left">South Lancashire</td> +<td align="right">12,130 feet</td> +</tr> + +<tr> +<td align="left">North Lancashire</td> +<td align="right">18,700 feet</td> +</tr> +</table> +</center> + +<p> </p> + +<hr> +<p class="page"><a name="page 396">[ 396 ]</a></p> + +<p>In central England, where the sedimentary beds are reduced to +about 3000 feet in all, the Carboniferous Limestone attains an +enormous thickness, as much as 4000 feet at Ashbourne, near Derby, +according to Mr. Hull’s estimate. To a certain extent, +therefore, we may consider the calcareous member of the formation +as having originated simultaneously with the accumulation of the +materials of grit, sandstone, and shale, with seams of coal; just +as strata of mud, sand, and pebbles, several thousand feet thick, +with layers of vegetable matter, are now in the process of +formation in the cypress swamps and delta of the Mississippi, while +coral reefs are forming on the coast of Florida and in the sea of +the Bermuda islands. For we may safely conclude that in the ancient +Carboniferous ocean those marine animals which were limestone +builders were never freely developed in areas where the rivers +poured in fresh water charged with sand or clay; and the limestone +could only become several thousand feet thick in parts of the ocean +which remained perfectly clear for ages.</p> + +<p>The calcareous strata of the Scotch coal-fields, those of +Lanarkshire, the Lothians, and Fife, for example, are very +insignificant in thickness when compared to those of England. They +consist of a few beds intercalated between the sandstones and +shales containing coal and ironstone, the combined thickness of all +the limestones amounting to no more than 150 feet. The vegetation +of some of these northern sedimentary beds containing coal may be +older than any of the coal-measures of central and southern +England, as being coeval with the Mountain Limestone of the south. +In Ireland the limestone predominates over the coal-bearing sands +and shales. We may infer the former continuity of several of the +coal-fields in northern and central England, not only from the +abrupt manner in which they are cut off at their outcrop, but from +their remarkable correspondence in the succession and character of +particular beds. But the limited extent to which these strata are +exposed at the surface is not merely owing to their former +denudation, but even in a still greater degree to their having been +largely covered by the New Red Sandstone, as in Cheshire, and here +and there by the Permian strata, as in Durham.</p> + +<p>It has long been the opinion of the most eminent geologists that +the coal-fields of Yorkshire and Lancashire were once united, the +upper Coal-measures and the overlying Millstone Grit and Yoredale +rocks having been subsequently removed; but what is remarkable, is +the ancient date now assigned to this denudation, for it seems that +a thickness of no less than</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 397">[ 397 ]</a></p> + +<p>10,000 feet of the coal-measures had been carried away before +the deposition even of the lower Permian rocks which were thrown +down upon the already disturbed truncated edges of the +coal-strata.* The carboniferous strata most productive of workable +coal have so often a basin-shaped arrangement that these troughs +have sometimes been supposed to be connected with the original +conformation of the surface upon which the beds were deposited. But +it is now admitted that this structure has been owing to movements +of the earth’s crust of upheaval and subsidence, and that the +flexure and inclination of the beds has no connection with the +original geographical configuration of the district.</p> + +<br> + + +<center><small>COAL-MEASURES.</small></center> + +<p>I shall now treat more particularly of the productive +coal-measures, and their mode of origin and organic remains.</p> + +<p><b>Coal formed on Land.</b>—In South Wales, already +alluded to, where the coal-measures attain a thickness of 12,000 +feet, the beds throughout appear to have been formed in water of +moderate depth, during a slow, but perhaps intermittent, depression +of the ground, in a region to which rivers were bringing a +never-failing supply of muddy sediment and sand. The same area was +sometimes covered with vast forests, such as we see in the deltas +of great rivers in warm climates, which are liable to be submerged +beneath fresh or salt water should the ground sink vertically a few +feet.</p> + +<p>In one section near Swansea, in South Wales, where the total +thickness of strata is 3246 feet, we learn from Sir H. De la Beche +that there are ten principal masses of sandstone. One of these is +500 feet thick, and the whole of them make together a thickness of +2125 feet. They are separated by masses of shale, varying in +thickness from 10 to 50 feet. The intercalated coal-beds, sixteen +in number, are generally from one to five feet thick, one of them, +which has two or three layers of clay interposed, attaining nine +feet. At other points in the same coal-field the shales predominate +over the sandstones. Great as is the diversity in the horizontal +extent of individual coal-seams, they all present one +characteristic feature, in having, each of them, what is called its +<i>underclay.</i> These underclays, co-extensive with every layer +of coal, consist of arenaceous shale, sometimes called fire-stone, +because it can be made into bricks which stand the fire of a +furnace. They vary in thickness from six inches to more than ten +feet; and Sir William Logan first announced to the scientific world +in 1841 that they were regarded by the colliers in South</p> + +<p class="fnote">* Edward Hull, Quart. Geol. Journ., vol. xxiv, p. +327.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 398">[ 398 ]</a></p> + +<p>Wales as an essential accompaniment of each of the eighty or +more seams of coal met with in their coal-field. They are said to +form the <i>floor</i> on which the coal rests; and some of them +have a slight admixture of carbonaceous matter, while others are +quite blackened by it.</p> + +<p>All of them, as Sir William Logan pointed out, are characterised +by inclosing a peculiar species of fossil vegetable called <i> +Stigmaria,</i> to the exclusion of other plants. It was also +observed that, while in the overlying shales, or “roof” +of the coal, ferns and trunks of trees abound without any <i> +Stigmariæ,</i> and are flattened and compressed, those +singular plants of the underclay most commonly retain their natural +forms, unflattened and branching freely, and sending out their +slender rootlets, formerly thought to be leaves, through the mud in +all directions. Several species of <i>Stigmaria</i> had long been +known to botanists, and described by them, before their position +under each seam of coal was pointed out, and before their true +nature as the roots of trees (some having been actually found +attached to the base of <i>Sigillaria</i> stumps) was recognised. +It was conjectured that they might be aquatic, perhaps floating +plants, which sometimes extended their branches and leaves freely +in fluid mud, in which they were finally enveloped.