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diff --git a/old/3772-h/files/ch30.html b/old/3772-h/files/ch30.html new file mode 100644 index 0000000..848d37e --- /dev/null +++ b/old/3772-h/files/ch30.html @@ -0,0 +1,780 @@ +<!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 536">[ 536 ]</a></p> + +<p> </p> + +<center><b>Chapter XXX</b><br> +<br> +AGE OF VOLCANIC ROCKS—<i>continued.</i></center> + +<p class="intro">Volcanic Rocks of the Upper Miocene Period. +— Madeira. — Grand Canary. — Azores. — +Lower Miocene Volcanic Rocks. — Isle of Mull. — Staffa +and Antrim. — The Eifel. — Upper and Lower Miocene +Volcanic Rocks of Auvergne. — Hill of Gergovia. — +Eocene Volcanic Rocks of Monte Bolca. — Trap of Cretaceous +Period. — Oolitic Period. — Triassic Period. — +Permian Period. — Carboniferous Period. — Erect Trees +buried in Volcanic Ash in the Island of Arran. — Old Red +Sandstone Period. — Silurian Period. — Cambrian Period. +— Laurentian Volcanic Rocks.</p> + +<p><b>Volcanic Rocks of the Upper Miocene +Period.</b>—<i>Madeira.</i>—The greater part of the +volcanic eruptions of Madeira, as we have already seen (<a href= +"ch29.html#page 532">p. 532</a>), belong to the Pliocene Period, +but the most ancient of them are of Upper Miocene date, as shown by +the fossil shells included in the marine tuffs which have been +upraised at San Vicente, in the northern part of the island, to the +height of 1300 feet above the level of the sea. A similar marine +and volcanic formation constitutes the fundamental portion of the +neighbouring island of Porto Santo, forty miles distant from +Madeira, and is there elevated to an equal height, and covered, as +in Madeira, with lavas of supra-marine origin.</p> + +<p>The largest number of fossils have been collected from the tuffs +and conglomerates and some beds of limestone in the island of +Baixo, off the southern extremity of Porto Santo. They amount in +this single locality to more than sixty in number, of which about +fifty are mollusca, but many of these are only casts. Some of the +shells probably lived on the spot during the intervals between +eruptions, and some may have been cast up into the water or air +together with muddy ejections, and, falling down again, have been +deposited on the bottom of the sea. The hollows in some of the +fragments of vesicular lava of which the breccias and conglomerates +are composed are partially filled with calc-sinter, being thus half +converted into amygdaloids. Among the fossil shells common to +Madeira and Porto Santo, large cones, strombs, and cowries are +conspicuous among the univalves, and <i>Cardium, Spondylus,</i> and +<i>Lithodomus</i> among the lamellibranchiate bivalves, and among +the <i>Echinoderms</i> the large Clypeaster called <i>C. altus,</i> +an extinct European Miocene fossil.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 537">[ 537 ]</a></p> + +<p>The largest list of fossils has been published by Mr. Karl +Meyer, in Hartung’s “Madeira;” but in the +collection made by myself, and in a still larger one formed by Mr. +J. Yate Johnson, several remarkable forms not in Meyer’s list +occur, as, for example, <i>Pholadomya,</i> and a large <i> +Terebra.</i> Mr. Johnson also found a fine specimen of <i>Nautilus +(Atruria) ziczac</i> (<a href="../images1/fig207.jpg">Fig. 211</a>), a +well-known Falunian fossil of Europe; and in the same volcanic tuff +of Baixo, the Echinoderm <i>Brissus Scillæ,</i> a living +Mediterranean species, found fossil in the Miocene strata of Malta. +Mr. Meyer identifies one-third of the Madeira shells with known +European Miocene (or Falunian) forms. The huge Strombus of San +Vicente and Porto Santo, <i>S. Italicus,</i> is an extinct shell of +the Sub-apennine or Older Pliocene formations. The mollusca already +obtained from various localities of Madeira and Porto Santo are not +less than one hundred in number, and, according to the late Dr. S. +P. Woodward, rather more than a third are of species still living, +but many of these are not now inhabitants of the neighbouring +sea.</p> + +<p>It has been remarked (<a href="ch14.html#page 212">p. 212</a>), +that in the Older Pliocene and Upper Miocene deposits of Europe +many forms occur of a more southern aspect than those now +inhabiting the nearest sea. In like manner the fossil corals, or +Zoantharia, six in number, which I obtained from Madeira, of the +genera <i>Astræa, Sarcinula, Hydnophora,</i> were pronounced +by Mr. Lonsdale to be forms foreign to the adjacent coasts, and +agreeing with the fauna of a sea warmer than that now separating +Madeira from the nearest part of the African coast. We learn, +indeed, from the observations made in 1859, by the Reverend R. T. +Lowe, that more than one-half, or fifty-three in ninety, of the +marine mollusks collected by him from the sandy beach of Mogador +are common British species, although Mogador is 18½ degrees +south of the nearest shores of England. The living shells of +Madeira and Porto Santo are in like manner those of a temperate +climate, although in great part differing specifically from those +of Mogador.