diff options
Diffstat (limited to 'old/3772-h/files/ch25.html')
| -rw-r--r-- | old/3772-h/files/ch25.html | 915 |
1 files changed, 915 insertions, 0 deletions
diff --git a/old/3772-h/files/ch25.html b/old/3772-h/files/ch25.html new file mode 100644 index 0000000..23d5b22 --- /dev/null +++ b/old/3772-h/files/ch25.html @@ -0,0 +1,915 @@ +<!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 439">[ 439 ]</a></p> + +<p> </p> + +<center><b>Chapter XXV</b><br> +<br> +DEVONIAN OR OLD RED SANDSTONE GROUP.</center> + +<p class="intro">Classification of the Old Red Sandstone in +Scotland and in Devonshire. — Upper Old Red Sandstone in +Scotland, with Fish and Plants. — Middle Old Red Sandstone. +— Classification of the Ichthyolites of the Old Red, and +their Relation to Living Types. — Lower Old Red Sandstone, +with Cephalaspis and Pterygotus. — Marine or Devonian Type of +Old Red Sandstone. — Table of Devonian Series. — Upper +Devonian Rocks and Fossils. — Middle. — Lower. — +Eifel Limestone of Germany. — Devonian of Russia. — +Devonian Strata of the United States and Canada. — Devonian +Plants and Insects of Canada.</p> + +<p><b>Classification of the two Types of Old Red +Sandstone.</b>—We have seen that the Carboniferous strata are +surmounted by the Permian and Trias, both originally included in +England under the name “New Red Sandstone,” from the +prevailing red colour of the strata. Under the coal came other red +sandstones and shales which were distinguished by the title of +“Old Red Sandstone.” Afterwards the name of +“Devonian” was given by Sir R. Murchison and Professor +Sedgwick to marine fossiliferous strata which, in the south of +England, occupy a similar position between the overlying coal and +the underlying Silurian formations.</p> + +<p>It may be truly said that in the British Isles the rocks of this +age present themselves in their mineral aspect, and even to some +extent in their fossil contents, under two very different forms; +the one as distinct from the other as are often lacustrine or +fluviatile from marine strata. It has indeed been suggested that by +far the greater part of the deposits belonging to what may be +termed the Old Red Sandstone type are of fresh-water origin. The +number of land-plants, the character of the fishes, and the fact +that the only shell yet discovered belongs to the genus <i> +Anodonta,</i> must be allowed to lend no small countenance to this +opinion. In this case the difficulty of classification when the +strata of this type are compared in different regions, even where +they are contiguous, may arise partly from their having been formed +in distinct hydrographical basins, or in the neighbourhood of the +land in shallow parts of the sea into which large bodies of +fresh-water entered, and where no marine mollusca or corals could +flourish. Under such geographical conditions the limited extent of +some kinds of sediment, as well as the</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 440">[ 440 ]</a></p> + +<p>absence of those marine forms by which we are able to identify +or contrast marine formations, may be explained, while the great +thickness of the rocks, which might seem at first sight to require +a corresponding depth of water, can often be shown to have been due +to the gradual sinking down of the bottom of the estuary or sea +where the sediment was accumulated.</p> + +<p>Another active cause of local variation in Scotland was the +frequency of contemporaneous volcanic eruptions; some of the rocks +derived from this source, as between the Grampians and the Tay, +having formed islands in the sea, and having been converted into +shingle and conglomerate, before the upper portions of the red +shales and sandstones were superimposed.</p> + +<p>The dearth of calcareous matter over wide areas is +characteristic of the Old Red Sandstone. This is, no doubt, in +great part due to the absence of shells and corals; but why should +these be so generally wanting in all sedimentary rocks the colour +of which is determined by the red oxide of iron? Some geologists +are of opinion that the waters impregnated with this oxide were +prejudicial to living beings, others that strata permeated with +this oxide would not preserve such fossil remains.</p> + +<p>In regard to the two types, the Old Red Sandstone and the +Devonian, I shall first treat of them separately, and then allude +to the proofs of their having been to a great extent +contemporaneous. That they constitute a series of rocks +intermediate in date between the lowest Carboniferous and the +uppermost Silurian is not disputed by the ablest geologists; and it +can no longer be contended that the Upper, Middle, and Lower Old +Red Sandstone preceded in date the three divisions to which, by aid +of the marine shells, the Devonian rocks have been referred, while, +on the other hand, we have not yet data for enabling us to affirm +to what extent the subdivisions of the one series may be the +equivalents in time of those of the other.