</p> + +<p>Now that all agree that these underclays are ancient soils, it +follows that in every instance where we find them they attest the +terrestrial nature of the plants which formed the overlying coal, +which consists of the trunks, branches, and leaves of the same +plants. The trunks have generally fallen prostrate in the coal, but +some of them still remain at right angles to the ancient soils (see +<a href="../images32/fig440.jpg">Fig. 440</a>). Professor Goppert, +after examining the fossil vegetables of the coal-fields of +Germany, has detected, in beds of pure coal, remains of plants of +every family hitherto known to occur fossil in the carboniferous +rocks. Many seams, he remarks, are rich in <i>Sigillariæ, +Lepidodendra,</i> and <i>Stigmariæ,</i> the latter in such +abundance as to appear to form the bulk of the coal. In some +places, almost all the plants were calamites, in others ferns.*</p> + +<p>Between the years 1837 and 1840, six fossil trees were +discovered in the coal-fields of Lancashire, where it is +intersected by the Bolton railway. They were all at right angles to +the plane of the bed, which dips about 15 degrees to the south. The +distance between the first and the last was more than 100 feet, and +the roots of all were imbedded in a soft argillaceous shale. In the +same plane with the roots is a bed of</p> + +<p class="fnote">* Quart. Geol. Journ., vol. v, Mem., p. 17.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 399">[ 399 ]</a></p> + +<p>coal, eight or ten inches thick, which has been found to extend +across the railway, or to the distance of at least ten yards. Just +above the covering of the roots, yet beneath the coal-seam, so +large a quantity of the <i>Lepidostrobus variabilis</i> was +discovered inclosed in nodules of hard clay, that more than a +bushel was collected from the small openings around the base of +some of the trees (see <a href="../images3/fig457.jpg">Fig. 457</a> of +this genus). The exterior trunk of each was marked by a coating of +friable coal, varying from one-quarter to three-quarters of an inch +in thickness; but it crumbled away on removing the matrix. The +dimensions of one of the trees is 15½ feet in circumference +at the base, 7½ feet at the top, its height being eleven +feet. All the trees have large spreading roots, solid and strong, +sometimes branching, and traced to a distance of several feet, and +presumed to extend much farther.</p> + +<p>In a colliery near Newcastle a great number of <i> +Sigillariæ</i> occur in the rock as if they had retained the +position in which they grew. No less than thirty, some of them four +or five feet in diameter, were visible within an area of 50 yards +square, the interior being sandstone, and the bark having been +converted into coal. Such vertical stems are familiar to our +coal-miners, under the name of coal-pipes. They are much dreaded, +for almost every year in the Bristol, Newcastle, and other +coal-fields, they are the cause of fatal accidents. Each +cylindrical cast of a tree, formed of solid sandstone, and +increasing gradually in size towards the base, and being without +branches, has its whole weight thrown downward, and receives no +support from the coating of friable coal which has replaced the +bark. As soon, therefore, as the cohesion of this external layer is +overcome, the heavy column falls suddenly in a perpendicular or +oblique direction from the roof of the gallery whence coal has been +extracted, wounding or killing the workman who stands below. It is +strange to reflect how many thousands of these trees fell +originally in their native forests in obedience to the law of +gravity; and how the few which continued to stand erect, obeying, +after myriads of ages, the same force, are cast down to immolate +their human victims.</p> + +<p>It has been remarked that if, instead of working in the dark, +the miner was accustomed to remove the upper covering of rock from +each seam of coal, and to expose to the day the soils on which +ancient forests grew, the evidence of their former growth would be +obvious. Thus in South Staffordshire a seam of coal was laid bare +in the year 1844, in what is called an open work at Parkfield +colliery, near Wolverhampton. In the space of about a quarter of an +acre the</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 400">[ 400 ]</a></p> + +<img src="../images3/fig429.jpg" width="279" height="274" alt= +"Fig. 429: Ground plan of fossil forest, Parkfield Colliery, near Wolverhampton, showing the position of 73 trees in a quarter of an ace." + align="left"> + +<p>stumps of no less than 73 trees with their roots attached +appeared, as shown in Fig. 429, some of them more than eight feet +in circumference. The trunks, broken off close to the root, were +lying prostrate in every direction, often crossing each other. One +of them measured 15, another 30 feet in length, and others less. +They were invariably flattened to the thickness of one or two +inches, and converted into coal. Their roots formed part of a +stratum of coal ten inches thick, which rested on a layer of clay +two inches thick, below which was a second forest resting on a +two-foot seam of coal. Five feet below this, again, was a third +forest with large stumps of <i>Lepidodendra, Calamites,</i> and +other trees.</p> + +<p><b>Blending of Coal-seams.</b>—Both in England and North +America seams of coal are occasionally observed to be parted from +each other by layers of clay and sand, and, after they have been +persistent for miles, to come together and blend in one single bed, +which is then found to be equal in the aggregate to the thickness +of the several seams. I was shown by Mr. H. D. Rogers a remarkable +example of this in Pennsylvania. In the Shark Mountain, near +Pottsville, in that State, there are thirteen seams of anthracite +coal, some of them more than six feet thick, separated by beds of +white quartzose grit and a conglomerate of quartz pebbles, often of +the size of a hen’s egg. Between Pottsville and the Lehigh +Summit Mine, seven of these seams of coal, at first widely +separated, are, in the course of several miles, brought nearer and +nearer together by the gradual thinning out of the intervening +coarse-grained strata and their accompanying shales, until at +length they successively unite and form one mass of coal between +forty and fifty feet thick, very pure on the whole, though with a +few thin partings of clay. This mass of coal I saw quarried in the +open air at Mauch</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 401">[ 401 ]</a></p> + +<p>Chunk, on the Bear Mountain. The origin of such a vast thickness +of vegetable remains, so unmixed, on the whole, with earthy +ingredients, can be accounted for in no other way than by the +growth, during thousands of years, of trees and ferns in the manner +of peat—a theory which the presence of the Stigmaria <i>in +situ</i> under each of the seven layers of anthracite fully bears +out. The rival hypothesis, of the drifting of plants into a sea or +estuary, leaves the non-intermixture of sediment, or of clay, sand, +and pebbles, with the pure coal wholly unexplained.</p> + +<p>The late Mr. Bowman was the first who gave a satisfactory +explanation of the manner in which distinct coal-seams, after +maintaining their independence for miles, may at length unite, and +then persist throughout another wide area with a thickness equal to +that which the separate seams had previously maintained.</p> + +<center><img src="../images3/fig430.jpg" width="396" height="97" alt= +"Fig. 430: Uniting of distinct coal-seams."