*</p> + +<p><i>Grand Canary.</i>—In the Canaries, especially in the +Grand Canary, the same marine Upper Miocene formation is found. +Stratified tuffs, with intercalated conglomerates and lavas, are +there seen in nearly horizontal layers in sea-cliffs about 300 feet +high, near Las Palmas. Mr. Hartung and I were unable to find marine +shells in these tuffs at a greater elevation than 400 feet above +the sea; but as the deposit to which they belong reaches to the +height of 1100 feet or more in the interior, we conceive that an +upheaval of at least that amount has</p> + +<p class="fnote">* Linnean Proceedings; Zoology, 1860.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 538">[ 538 ]</a></p> + +<p>taken place. The <i>Clypeaster altus, Spondylus gæderopus, +Pectunculus pilosus, Cardita calyculata,</i> and several other +shells, serve to identify this formation with that of the Madeiras, +and <i>Ancillaria glandiformis,</i> which is not rare, and some +other fossils, remind us of the faluns of Touraine.</p> + +<p>The sixty-two Miocene species which I collected in the Grand +Canary were referred by the late Dr. S. P. Woodward to forty-seven +genera, ten of which are no longer represented in the neighbouring +sea, namely <i>Corbis,</i> an African form, Hinnites, now living in +Oregon, <i>Thecidium</i> (<i>T. Mediterranean,</i> identical with +the Miocene fossil of St. Juvat, in Brittany), <i>Calyptræa, +Hipponyx, Nerita, Erato, Oliva, Ancillaria,</i> and <i> +Fasciolaria.</i></p> + +<p>These tuffs of the southern shores of the Grand Canary, +containing the Upper Miocene shells, appear to be about the same +age as the most ancient volcanic rocks of the island, composed of +slaty diabase, phonolite, and trachyte. Over the marine lavas and +tuffs trachytic and basaltic products of subaërial volcanic +origin, between 4000 and 5000 feet in thickness, have been piled, +the central parts of the Grand Canary reaching the height of about +6000 feet above the level of the sea. A large portion of this mass +is of Pliocene date, and some of the latest lavas have been poured +out since the time when the valleys were already excavated to +within a few feet of their present depth.</p> + +<p>On the whole, the rocks of the Grand Canary, an island of a +nearly circular shape, and 6½ geographical miles diameter, +exhibit proofs of a long series of eruptions beginning like those +of Madeira, Porto Santo, and the Azores, in the Upper Miocene +period, and continued to the Post-Pliocene. The building up of the +Grand Canary by subaërial eruptions, several thousand feet +thick, went on simultaneously with the gradual upheaval of the +earliest products of submarine eruptions, in the same manner as the +Pliocene marine strata of the oldest parts of Vesuvius and Etna +have been upraised during eruptions of Post-tertiary date.</p> + +<p>In proof that movements of elevation have actually continued +down to Post-tertiary times, I may remark that I found raised +beaches containing shells of the Recent Period in the Grand Canary, +Teneriffe, and Porto Santo. The most remarkable raised beach which +I observed in the Grand Canary, in the study of which I was +assisted by Don Pedro Maffiotte, is situated in the north-eastern +part of the island at San Catalina, about a quarter of a mile north +of Las Palmas. It intervenes between the base of the high cliff +formed of the tuffs with Miocene shells and the sea-shore. From</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 539">[ 539 ]</a></p> + +<p>this beach, at an elevation of twenty-five feet above high-water +mark, and at a distance of about 150 feet from the present shore, I +obtained more than fifty species of living marine shells. Many of +them, according to Dr. S. P. Woodward, are no longer inhabitants of +the contiguous sea, as, for example, <i>Strombus bubonius,</i> +which is still living on the West Coast of Africa, and <i>Cerithium +procerum,</i> found at Mozambique; others are Mediterranean +species, as <i>Pecten Jacobæus</i> and <i>P. polymorphus.</i> +Some of these testacea, such as <i>Cardita squamosa,</i> are +inhabitants of deep water, and the deposit on the whole seems to +indicate a depth of water exceeding a hundred feet.</p> + +<p><i>Azores.</i>—In the island of St. Mary’s, one of +the Azores, marine fossil shells have long been known. They are +found on the north-east coast on a small projecting promontory +called Ponta do Papagaio (or Point-Parrot), chiefly in a limestone +about twenty feet thick, which rests upon, and is again covered by, +basaltic lavas, scoriæ, and conglomerates. The pebbles in the +conglomerate are cemented together with carbonate of lime.</p> + +<p>Mr. Hartung, in his account of the Azores, published in 1860, +describes twenty-three shells from St. Mary’s,* of which +eight perhaps are identical with living species, and twelve are +with more or less certainty referred to European Tertiary forms, +chiefly Upper Miocene. One of the most characteristic and abundant +of the new species, <i>Cardium Hartungi,</i> not known as fossil in +Europe, is very common in Porto Santo and Baixo, and serves to +connect the Miocene fauna of the Azores and the Madeiras. In some +of the Azores, as well as in the Canary islands, the volcanic fires +are not yet extinct, as the recorded eruptions of Lanzerote, +Teneriffe, Palma, St. Michael’s, and others, attest.</p> + +<p><b>Lower Miocene Volcanic Rocks.</b>—<i>Isle of Mull and +Antrim.</i>—I may refer the reader to the account already +given (<a href="ch15.html#page 247">p. 247</a>) of leaf-beds at +Ardtun, in the Isle of Mull in the Hebrides, which bear a relation +to the associated volcanic rocks of Lower Miocene date analogous to +that which the Madeira leaf-bed, above described (<a href= +"ch29.html#page 532">p. 532</a>), bears to the Pliocene lavas of +that island. Mr. Geikie has shown that the volcanic rocks in Mull +are above 3000 feet in thickness. There seems little doubt that the +well-known columnar basalt of Staffa, as well as that of Antrim in +Ireland, are of the same age, and not of higher antiquity, as once +suspected.</p> + +<p><i>The Eifel.</i>—A large portion of the volcanic rocks of +the</p> + +<p class="fnote">* Hartung, Die Azoren, 1860; also Insel Gran +Canaria, Madeira und Porto Santo, 1864, Leipsig.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 540">[ 540 ]</a></p> + +<p>Lower Rhine and the Eifel are coeval with the Lower Miocene +deposits to which most of the “Brown-Coal” of Germany +belongs. The Tertiary strata of that age are seen on both sides of +the Rhine, in the neighbourhood of Bonn, resting unconformably on +highly inclined and vertical strata of Silurian and Devonian rocks. +The Brown-Coal formation of that region consists of beds of loose +sand, sandstone, and conglomerate, clay with nodules of +clay-iron-stone, and occasionally silex. Layers of light brown and +sometimes black lignite are interstratified with the clays and +sands, and often irregularly diffused through them. They contain +numerous impressions of leaves and stems of trees, and are +extensively worked for fuel, whence the name of the formation. In +several places layers of trachytic tuff are interstratified, and in +these tuffs are leaves of plants identical with those found in the +brown-coal, showing that, during the period of the accumulation of +the latter, some volcanic products were ejected. The igneous rocks +of the Westerwald, and of the mountains called the Siebengebirge, +consist partly of basaltic and partly of trachytic lavas, the +latter being in general the more ancient of the two. There are many +varieties of trachyte, some of which are highly crystalline, +resembling a coarse-grained granite, with large separate crystals +of feldspar. Trachytic tuff is also very abundant.</p> + +<p>M. Von Dechen, in his work on the Siebengebirge,* has given a +copious list of the animal and vegetable remains of the fresh-water +strata associated with the brown-coal of that part of Germany. +Plants of the genera <i>Flabellaria, Ceanothus,</i> and <i> +Daphnogene,</i> including <i>D. cinnamomifolia</i> (<a href= +"../images1/fig155.jpg">Fig. 155</a>), occur in these beds, with +nearly 150 other plants. The fishes of the brown-coal near Bonn are +found in a bituminous shale, called paper-coal, from being +divisible into extremely thin leaves. The individuals are very +numerous; but they appear to belong to a small number of species, +some of which were referred by Agassiz to the genera <i>Leuciscus, +Aspius,</i> and <i>Perca.</i> The remains of frogs also, of extinct +species, have been discovered in the paper-coal; and a complete +series may be seen in the museum at Bonn, from the most imperfect +state of the tadpole to that of the full-grown animal. With these a +salamander, scarcely distinguishable from the recent species, has +been found, and the remains of many insects.</p> + +<p><b>Upper and Lower Miocene Volcanic Rocks of +Auvergne.</b>—The extinct volcanoes of Auvergne and Cantal, +in central France, seem to have commenced their eruptions in the +Lower</p> + +<p class="fnote">* Geognost. Beschreib. des Siebengebirges am +Rhein. Bonn, 1852.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 541">[ 541 ]</a></p> + +<p>Miocene period, but to have been most active during the Upper +Miocene and Pliocene eras. I have already alluded to the grand +succession of events of which there is evidence in Auvergne since +the last retreat of the sea (see <a href="ch29.html#page 527">p. +527</a>).