></center> + +<p>Let A C (Fig. 430) be a three-foot seam of coal originally laid +down as a mass of vegetable matter on the level area of an +extensive swamp, having an under-clay, <i>f g,</i> through which +the Stigmariæ or roots of the trees penetrate as usual. One +portion, B C, of this seam of coal is now inclined; the area of the +swamp having subsided as much as 25 feet at E C, and become for a +time submerged under salt, fresh, or brackish water. Some of the +trees of the original forest A B C fell down, others continued to +stand erect in the new lagoon, their stumps and part of their +trunks becoming gradually enveloped in layers of sand and mud, +which at length filled up the new piece of water C E.</p> + +<p>When this lagoon has been entirely silted up and converted into +land, the forest-covered surface A B will extend once more over the +whole area A B E, and a second mass of vegetable matter, D E, +forming three feet more of coal, will accumulate. We then find in +the region E C two seams of coals, each three feet thick, with +their respective under-clays, with erect buried trees based upon +the surface of the lower coal, the two seams being separated by 25 +feet of intervening shale and sandstone. Whereas in the region A B, +where the growth of the forest has never been interrupted by +submergence, there will simply be one seam, two yards thick, +corresponding to the united thickness of the beds B E and</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 402">[ 402 ]</a></p> + +<p>B C. It may be objected that the uninterrupted growth of plants +during the interval of time required for the filling up of the +lagoon will have caused the vegetable matter in the region D A B to +be thicker than the two distinct seams E and C, and no doubt there +would actually be a slight excess representing one or more +generation of trees and plants forming the undergrowth; but this +excess of vegetable matter, when compressed into coal, would be so +insignificant in thickness that the miner might still affirm that +the seam D A throughout the area D A B was equal to the two seams C +and E.</p> + +<p><b>Cause of the Purity of Coal.</b>—The purity of the coal +itself, or the absence in it of earthy particles and sand, +throughout areas of vast extent, is a fact which appears very +difficult to explain when we attribute each coal-seam to a +vegetation growing in swamps. It has been asked how, during river +inundations capable of sweeping away the leaves of ferns and the +stems and roots of <i>Sigillariæ</i> and other trees, could +the waters fail to transport some fine mud into the swamps? One +generation after another of tall trees grew with their roots in +mud, and their leaves and prostrate trunks formed layers of +vegetable matter, which was afterwards covered with mud since +turned to shale. Yet the coal itself, or altered vegetable matter, +remained all the while unsoiled by earthy particles. This enigma, +however perplexing at first sight, may, I think, be solved by +attending to what is now taking place in deltas. The dense growth +of reeds and herbage which encompasses the margins of +forest-covered swamps in the valley and delta of the Mississippi is +such that the fluviatile waters, in passing through them, are +filtered and made to clear themselves entirely before they reach +the areas in which vegetable matter may accumulate for centuries, +forming coal if the climate be favourable. There is no possibility +of the least intermixture of earthy matter in such cases. Thus in +the large submerged tract called the “Sunk Country,” +near New Madrid, forming part of the western side of the valley of +the Mississippi, erect trees have been standing ever since the year +1811-12, killed by the great earthquake of that date; lacustrine +and swamp plants have been growing there in the shallows, and +several rivers have annually inundated the whole space, and yet +have been unable to carry in any sediment within the outer +boundaries of the morass, so dense is the marginal belt of reeds +and brush-wood. It may be affirmed that generally, in the +“cypress swamps” of the Mississippi, no sediment +mingles with the vegetable matter accumulated there from the decay +of trees and semi-aquatic plants. As a singular proof of this</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 403">[ 403 ]</a></p> + +<p>fact, I may mention that whenever any part of a swamp in +Louisiana is dried up, during an unusually hot season, and the wood +set on fire, pits are burnt into the ground many feet deep, or as +far down as the fire can descend without meeting with water, and it +is then found that scarcely any residuum or earthy matter is left. +At the bottom of all these “cypress swamps” a bed of +clay is found, with roots of the tall cypress (<i>Taxodium +distichum</i>), just as the under-clays of the coal are filled with +<i>Stigmaria.</i></p> + +<p><b>Conversion of Coal into Anthracite.</b>—It appears from +the researches of Liebig and other eminent chemists, that when wood +and vegetable matter are buried in the earth exposed to moisture, +and partially or entirely excluded from the air, they decompose +slowly and evolve carbonic acid gas, thus parting with a portion of +their original oxygen. By this means they become gradually +converted into lignite or wood-coal, which contains a larger +proportion of hydrogen than wood does. A continuance of +decomposition changes this lignite into common or bituminous coal, +chiefly by the discharge of carbureted hydrogen, or the gas by +which we illuminate our streets and houses. According to Bischoff, +the inflammable gases which are always escaping from mineral coal, +and are so often the cause of fatal accidents in mines, always +contain carbonic acid, carbureted hydrogen, nitrogen, and olefiant +gas. The disengagement of all these gradually transforms ordinary +or bituminous coal into anthracite, to which the various names of +glance-coal, coke, hard-coal, culm, and many others, have been +given.</p> + +<p>There is an intimate connection between the extent to which the +coal has in different regions parted with its gaseous contents, and +the amount of disturbance which the strata have undergone. The +coincidence of these phenomena may be attributed partly to the +greater facility afforded for the escape of volatile matter, when +the fracturing of the rocks has produced an infinite number of +cracks and crevices. The gases and water which are made to +penetrate these cracks are probably rendered the more effective as +metamorphic agents by increased temperature derived from the +interior. It is well known that, at the present period, thermal +waters and hot vapours burst out from the earth during earthquakes, +and these would not fail to promote the disengagement of volatile +matter from the Carboniferous rocks.</p> + +<p>In Pennsylvania the strata of coal are horizontal to the +westward of the Alleghany Mountains, where the late Professor H. D. +Rogers pointed out that they were most</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 404">[ 404 ]</a></p> + +<p>bituminous; but as we travel south-eastward, where they no +longer remain level and unbroken, the same seams become +progressively debitumenized in proportion as the rocks become more +bent and distorted. At first, on the Ohio River, the proportion of +hydrogen, oxygen, and other volatile matters ranges from forty to +fifty per cent. Eastward of this line, on the Monongahela, it still +approaches forty per cent, where the strata begin to experience +some gentle flexures. On entering the Alleghany Mountains, where +the distinct anticlinal axes begin to show themselves, but before +the dislocations are considerable, the volatile matter is generally +in the proportion of eighteen or twenty per cent. At length, when +we arrive at some insulated coal-fields associated with the boldest +flexures of the Appalachian chain, where the strata have been +actually turned over, as near Pottsville, we find the coal to +contain only from six per cent of volatile matter, thus becoming a +genuine anthracite.