</p> + +<p>The earliest monuments of the Tertiary Period in that region are +lacustrine deposits of great thickness, in the lowest conglomerates +of which are rounded pebbles of quartz, mica-schist, granite, and +other non-volcanic rocks, without the slightest intermixture of +igneous products. To these conglomerates succeed argillaceous and +calcareous marls and limestones, containing Lower Miocene shells +and bones of mammalia, the higher beds of which sometimes alternate +with volcanic tuff of contemporaneous origin. After the filling up +or drainage of the ancient lakes, huge piles of trachytic and +basaltic rocks, with volcanic breccias, accumulated to a thickness +of several thousand feet, and were superimposed upon granite, or +the contiguous lacustrine strata. The greater portion of these +igneous rocks appear to have originated during the Upper Miocene +and Pliocene periods; and extinct quadrupeds of those eras, +belonging to the genera Mastodon, Rhinoceros, and others, were +buried in ashes and beds of alluvial sand and gravel, which owe +their preservation to overspreading sheets of lava.</p> + +<p>In Auvergne, the most ancient and conspicuous of the volcanic +masses is Mont Dor, which rests immediately on the granitic rocks +standing apart from the fresh-water strata. This great mountain +rises suddenly to the height of several thousand feet above the +surrounding platform, and retains the shape of a flattened and +somewhat irregular cone, the slope of which is gradually lost in +the high plain around. This cone is composed of layers of +scoriæ, pumice-stones, and their fine detritus, with +interposed beds of trachyte and basalt, which descend often in +uninterrupted sheets until they reach and spread themselves round +the base of the mountain.* Conglomerates, also, composed of angular +and rounded fragments of igneous rocks, are observed to alternate +with the above; and the various masses are seen to dip off from the +central axis, and to lie parallel to the sloping flanks of the +mountain. The summit of Mont Dor terminates in seven or eight rocky +peaks, where no regular crater can now be traced, but where we may +easily imagine one to have existed, which may have been shattered +by earthquakes, and have suffered degradation by aqueous agents. +Originally, perhaps, like the highest crater of Etna, it may have +formed</p> + +<p class="fnote">* Scrope’s Central France, p. 98.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 542">[ 542 ]</a></p> + +<p>an insignificant feature in the great pile, and, like it, may +frequently have been destroyed and renovated.</p> + +<p>Respecting the age of the great mass of Mont Dor, we can not +come at present to any positive decision, because no organic +remains have yet been found in the tuffs, except impressions of the +leaves of trees of species not yet determined. It has already been +stated (<a href="ch15.html#page 234">p. 234</a>) that the earliest +eruptions must have been posterior in origin to those grits and +conglomerates of the fresh-water formation of the Limagne which +contain no pebbles of volcanic rocks. But there is evidence at a +few points, as in the hill of Gergovia, presently to be mentioned, +that some eruptions took place before the great lakes were drained, +while others occurred after the desiccation of those lakes, and +when deep valleys had already been excavated through fresh-water +strata.</p> + +<p>The valley in which the cone of Tartaret, above-mentioned (<a +href="ch29.html#page 527">p. 527</a>), is situated affords an +impressive monument of the very different dates at which the +igneous eruptions of Auvergne have happened; for while the cone +itself is of Post-Pliocene date, the valley is bounded by lofty +precipices composed of sheets of ancient columnar trachyte and +basalt, which once flowed from the summit of Mont Dor in some part +of the Miocene period. These Miocene lavas had accumulated to a +thickness of nearly 1000 feet before the ravine was cut down to the +level of the river Couze, a river which was at length dammed up by +the modern cone and the upper part of its course transformed into a +lake.</p> + +<p><i>Gergovia.</i>—It has been supposed by some observers +that there is an alternation of a contemporaneous sheet of lava +with fresh-water strata in the hill of Gergovia, near Clermont.</p> + +<center><img src="../images5/fig604.jpg" width="406" height="237" alt= +"Fig. 604: Hill of Gergovia."></center> + +<p> </p> + +<hr> +<p class="page"><a name="page 543">[ 543 ]</a></p> + +<p>But this idea has arisen from the intrusion of the dike +represented in Fig. 604, which has altered the green and white +marls both above and below. Nevertheless, there is a real +alternation of volcanic tuff with strata containing Lower Miocene +fresh-water shells, among others a Melania allied to <i>M. +inquinata</i> (<a href="../images1/fig216.jpg">Fig. 217</a>), with a +Melanopsis and a Unio; there can, therefore, be no doubt that in +Auvergne some volcanic explosions took place before the drainage of +the lakes, and at a time when the Lower Miocene species of animals +and plants still flourished.</p> + +<p><b>Eocene Volcanic Rocks.</b>—<i>Monte +Bolca.</i>—The fissile limestone of Monte Bolca, near Verona, +has for many centuries been celebrated in Italy for the number of +perfect Ichthyolites which it contains. Agassiz has described no +less than 133 species of fossil fish from this single deposit, and +the multitude of individuals by which many of the species are +represented is attested by the variety of specimens treasured up in +the principal museums of Europe. They have been all obtained from +quarries worked exclusively by lovers of natural history, for the +sake of the fossils. Had the lithographic stone of Solenhofen, now +regarded as so rich in fossils, been in like manner quarried solely +for scientific objects, it would have remained almost a sealed book +to palæontologists, so sparsely are the organic remains +scattered through it. When I visited Monte Bolca, in company with +Sir Roderick Murchison, in 1828, we ascertained that the +fish-bearing beds were of Eocene date, containing well-known +species of Nummulites, and that a long series of submarine volcanic +eruptions, evidently contemporaneous, had produced beds of tuff, +which are cut through by dikes of basalt. There is evidence here of +a long series of submarine volcanic eruptions of Eocene date, and +during some of them, as Sir R. Murchison has suggested, shoals of +fish were probably destroyed by the evolution of heat, noxious +gases, and tufaceous mud, just as happened when Graham’s +Island was thrown up between Sicily and Africa in 1831, at which +time the waters of the Mediterranean were seen to be charged with +red mud, and covered with dead fish over a wide area.*</p> + +<p>Associated with the marls and limestones of Monte Bolca are beds +containing lignite and shale with numerous plants, which have been +described by Unger and Massalongo, and referred by them to the +Eocene period. I have already cited (<a href= +"ch16.html#page 263">p. 263</a>) Professor Heer’s remark, +that several of the species are common to Monte Bolca and the white +clay of Alum Bay, a Middle Eocene deposit; and the same botanist +dwells on</p> + +<p class="fnote">* Principles of Geology, chap. xxvi, 9th ed., p. +432.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 544">[ 544 ]</a></p> + +<p>the tropical character of the flora of Monte Bolca and its +distinctness from the sub-tropical flora of the Lower Miocene of +Switzerland and Italy, in which last there is a far more +considerable mixture of forms of a temperate climate, such as the +willow, poplar, birch, elm, and others. That scarcely any one of +the Monte Bolca fish should have been found in any other locality +in Europe, is a striking illustration of the extreme imperfection +of the palæontological record. We are in the habit of +imagining that our insight into the geology of the Eocene period is +more than usually perfect, and we are certainly acquainted with an +almost unbroken succession of assemblages of shells passing one +into the other from the era of the Thanet sands to that of the +Bembridge beds or Paris gypsum. The general dearth, therefore, of +fish in the different members of the Eocene series, Upper, Middle, +and Lower, might induce a hasty reasoner to conclude that there was +a poverty of ichthyic forms during this period; but when a local +accident, like the volcanic eruptions of Monte Bolca, occurs, +proofs are suddenly revealed to us of the richness and variety of +this great class of vertebrata in the Eocene sea. The number of +genera of Monte Bolca fish is, according to Agassiz, no less than +seventy-five, twenty of them peculiar to that locality, and only +eight common to the antecedent Cretaceous period. No less than +forty-seven out of the seventy-five genera make their appearance +for the first time in the Monte Bolca rocks, none of them having +been met with as yet in the antecedent formations. They form a +great contrast to the fish of the secondary strata, as, with the +exception of the Placoids, they are all Teleosteans, only one +genus, <i>Pycnodus,</i> belonging to the order of Ganoids, which +form, as before stated, the vast majority of the ichthyolites +entombed in the secondary are Mesozoic rocks.</p> + +<p><b>Cretaceous Period.</b>—M. Virlet, in his account of the +geology of the Morea, p. 205, has clearly shown that certain traps +in Greece are of Cretaceous date; as those, for example, which +alternate conformably with cretaceous limestone and greensand +between Kastri and Damala, in the Morea. They consist in great part +of diallage rocks and serpentine, and of an amygdaloid with +calcareous kernels, and a base of serpentine. In certain parts of +the Morea, the age of these volcanic rocks is established by the +following proofs: first, the lithographic limestones of the +Cretaceous era are cut through by trap, and then a conglomerate +occurs, at Nauplia and other places, containing in its calcareous +cement many well-known fossils of the chalk and greensand, together +with pebbles</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 545">[ 545 ]</a></p> + +<p>ormed of rolled pieces of the same serpentinous trap, which +appear in the dikes above alluded to.</p> + +<p><b>Period of Oolite and Lias.