</p> + +<p><b>Clay-ironstone.</b>—Bands and nodules of clay-ironstone +are common in coal-measures, and are formed, says Sir H. De la +Beche, of carbonate of iron mingled mechanically with earthy +matter, like that constituting the shales. Mr. Hunt, of the Museum +of Practical Geology, instituted a series of experiments to +illustrate the production of this substance, and found that +decomposing vegetable matter, such as would be distributed through +all coal strata, prevented the further oxidation of the proto-salts +of iron, and converted the peroxide into protoxide by taking a +portion of its oxygen to form carbonic acid. Such carbonic acid, +meeting with the protoxide of iron in solution, would unite with it +and form a carbonate of iron; and this mingling with fine mud, when +the excess of carbonic acid was removed, might form beds or nodules +of argillaceous ironstone.*</p> + +<p><b>Intercalated Marine Beds in Coal.</b>—Both in the +coal-fields of Europe and America the association of fresh, +brackish-water, and marine strata with coal-seams of terrestrial +origin is frequently recognised. Thus, for example, a deposit near +Shrewsbury, probably formed in brackish water, has been described +by Sir R. Murchison as the youngest member of the coal-measures of +that district, at the point where they are in contact with the +overlying Permian group. It consists of shales and sandstones about +150 feet thick, with coal and traces of plants; including a bed of +limestone varying from two to nine feet in thickness, which is +cellular, and resembles some lacustrine limestones of France and +Germany. It has been traced for 30 miles in a straight line, and +can be recognised</p> + +<p class="fnote">* Memoirs of the Geol. Survey, pp. 51, 255, +etc.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 405">[ 405 ]</a></p> + +<img src="../images3/fig431.jpg" width="241" height="420" alt= +"Fig. 431: Microconchus (Spirorbis) carbonarius. Fig. 432: Cythere (Leperditia) inflata. Fig. 433: Goniatites Listeri. Fig. 434: Aviculopecten papyraceus." + align="left"> + +<p>at still more distant points. The characteristic fossils are a +small bivalve, having the form of a <i>Cyclas</i> or <i>Cyrena,</i> +also a small entomostracan, <i>Cythere inflata</i> (Fig. 432), and +the microscopic shell of an annelid of an extinct genus called <i> +Microconchus</i> (Fig. 431), allied to <i>Spirorbis.</i> In the +coal-field of Yorkshire there are fresh-water strata, some of which +contain shells referred to the family Unionidæ; but in the +midst of the series there is one thin but very widely-spread +stratum, abounding in fishes and marine shells, such as <i> +Goniatites Listeri</i> (Fig. 433), <i>Orthoceras,</i> and <i> +Aviculopecten papyraceus,</i> Goldf. (Fig. 434).</p> + +<p><b>Insects in European Coal.</b>—Articulate animals of the +genus Scorpion were found by Count Sternberg in 1835 in the +coal-measures of Bohemia, and about the same time in those of +Coalbrook Dale by Mr. Prestwich, were also true insects, such as +beetles of the family <i>Curculionidæ,</i> a neuropterous +insect of the genus <i>Corydalis,</i> and another related to the +<i>Phasmidæ,</i> have been found.</p> + +<p>From the coal of Wetting, in Westphalia, several specimens</p> + +<center><img src="../images3/fig435.jpg" width="345" height="186" alt= +"Fig. 435: Wing of a Grasshopper. Gryllacris lithanthraca."> +</center> + +<p> </p> + +<hr> +<p class="page"><a name="page 406">[ 406 ]</a></p> + +<p>of the cockroach or <i>Blatta</i> family, and the wing of a +cricket (<i>Acridites</i>) have been described by Germar. Professor +Goldenberg published, in 1854, descriptions of no less than twelve +species of insects from the nodular clay-ironstone of +Saarbrück, near Trèves.* Among them are several <i> +Blattinæ,</i> three species of <i>Neuroptera,</i> one beetle +of the <i>Scarabæus</i> family, a grasshopper or locust, <i> +Gryllacris</i> (see Fig. 435), and several white ants or Termites. +Professor Goldenberg showed me, in 1864, the wing of a white ant, +found low down in the productive coal-measures of Saarbrück, +in the interior of a flattened Lepidodendron. It is much larger +than that of any known living species of the same genus.</p> + +<img src="../images3/fig436.jpg" width="257" height="432" alt= +"Fig. 436: Archegosaurus minor. Fossil reptile from the coal-measures, Saarbrück." + align="left"> + +<p><b>Batrachian Reptiles in Coal.</b>—No vertebrated animals +more highly organised than fish were known in rocks of higher +antiquity than the Permian until the year 1844, when the <i>Apateon +pedestris,</i> Meyer, was discovered in the coal-measures of +Munster-Appel in Rhenish Bavaria, and three years later, in 1847, +Professor von Dechen found three other distinct species of the same +family of Amphibia in the Saarbruck coal-field above alluded to. +These were described by the late Professor Goldfuss under the +generic name of <i>Archegosaurus.</i> The skulls, teeth, and the +greater portions of the skeleton, nay, even a large part of the +skin, of two of these reptiles have been faithfully preserved in +the centre of spheroidal concretions of clay-ironstone. The largest +of these, <i>Archegosaurus Decheni,</i> must</p> + +<p class="fnote">* Dunker and V. Meyer, Palæont., vol. iv, p. +17.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 407">[ 407 ]</a></p> + +<p>have been three feet six inches long. Figure 436 represents the +skull and neck bones of the smallest of the three, of the natural +size. They were considered by Goldfuss as saurians, but by Herman +von Meyer as most nearly allied to the <i>Labyrinthodon</i> before +mentioned (<a href="ch21.html#page 371">p. 371</a>), and the +remains of the extremities leave no doubt they were quadrupeds, +“provided,” says Von Meyer, “with hands and feet +terminating in distinct toes; but these limbs were weak, serving +only for swimming or creeping.” The same anatomist has +pointed out certain points of analogy between their bones and those +of the <i>Proteus anguinus</i>; and Professor Owen has observed +that they make an approach to the <i>Proteus</i> in the shortness +of their ribs. Two specimens of these ancient reptiles retain a +large part of the outer skin, which consisted of long, narrow, +wedge-shaped, tile-like, and horny scales, arranged in rows (see +Fig. 437).</p> + +<img src="../images3/fig437.jpg" width="182" height="123" alt= +"Fig. 437: Imbricated covering of skin of Archegosaurus medius." +align="right"> + +<p>In 1865, several species belonging to three different genera of +the same family of perennibranchiate Batrachians were found in the +coal-field of Kilkenny in bituminous shale at the junction of the +coal with the underlying Stigmaria-bearing clay. They were, +probably, inhabitants of a marsh, and the large processes +projecting from the vertebræ of their tail imply, according +to Professor Huxley, great powers of swimming. They were of the +Labyrinthodont family, and their association with the fish of the +coal, of which so large a proportion are ganoids, reminds us that +the living perennibranchiate amphibia of America frequent the same +rivers as the ganoid Lepidostei or bony pikes.