</b>—Although the green and +serpentinous trap-rocks of the Morea belong chiefly to the +Cretaceous era, as before mentioned, yet it seems that some +eruptions of similar rocks began during the Oolitic period;* and it +is probable that a large part of the trappean masses, called +ophiolites in the Apennines, and associated with the limestone of +that chain, are of corresponding age.</p> + +<p><b>Trap of the New Red Sandstone Period.</b>—In the +southern part of Devonshire, trappean rocks are associated with New +Red Sandstone, and, according to Sir H. De la Beche, have not been +intruded subsequently into the sandstone, but were produced by +contemporaneous volcanic action. Some beds of grit, mingled with +ordinary red marl, resemble sands ejected from a crater; and in the +stratified conglomerates occurring near Tiverton are many angular +fragments of trap porphyry, some of them one or two tons in weight, +intermingled with pebbles of other rocks. These angular fragments +were probably thrown out from volcanic vents, and fell upon +sedimentary matter then in the course of deposition.†</p> + +<p><b>Trap of the Permian Period.</b>—The recent +investigations of Mr. Archibald Geikie in Ayrshire have shown that +some of the volcanic rocks in that county are of Permian age, and +it appears highly probable that the uppermost portion of +Arthur’s Seat in the suburbs of Edinburgh marks the site of +an eruption of the same era.</p> + +<p><b>Trap of the Carboniferous Period.</b>—Two classes of +contemporaneous trap-rocks occur in the coal-field of the Forth, in +Scotland. The newest of these, connected with the higher series of +coal-measures, is well exhibited along the shores of the Forth, in +Fifeshire, where they consist of basalt with olivine, amygdaloid, +greenstone, wacke, and tuff. They appear to have been erupted while +the sedimentary strata were in a horizontal position, and to have +suffered the same dislocations which those strata have subsequently +undergone. In the volcanic tuffs of this age are found not only +fragments of limestone, shale, flinty slate, and sandstone, but +also pieces of coal. The other or older class of carboniferous +traps are traced along the south margin of Stratheden, and +constitute a ridge parallel with the Ochils, and extending from +Stirling to near St. Andrews. They consist almost exclusively of +greenstone, becoming, in a few instances, earthy and amygdaloidal. +They are regularly interstratified with the</p> + +<p class="fnote">* Boblaye and Virlet, Morea, p. 23.<br> +† De la Beche, Geol. Proceedings, vol. ii, p. 198.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 546">[ 546 ]</a></p> + +<p>sandstone, shale, and iron-stone of the lower coal-measures, +and, on the East Lomond, with Mountain Limestone. I examined these +trap-rocks in 1838, in the cliffs south of St. Andrews, where they +consist in great part of stratified tuffs, which are curved, +vertical, and contorted, like the associated coal-measures. In the +tuff I found fragments of carboniferous shale and limestone, and +intersecting veins of greenstone.</p> + +<p><i>Fife—Flisk Dike.</i>—A trap dike was pointed out +to me by Dr. Fleming, in the parish of Flisk, in the northern part +of the county of Fife, which cuts through the grey sandstone and +shale, forming the lowest part of the Old Red Sandstone, but which +may probably be of carboniferous date. It may be traced for many +miles, passing through the amygdaloidal and other traps of the hill +called Norman’s Law in that parish. In its course it affords +a good exemplification of the passage from the trappean into the +Plutonic, or highly crystalline texture. Professor Gustavus Rose, +to whom I submitted specimens of this dike, found it to be +dolerite, and composed of greenish black augite and Labrador +feldspar, the latter being the most abundant ingredient. A small +quantity of magnetic iron, perhaps titaniferous, is also present. +The result of this analysis is interesting, because both the +ancient and modern lavas of Etna consist in like manner of augite, +Labradorite, and titaniferous iron.</p> + +<p><i>Erect Trees buried in Volcanic Ash at Arran.</i>—An +interesting discovery was made in 1867 by Mr. E. A. Wünsch in +the carboniferous strata of the north-eastern part of the island of +Arran. In the sea-cliff about five miles north of Corrie, near the +village of Laggan, strata of volcanic ash occur, forming a solid +rock cemented by carbonate of lime and enveloping trunks of trees, +determined by Mr. Binney to belong to the genera Sigillaria and +Lepidodendron. Some of these trees are at right angles to the +planes of stratification, while others are prostrate and +accompanied by leaves and fruits of the same genera. I visited the +spot in company with Mr. Wünsch in 1870, and saw that the +trees with their roots, of which about fourteen had been observed, +occur at two distinct levels in volcanic tuffs parallel to each +other, and inclined at an angle of about 40°, having