</p> + +<p><i>Labyrinthodont footprints in coal-measures.</i>—In +1844, the very year when the Apateon, before mentioned, of the coal +was first met with in the country between the Moselle and the +Rhine, Dr. King published an account of the footprints of a large +reptile discovered by him in North America. These occur in the +coal-strata of Greensburg, in Westmoreland County, Pennsylvania; +and I had an opportunity of examining them when in that country in +1846. The footmarks were first observed standing out in relief from +the lower surface of slabs of sandstone, resting on thin layers of +fine unctuous clay. I brought away one of these masses, which is +represented in Fig. 438. It displays, together with footprints, the +casts of cracks (<i>a, a'</i>) of various sizes. The origin of such +cracks in</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 408">[ 408 ]</a></p> + +<center><img src="../images3/fig438.jpg" width="404" height="528" alt= +"Fig. 438: Slab of sandstone from the coal-measures of Pennsylvania, with foot-prints of air-breathing reptile and casts of cracks."> +</center> + +<p>clay, and casts of the same, has before been explained, and +referred to the drying and shrinking of mud, and the subsequent +pouring of sand into open crevices. It will be seen that some of +the cracks, as at <i>b, c,</i> traverse the footprints, and produce +distortion in them, as might have been expected, for the mud must +have been soft when the animal walked over it and left the +impressions; whereas, when it afterwards dried up and shrank, it +would be too hard to receive such indentations.</p> + +<p>We may assume that the reptile which left these prints</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 409">[ 409 ]</a></p> + +<p>on the ancient sands of the coal-measures was an air-breather, +because its weight would not have been sufficient under water to +have made impressions so deep and distinct. The same conclusion is +also borne out by the casts of the cracks above described, for they +show that the clay had been exposed to the air and sun, so as to +have dried and shrunk.</p> + +<p><b>Nova Scotia Coal-measures.</b>—The sedimentary strata +in which thin seams of coal occur attain a thickness, as we have +seen, of 18,000 feet in the north of England exclusive of the +Mountain Limestone, and are estimated by Von Dechen at over 20,000 +feet in Rhenish Prussia. But the finest example in the world of a +natural exposure in a continuous section ten miles long, occurs in +the sea-cliffs bordering a branch of the Bay of Fundy, in Nova +Scotia. These cliffs, called the “South Joggins,” which +I first examined in 1842, and afterwards with Dr. Dawson in 1845, +have lately been admirably described by the last-mentioned +geologist* in detail, and his evidence is most valuable as showing +how large a portion of this dense mass was formed on land, or in +swamps where terrestrial vegetation flourished, or in fresh-water +lagoons. His computation of the thickness of the whole series of +carboniferous strata as exceeding three miles, agrees with the +measurement made independently by Sir William Logan in his survey +of this coast.</p> + +<p>There is no reason to believe that in this vast succession of +strata, comprising some marine as well as many fresh-water and +terrestrial formations, there is any repetition of the same beds. +There are no faults to mislead the geologist, and cause him to +count the same beds over more than once, while some of the same +plants have been traced from the top to the bottom of the whole +series, and are distinct from the flora of the antecedent Devonian +formation of Canada. Eighty-one seams of coal, varying in thickness +from an inch to about five feet, have been discovered, and no less +than seventy-one of these have been actually exposed in the +sea-cliffs.</p> + +<p>In the section (Fig. 439), which I examined in 1842, the beds +from <i>c</i> to <i>i</i> are seen all dipping the same way, their +average inclination being at an angle of 24° S.S.W. The +vertical height of the cliffs is from 150 to 200 feet; and between +<i>d</i> and <i>g</i>—in which space I observed seventeen +trees in an upright position, or, to speak more correctly, at right +angles to the planes of stratification—I counted nineteen +seams of coal, varying in thickness from two inches to four feet. +At low tide a fine horizontal section of the same beds is exposed +to view on the beach, which at low tide extends sometimes 200</p> + +<p class="fnote">* Acadian Geology, 2nd edit., 1868.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 410">[ 410 ]</a></p> + +<p>yards from the base of the cliff. The thickness of the beds +alluded to, between <i>d</i> and <i>g,</i> is about 2500 feet, the +erect trees consisting chiefly of large <i>Sigillariæ,</i> +occurring at ten distinct levels, one above the other. The usual +height of the buried trees seen by me was from six to eight feet; +but one trunk was about 25 feet high and four feet in diameter, +with a considerable bulge at the base. In no instance could I +detect any trunk intersecting a layer of coal, however thin; and +most of the trees terminated downward in seams of coal. Some few +only were based on clay and shale; none of them, except <i> +Calamites,</i> on sandstone. The erect trees, therefore, appeared +in general to have grown on beds of vegetable matter. In the +underclays <i>Stigmaria</i> abounds.</p> + +<center><img src="../images3/fig439.jpg" width="394" height="142" alt= +"Fig. 439: Section of the cliffs of the South Joggins, near Minudie, Nova Scotia."> +</center> + +<p>These root-bearing beds have been found under all the +coal-seams, and such old soils are at present the most destructible +masses in the whole cliff, the sandstones and laminated shales +being harder and more capable of resisting the action of the waves +and the weather. Originally the reverse was doubtless true, for in +the existing delta of the Mississippi those clays in which the +innumerable roots of the deciduous cypress and other swamp trees +ramify in all directions are seen to withstand far more effectually +the undermining power of the river, or of the sea at the base of +the delta, than do beds of loose sand or layers of mud not +supporting trees. It is obvious that if this sand or mud be +afterwards consolidated and turned to sandstone and hard shale, it +would be the least destructible.</p> + +<p>In regard to the plants, they belonged to the same genera, and +most of them to the same species, as those met with in the distant +coal-fields of Europe. Dr. Dawson has enumerated more than 150 +species, two-thirds of which are European, a greater agreement than +can be said to exist between the same Nova Scotia flora and that of +the coal-fields of the United States. By referring to the section, +Fig. 439, the position of the four-foot coal will be perceived, and +in Fig. 440 (a section made by me in 1842 of a small portion) that +from <i>e</i> to <i>f</i></p> + +<p> </p> + +<hr> +<p class="page"><a name="page 411">[ 411 ]</a></p> + +<p>of the same cliff is exhibited, in order to show the manner of +occurrence of erect fossil trees at right angles to the planes of +the inclined strata.</p> + +<img src="../images3/fig440.jpg" width="284" height="207" alt= +"Fig. 440: Erect fossil trees, Coal-measures, Nova Scotia." align= +"right"> + +<p>In the sandstone which filled their interiors, I frequently +observed fern-leaves, and sometimes fragments of <i>Stigmaria,</i> +which had evidently entered together with sediment after the trunk +had decayed and become hollow, and while it was still standing +under water. Thus the tree, <i>a,</i> Fig. 440, represented in the +bed <i>e</i> in the section, Fig. 439, is a hollow trunk five feet +eight inches in length, traversing various strata, and cut off at +the top by a layer of clay two feet thick, on which rests a seam of +coal (<i>b,</i> Fig. 440) one foot thick. On this coal again stood +two large trees (<i>c</i> and <i>d</i>), while at a greater height +the trees <i>f</i> and <i>g</i> rest upon a thin seam of coal +(<i>e</i>), and above them is an underclay, supporting the +four-foot coal.</p> + +<p>Occasionally the layers of matter in the inside of the tree are +more numerous than those without; but it is more common in the +coal-measures of all countries to find a cylinder of pure +sandstone—the cast of the interior of a +tree—intersecting a great many alternating beds of shale and +sandstone, which originally enveloped the trunk as it stood erect +in the water. Such a want of correspondence in the materials +outside and inside, is just what we might expect if we reflect on +the difference of time at which the deposition of sediment will +take place in the two cases; the imbedding of the tree having gone +on for many years before its decay had made much progress. In many +places distinct proof is seen that the enveloping strata took years +to accumulate, for some of the sandstones surrounding erect +sigillarian trunks support at different levels roots and stems of +<i>Calamites</i>; the <i>Calamites</i> having begun to grow after +the older <i>Sigillariæ</i> had been partially buried.</p> + +<p>The general absence of structure in the interior of the large +fossil trees of the Coal implies the very durable nature of their +bark, as compared with their woody portion. The</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 412">[ 412 ]</a></p> + +<p>same difference of durability of bark and wood exists in modern +trees, and was first pointed out to me by Dr. Dawson, in the +forests of Nova Scotia, where the Canoe Birch (<i>Betula +papyracea</i>) has such tough bark that it may sometimes be seen in +the swamps looking externally sound and fresh, although consisting +simply of a hollow cylinder with all the wood decayed and gone. +When portions of such trunks have become submerged in the swamps +they are sometimes found filled with mud. One of the erect fossil +trees of the South Joggins fifteen feet in height, occurring at a +higher level than the main coal, has been shown by Dr. Dawson to +have a coniferous structure, so that some <i>Coniferæ</i> of +the Coal period grew in the same swamps as <i>Sigillariæ,</i> +just as now the deciduous Cypress (<i>Taxodium distichum</i>) +abounds in the marshes of Louisiana even to the edge of the +sea.</p> + +<p>When the carboniferous forests sank below high-water mark, a +species of <i>Spirorbis</i> or <i>Serpula</i> (<a href= +"../images3/fig431.jpg">Fig. 431</a>), attached itself to the outside +of the stumps and stems of the erect trees, adhering occasionally +even to the interior of the bark—another proof that the +process of envelopment was very gradual. These hollow upright +trees, covered with innumerable marine annelids, reminded me of a +“cane-brake,” as it is commonly called, consisting of +tall reeds, <i>Arundinaria macrosperma,</i> which I saw in 1846, at +the Balize, or extremity of the delta of the Mississippi. Although +these reeds are fresh-water plants, they were covered with +barnacles, having been killed by an incursion of salt-water over an +extent of many acres, where the sea had for a season usurped a +space previously gained from it by the river. Yet the dead reeds, +in spite of this change, remained standing in the soft mud, +enabling us to conceive how easily the larger <i> +Sigillariæ,</i> hollow as they were but supported by strong +roots, may have resisted an incursion of the sea.</p> + +<p>The high tides of the Bay of Fundy, rising more than 60 feet, +are so destructive as to undermine and sweep away continually the +whole face of the cliffs, and thus a new crop of erect fossil trees +is brought into view every three or four years. They are known to +extend over a space between two and three miles from north to +south, and more than twice that distance from east to west, being +seen in the banks of streams intersecting the coal-field.</p> + +<p><i>Structure of Coal.</i>—The bituminous coal of Nova +Scotia is similar in composition and structure to that of Great +Britain, being chiefly derived from sigillarioid trees mixed with +leaves of ferns and of a Lycopodiaceous tree called <i> +Cordaites</i></p> + +<p> </p> + +<hr> +<p class="page"><a name="page 413">[ 413 ]</a></p> + +<p>(<i>Noeggerathia,</i> etc., for genus, see <a href= +"../images3/fig428.jpg">Fig. 428</a>), supposed by Dawson to have been +deciduous, and which had broad parallel veined leaves without a +mid-rib. On the surface of the seams of coal are large quantities +of mineral charcoal, which doubtless consist, as Dr. Dawson +suggests, of fragments of wood which decayed in the open air, as +would naturally be expected in swamps where so many erect trees +were preserved. Beds of cannel-coal display, says Dr. Dawson, such +a microscopical structure and chemical composition as shows them to +have been of the nature of fine vegetable mud such as accumulates +in the shallow ponds of modern swamps. The underclays are loamy +soils, which must have been sufficiently above water to admit of +drainage, and the absence of sulphurets, and the occurrence of +carbonate of iron in them, prove that when they existed as soils, +rain-water, and not sea-water, percolated them. With the exception, +perhaps, of <i>Asterophyllites</i> (see <a href= +"../images3/fig460.jpg">Fig. 461</a>), there is a remarkable absence +from the coal-measures of any form of vegetation properly aquatic, +the true coal being a sub-aërial accumulation in soil that was +wet and swampy but not permanently submerged.</p> + +<p><b>Air-breathers of the Coal.