between +them beds of shale and coaly matter seven feet thick. It is evident +that the trees were overwhelmed by a shower of ashes from some +neighbouring volcanic vent, as Pompeii was buried by matter ejected +from Vesuvius. The trunks, several of them from three to five feet +in circumference, remained with their Stigmarian roots spreading +through the stratum below, which had served as a soil. The trees +must have continued for</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 547">[ 547 ]</a></p> + +<p>years in an upright position after they were killed by the +shower of burning ashes, giving time for a partial decay of the +interior, so as to afford hollow cylinders into which the spores of +plants were wafted. These spores germinated and grew, until finally +their stems were petrified by carbonate of lime like some of the +remaining portions of the wood of the containing Sigillaria. Mr. +Carruthers has discovered that sometimes the plants which had thus +grown and become fossil in the inside of a single trunk belonged to +several distinct genera. The fact that the tree-bearing deposits +now dip at an angle of 40° is the more striking, as they must +clearly have remained horizontal and undisturbed during a long +period of intermittent and contemporaneous volcanic action.</p> + +<p>In some of the associated carboniferous shales, ferns and +calamites occur, and all the phenomena of the successive buried +forests remind us of the sections in <a href="ch23.html#page 410"> +pp. 410 and 411</a> of the Nova Scotia coal-measures, with this +difference only, that in the case of the South Joggins the +fossilisation of the trees was effected without the eruption of +volcanic matter.</p> + +<p><b>Trap of the Old Red Sandstone Period.</b>—By referring +to the section explanatory of the structure of Forfarshire, already +given (<a href="ch5.html#page 74">p. 74</a>), the reader will +perceive that beds of conglomerate, No. 3, occur in the middle of +the Old Red Sandstone system, 1, 2, 3, 4. The pebbles in these +conglomerates are sometimes composed of granitic and quartzose +rocks, sometimes exclusively of different varieties of trap, which +last, although purposely omitted in the section referred to, is +often found either intruding itself in amorphous masses and dikes +into the old fossiliferous tilestones, No. 4, or alternating with +them in conformable beds. All the different divisions of the red +sandstone, 1, 2, 3, 4, are occasionally intersected by dikes, but +they are very rare in Nos. 1 and 2, the upper members of the group +consisting of red shale and red sandstone. These phenomena, which +occur at the foot of the Grampians, are repeated in the Sidlaw +Hills; and it appears that in this part of Scotland volcanic +eruptions were most frequent in the earlier part of the Old Red +Sandstone period. The trap-rocks alluded to consist chiefly of +feldspathic porphyry and amygdaloid, the kernels of the latter +being sometimes calcareous, often chalcedonic, and forming +beautiful agates. We meet also with claystone, greenstone, compact +feldspar, and tuff. Some of these rocks look as if they had flowed +as lavas over the bottom of the sea, and enveloped quartz pebbles +which were lying there, so as to form conglomerates with a base of +greenstone, as is seen in Lumley Den, in the Sidlaw Hills. On +either side of the axis of this chain of hills</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 548">[ 548 ]</a></p> + +<p>(see <a href="../images/fig55.jpg">Fig. 55</a>), the beds of +massive trap, and the tuffs composed of volcanic sand and ashes, +dip regularly to the south-east or north-west, conformably with the +shales and sandstones.</p> + +<p>But the geological structure of the Pentland Hills, near +Edinburgh, shows that igneous rocks were there formed during the +newer part of the Devonian or “Old Red” period. These +hills are 1900 feet high above the sea, and consist of +conglomerates and sandstones of Upper Devonian age, resting on the +inclined edges of grits and slates of Lower Devonian and Upper +Silurian date. The contemporaneous volcanic rocks intercalated in +this Upper Old Red consist of feldspathic lavas, or feldstones, +with associated tuffs or ashy beds. The lavas were some of them +originally compact, others vesicular, and these last have been +converted into amygdaloids. They consist chiefly of feldstone or +compact feldspar. The Pentland Hills, say Messrs. Maclaren and +Geikie, afford evidence that at the time of the Upper Old Red +Sandstone, the district to the south-west of Edinburgh was for a +long while the seat of a powerful volcano, which sent out massive +streams of lava and showers of ash, and continued active until +well-nigh the dawn of the Carboniferous period.*</p> + +<p><b>Silurian Volcanic Rocks.