</b>—If we have rightly +interpreted the evidence of the former existence at more than +eighty different levels of forests of trees, some of them of vast +extent, and which lasted for ages, giving rise to a great +accumulation of vegetable matter, it is natural to ask whether +there were not many air-breathing inhabitants of these same +regions. As yet no remains of mammalia or birds have been found, a +negative character common at present to all the Palæozoic +formations; but in 1852 the osseous remains of a reptile, the first +ever met with in the carboniferous strata of the American +continent, were found by Dr. Dawson and myself. We detected them in +the interior of one of the erect Sigillariæ before alluded to +as of such frequent occurrence in Nova Scotia. The tree was about +two feet in diameter, and consisted of an external cylinder of +bark, converted into coal, and an internal stony axis of black +sandstone, or rather mud and sand stained black by carbonaceous +matter, and cemented together with fragments of wood into a rock. +These fragments were in the state of charcoal, and seem to have +fallen to the bottom of the hollow tree while it was rotting away. +The skull, jaws, and vertebræ of a reptile, probably about +2½ feet in length (<i>Dendrerpeton Acadianum,</i> Owen), +were scattered through this stony matrix. The shell, also, of a <i> +Pupa</i> (see <a href="../images3/fig442.jpg">Fig. 442</a>), the first +land-shell ever met with in the coal or in beds older than the +tertiary, was observed in the</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 414">[ 414 ]</a></p> + +<p>same stony mass. Dr. Wyman of Boston pronounced the reptile to +be allied in structure to <i>Menobranchus</i> and <i>Menopoma,</i> +species of batrachians, now inhabiting the North American rivers. +The same view was afterwards confirmed by Professor Owen, who also +pointed out the resemblance of the cranial plates to those seen in +the skull of <i>Archegosaurus</i> and <i>Labyrinthodon.</i>* +Whether the creature had crept into the hollow tree while its top +was still open to the air, or whether it was washed in with mud +during a flood, or in whatever other manner it entered, must be +matter of conjecture.</p> + +<p>Footprints of two reptiles of different sizes had previously +been observed by Dr. Harding and Dr. Gesner on ripple-marked flags +of the lower coal-measures in Nova Scotia (No. 2, <a href= +"../images3/fig447.jpg">Fig. 447</a>), evidently made by quadrupeds +walking on the ancient beach, or out of the water, just as the +recent Menopoma is sometimes observed to do. The remains of a +second and smaller species of Dendrerpeton, <i>D. Oweni,</i> were +also found accompanying the larger one, and still retaining some of +its dermal appendages; and in the same tree were the bones of a +third small lizard-like reptile, <i>Hylonomus Lyelli,</i> seven +inches long, with stout hind limbs, and fore limbs comparatively +slender, supposed by Dr. Dawson to be capable of walking and +running on land.†</p> + +<center><img src="../images3/fig441.jpg" width="386" height="160" alt= +"Fig. 441: Xylobius Sigillariæ. Coal, Nova Scotia."></center> + +<p>In a second specimen of an erect stump of a hollow tree 15 +inches in diameter, the ribbed bark of which showed that it was a +Sigillaria, and which belonged to the same forest as the specimen +examined by us in 1852, Dr. Dawson obtained not only fifty +specimens of Pupa vetusta (Fig. 442), and nine skeletons of +reptiles belonging to four species, but also several examples of an +articulated animal resembling the recent centipede or gally-worm, a +creature which feeds on decayed vegetable matter (see Fig. 441). +Under the microscope, the</p> + +<p class="fnote">* Quart. Geol. Journ., vol. ix, p. 58.<br> +† Dawson, Air-Breathers of the Coal in Nova Scotia, +Montreal, 1863.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 415">[ 415 ]</a></p> + +<p>head, with the eyes, mandible, and labrum, are well seen. It is +interesting, as being the earliest known representative of the +myriapods, none of which had previously been met with in rocks +older than the oolite or lithographic slate of Germany.</p> + +<img src="../images3/fig442.jpg" width="118" height="255" alt= +"Fig. 442: Pupa vetusta." align="right"> + +<p>Some years after the discovery of the first Pupa, Dr. Dawson, +carefully examining the same great section containing so many +buried forests in the cliffs of Nova Scotia, discovered another +bed, separated from the tree containing Dendrerpeton by a mass of +strata more than 1200 feet thick. As there were 21 seams of coal in +this intervening mass, the length of time comprised in the interval +is not to be measured by the mere thickness of the sandstones and +shales. This lower bed is an underclay seven feet thick, with +stigmarian rootlets, and the small land-shells occurring in it are +in all stages of growth. They are chiefly confined to a layer about +two inches thick, and are unmixed with any aquatic shells. They +were all originally entire when imbedded, but are most of them now +crushed, flattened, and distorted by pressure; they must have been +accumulated, says Dr. Dawson, in mud deposited in a pond or +creek.</p> + +<img src="../images3/fig443.jpg" width="126" height="134" alt= +"Fig. 443: Zonites (Conulus) priseus." align="left"> + +<p>The surface striæ of <i>Pupa vetusta,</i> when magnified +50 diameters, present exactly the same appearance as a portion +corresponding in size of the common English <i>Pupa juniperi,</i> +and the internal hexagonal cells, magnified 500 diameters, show the +internal structure of the fossil and recent Pupa to be identical. +In 1866* Dr. Dawson discovered in this lower bed, so full of the +Pupa, another land-shell of the genus Helix (sub-genus Zonites), +see Fig. 443.</p> + +<p>None of the reptiles obtained from the coal-measures of the +South Joggins are of a higher grade than the Labyrinthodonts, but +some of these were of very great size, two caudal vertebræ +found by Mr. Marsh in 1862 measuring two and a half inches in +diameter, and implying a gigantic aquatic reptile with a powerful +swimming tail.</p> + +<p>Except some obscure traces of an insect found by Dr.</p> + +<p class="fnote">* Dawson, Acadian Geology, 1868, p. 385.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 416">[ 416 ]</a></p> + +<p>Dawson in a coprolite of a terrestrial reptile occurring in a +fossil tree, no specimen of this class has been brought to light in +the Joggins. But Mr. James Barnes found in a bed of shale at Little +Grace Bay, Cape Breton, the wing of an Ephemera, which must have +measured seven inches from tip to tip of the expanded +wings—larger than any known living insect of the Neuropterous +family.</p> + +<p>That we should have made so little progress in obtaining a +knowledge of the terrestrial fauna of the Coal is certainly a +mystery, but we have no reason to wonder at the extreme rarity of +insects, seeing how few are known in the carboniferous rocks of +Europe, worked for centuries before America was discovered, and now +quarried on so enormous a scale. These European rocks have not yet +produced a single land-shell, in spite of the millions of tons of +coal annually extracted, and the many hundreds of soils replete +with the fossil roots of trees, and the erect trunks and stumps +preserved in the position in which they grew. In many large +coal-fields we continue as much in the dark respecting the +invertebrate air-breathers then living, as if the coal had been +thrown down in mid-ocean. The early date of the carboniferous +strata can not explain the enigma, because we know that while the +land supported a luxuriant vegetation, the contemporaneous seas +swarmed with life—with Articulata, Mollusca, Radiata, and +Fishes. The perplexity in which we are involved when we attempt to +solve this problem may be owing partly to our want of diligence as +collectors, but still more perhaps to ignorance of the laws which +govern the fossilisation of land-animals, whether of high or low +degree.</p> + +<p><b>Carboniferous Rain-prints.</b>—At various levels in the +coal measures of Nova Scotia, ripple-marked sandstones, and shales +with rain-prints, were seen by Dr. Dawson and myself, but still +more perfect impressions of rain were discovered by Mr. Brown, near +Sydney, in the adjoining island of cape Breton. They consist of +very delicate markings on greenish slates, accompanied by +worm-tracks (<i>a, b,</i> Fig. 444), such as are often seen between +high and low water mark on the recent mud of the Bay of Fundy.</p> + +<p>The great humidity of the climate of the Coal period had been +previously inferred from the number of its ferns and the continuity +of its forests for hundreds of miles; but it is satisfactory to +have at length obtained such positive proofs of showers of rain, +the drops of which resembled in their average size those which now +fall from the clouds. From such data we may presume that the +atmosphere of the Carboniferous period corresponded in density with +that now investing</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 417">[ 417 ]</a></p> + +<p>the globe, and that different currents of air varied then as now +in temperature, so as to give rise, by their mixture, to the +condensation of aqueous vapour.</p> + +<center><img src="../images3/fig444.jpg" width="411" height="306" alt= +"Fig. 444: Carboniferous rain-prints with worm tracks on green shale, from Cape Brton, Nova Scotia. Fig. 445: Casts of rain-prints on a portion of the same slab (Fig. 444), seen to project on the underside of an incumbent layer of arenaceous shale."> +</center> + +<p><b>Folding and Denudation of the Beds indicated by the Nova +Scotia Coal-strata.</b>—The series of events which are +indicated by the great section of the coal-strata in Nova Scotia +consist of a gradual and long-continued subsidence of a tract which +throughout most of the period was in the state of a delta, though +occasionally submerged beneath a sea of moderate depth. Deposits of +mud and sand were first carried down into a shallow sea on the low +shores of which the footprints of reptiles were sometimes impressed +(see <a href="#page 407">p. 407</a>).</p> + +<img src="../images3/fig446.jpg" width="149" height="216" alt= +"Fig. 446: Cone and branch of Lepidodendron corrugatum." align= +"right"> + +<p>Though no regular seams of coal were formed, the characteristic +imbedded coal-plants are of the genera <i>Cyclopteris</i> and <i> +Alethopteris,</i> agreeing with species occurring at much higher +levels, and distinct from those of the antecedent Devonian group. +The <i>Lepidodendron corrugatum</i> (see Fig. 446), a plant +predominating in the Lower Carboniferous group of Europe, is also +conspicuous in these shallow-water beds, together with many fishes +and entomostracans. A more rapid rate of subsidence sometimes +converted part of the sea into deep clear water, in which there</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 418">[ 418 ]</a></p> + +<p>was a growth of coral which was afterwards turned into +crystalline limestone, and parts of it, apparently by the action of +sulphuric acid, into gypsum. In spite of continued sinking, +amounting to several thousand feet, the sea might in time have been +rendered shallow by the growth of coral, had not its conversion +into land or swampy ground been accelerated by the pouring in of +sand and the advance of the delta accompanied with such fluviatile +and brackish-water formations as are common in lagoons.</p> + +<p>The amount to which the bed of the sea sank down in order to +allow of the formation of so vast a thickness of rock of +sedimentary and organic origin is expressed by the total thickness +of the Carboniferous strata, including the coal-measures, No. 1, +and the rocks which underlie them, No. 2, Fig. 447.</p> + +<center><img src="../images3/fig447.jpg" width="400" height="187" alt= +"Fig. 447: Diagram showing the curvature and supposed denudation of the Carboniferous strata in Nova Scotia."> +</center> + +<p>After the strata No. 2 had been elaborated, the conditions +proper to a great delta exclusively prevailed, the subsidence still +continuing so that one forest after another grew and was submerged +until their under-clays with roots, and usually seams of coal, were +left at more than eighty distinct levels. Here and there, also, +deposits bearing testimony to the existence of fresh or +brackish-water lagoons, filled with calcareo-bituminous mud, were +formed. In these beds (<i>h</i> and <i>i,</i> <a href= +"../images3/fig439.jpg">Fig. 439</a>) are found fresh-water bivalves +or mussels allied to Anodon, though not identical with that or any +living genus, and called <i>Naiadites carbonarius</i> by Dawson. +They are associated with small entomostracous crustaceans of the +genus Cythere, and scales of small fishes. Occasionally some of the +calamite brakes and forests of Sigillariæ and Coniferæ +were exposed in the flood season, or sometimes, perhaps, by slight +elevatory movements to the denuding action of the river or the +sea.</p> + +<p>In order to interpret the great coast section exposed to view on +the shores of the Bay of Fundy, the student must,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 418">[ 418 ]</a></p> + +<p>in the first place, understand that the newest or last-mentioned +coal formations would have been the only ones known to us (for they +would have covered all the others), had there not been two great +movements in opposite directions, the first consisting of a general +sinking of three miles, which took place during the Carboniferous +Period, and the second an upheaval of more limited horizontal +extent, by which the anticlinal axis A was formed. That the first +great change of level was one of subsidence is proved by the fact +that there are shallow-water deposits at the base of the +Carboniferous series, or in the lowest beds of No. 2.</p> + +<p>Subsequent movements produced in the Nova Scotia and the +adjoining New Brunswick coal-fields the usual anticlinal and +synclinal flexures. In order to follow these, we must survey the +country for about thirty miles round the South Joggins, or the +region where the erect trees described in the foregoing pages are +seen. As we pass along the cliffs for miles in a southerly +direction, the beds containing these fossil trees, which were +mentioned as dipping about 18° south, are less and less +inclined, until they become nearly horizontal in the valley of a +small river called the Shoulie, as ascertained by Dr. Dawson. After +passing this synclinal line the beds begin to dip in an opposite or +north-easterly direction, acquiring a steep dip where they rest +unconformably on the edges of the Upper Silurian strata of the +Cobequid Hills, as shown in Fig. 447. But if we travel northward +towards Minudie from the region of the coal-seams and buried +forests, we find the dip of the coal-strata increasing from an +angle of 18° to one of more than 40°, lower beds being +continually exposed to view until we reach the anticlinal axis A +and see the lower Carboniferous formation, No. 2, at the surface. +The missing rocks removed by denudation are expressed by the faint +lines at A, and thus the student will see that, according to the +principles laid down in the seventh chapter, we are enabled, by the +joint operations of upheaval and denudation, to look, as it were, +about three miles into the interior of the earth without passing +beyond the limits of a single formation.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch22.html"> +Chapter XXII</a> / <a href="ch24.html">Chapter XXIV</a></small> +</body> +</html> + |