</b>—It appears from the +investigations of Sir R. Murchison in Shropshire, that when the +Lower Silurian strata of that country were accumulating, there were +frequent volcanic eruptions beneath the sea; and the ashes and +scoriæ then ejected gave rise to a peculiar kind of tufaceous +sandstone or grit, dissimilar to the other rocks of the Silurian +series, and only observable in places where syenitic and other +trap-rocks protrude. These tuffs occur on the flanks of the Wrekin +and Caer Caradoc, and contain Silurian fossils, such as casts of +encrinites, trilobites, and mollusca. Although fossiliferous, the +stone resembles a sandy claystone of the trap family.†</p> + +<p>Thin layers of trap, only a few inches thick, alternate in some +parts of Shropshire and Montgomeryshire with sedimentary strata of +the Lower Silurian system. This trap consists of slaty porphyry and +granular feldspar rock, the beds being traversed by joints like +those in the associated sandstone, limestone, and shale, and having +the same strike and dip.‡</p> + +<p>In Radnorshire there is an example of twelve bands of stratified +trap, alternating with Silurian schists and flagstones,</p> + +<p class="fnote">* Maclaren, Geology of Fife and Lothians. Geikie, +Trans. Royal Soc. Edinburgh, 1860-1861.<br> +† Murchison, Silurian System, etc., p. 230.<br> +‡ Ibid., p. 212.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 549">[ 549 ]</a></p> + +<p>in a thickness of 350 feet. The bedded traps consist of feldspar +porphyry, and other varieties; and the interposed Llandeilo flags +are of sandstone and shale, with trilobites and graptolites.*</p> + +<p>The Snowdonian hills in Carnarvonshire consist in great part of +volcanic tuffs, the oldest of which are interstratified with the +Bala and Llandeilo beds. There are some contemporaneous feldspathic +lavas of this era, which, says Professor Ramsay, alter the slates +on which they repose, having doubtless been poured out over them, +in a melted state, whereas the slates which overlie them having +been subsequently deposited after the lava had cooled and +consolidated, have entirely escaped alteration. But there are +greenstones associated with the same formation, which, although +they are often conformable to the slates, are in reality intrusive +rocks. They alter the stratified deposits both above and below +them, and when traced to great distances are sometimes seen to cut +through the slates, and to send off branches. Nevertheless, these +greenstones appear to belong, like the lavas, to the Lower Silurian +period.</p> + +<p><b>Cambrian Volcanic Rocks.</b>—The Lingula beds in North +Wales have been described as 5000 feet in thickness. In the upper +portion of these deposits volcanic tuffs or ashy materials are +interstratified with ordinary muddy sediment, and here and there +associated with thick beds of feldspathic lava. These rocks form +the mountains called the Arans and the Arenigs; numerous +greenstones are associated with them, which are intrusive, although +they often run in the lines of bedding for a space. “Much of +the ash,” says Professor Ramsay, “seems to have been +subaërial. Islands, like Graham’s Island, may have +sometimes raised their craters for various periods above the water, +and by the waste of such islands some of the ashy matter became +waterworn, whence the ashy conglomerate. Viscous matter seems also +to have been shot into the air as volcanic bombs, which fell among +the dust and broken crystals (that often form the ashes) before +perfect cooling and consolidation had taken +place.”†</p> + +<p><b>Laurentian Volcanic Rocks.</b>—The Laurentian rocks in +Canada, especially in Ottawa and Argenteuil, are the oldest +intrusive masses yet known. They form a set of dikes of a +fine-grained dark greenstone or dolerite, composed of feldspar and +pyroxene, with occasional scales of mica and grains of pyrites. +Their width varies from a few feet to a hundred yards, and they +have a columnar structure, the columns</p> + +<p class="fnote">* Murchison, Silurian System, etc., p. 325.<br> +† Quart. Geol. Journ., vol. ix, p. 170, 1852.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 550">[ 550 ]</a></p> + +<p>being truly at right angles to the plane of the dike. Some of +the dikes send off branches. These dolerites are cut through by +intrusive syenite, and this syenite, in its turn, is again cut and +penetrated by feldspar porphyry, the base of which consists of +petrosilex, or a mixture of orthoclase and quartz. All these +trap-rocks appear to be of Laurentian date, as the Cambrian and +Huronian rocks rest unconformably upon them.* Whether some of the +various conformable crystalline rocks of the Laurentian series, +such as the coarse-grained granitoid and porphyritic varieties of +gneiss, exhibiting scarcely any signs of stratification, and some +of the serpentines, may not also be of volcanic origin, is a point +very difficult to determine in a region which has undergone so much +metamorphic action.</p> + +<p class="fnote">* Logan, Geology of Canada, 1863.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch29.html"> +Chapter XXIX</a> / <a href="ch31.html">Chapter XXXI</a></small> +</body> +</html> + |