</p> + +<p><b>Upper Old Red Sandstone.</b>—The highest beds of the +series in Scotland, lying immediately below the coal in Fife, are +composed of yellow sandstone well seen at Dura Den, near Coupar, in +Fife, where, although the strata contain no mollusca, fish have +been found abundantly, and have been referred to the genera <i> +Holoptychius, Pamphractus, Glyptopomus,</i> and many others. In the +county of Cork, in Ireland, a similar yellow sandstone occurs +containing fish of genera characteristic of the Scotch Old Red +Sandstone, as for example Coccosteus (a form represented by many +species in the</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 441">[ 441 ]</a></p> + +<img src="../images4/fig494.jpg" width="212" height="150" alt= +"Fig. 494: Anodonta Jukesii." align="right"> + +<p>Old Red Sandstone and by one only in the Carboniferous group), +and <i>Glytolepis</i> and <i>Asterolepis,</i> both exclusively +confined to the “Old Red.” In the same Irish sandstone +at Kiltorkan has been found an <i>Anodonta</i> or fresh-water +mussel, the only shell hitherto discovered in the Old Red Sandstone +of the British Isles (see Fig. 494).</p> + +<img src="../images4/fig495.jpg" width="124" height="271" alt= +"Fig. 495: Bifurcating branch of Lepidendron Griffithsii." align= +"left"> <img src="../images4/fig496.jpg" width="114" height="335" alt= +"Fig. 496: Palæopteris Hibernia." align="right"> + +<p>In the same formation are found the fern (Fig. 496) and the <i> +Lepidodendron</i> (Fig. 495), and other species of plants, some of +which, Professor Heer remarks, agree specifically with species from +the lower carboniferous beds. This induces him to lean to the +opinion long ago advocated by Sir Richard Griffiths, that the +yellow sandstone, in spite of its fish remains, should be classed +as Lower Carboniferous, an opinion which I am not yet prepared to +adopt. Between the Mountain Limestone and the yellow sandstone in +the south-west of Ireland there intervenes a formation no less than +5000 feet thick, called the “Carboniferous slate,” and +at the base of this, in some places, are local deposits, such as +the Glengariff Grits, which appear to be beds of passage between +the Carboniferous and Old Red Sandstone groups.</p> + +<p>It is a remarkable result of the recent examination of the +fossil flora of Bear Island, latitude 74° 30' N., that +Professor Heer has described as occurring in that part of the +Arctic region (nearly twenty-six degrees to the north of the Irish +locality) a flora agreeing in several of its species with that of +the yellow sandstones of Ireland. This Bear Island flora is +believed by Professor Heer to comprise species of plants some of +which ascend even to the higher stages of the European +Carboniferous formation, or as high as the Mountain Limestone and +Millstone Grit. Palæontologists have long maintained that</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 442">[ 442 ]</a></p> + +<p>the same species which have a wide range in space are also the +most persistent in time, which may prepare us to find that some +plants having a vast geographical range may also have endured from +the period of the Upper Devonian to that of the Millstone Grit.</p> + +<img src="../images4/fig497.jpg" width="141" height="165" alt= +"Fig. 497: Scale of Holoptychius nobilissimus." align="left"> + +<p>Outliers of the Upper “Old Red” occur unconformably +on older members of the group, and the formation represented at +Whiteness, near Arbroath, <i>a,</i> <a href="../images/fig55.jpg">Fig. +55,</a> may probably be one of these outliers, though the want of +organic remains renders this uncertain. It is not improbable that +the beds given in this section as Nos. 1, 2, and 3, may all belong +to the early part of the period of the Upper Old Red, as some +scales of <i>Holoptychius nobilissimus</i> have been found +scattered through these beds, No. 2, in Strathmore. Another nearly +allied <i>Holoptychius</i> occurs in Dura Den, see Fig. 498 of this +fish and also Fig. 497 of one of its scales, as these last are +often the only parts met with; being scattered in Forfarshire +through red-coloured shales and sandstones, as are scales of a +large species of the same genus in a corresponding matrix in +Herefordshire.* The number of fish obtained from the British Upper +Old Red Sandstone amounts to fifteen species referred to eleven +genera.</p> + +<center><img src="../images4/fig498.jpg" width="389" height="230" alt= +"Fig. 498: Holoptychius, as restored by Professor Huxley."> +</center> + +<p>Sir R. Murchison groups with this upper division of the Old Red +of Scotland certain light-red and yellow sandstones and grits which +occur in the northernmost part of the mainland, and extend also +into the Orkney and Shetland Islands.</p> + +<p class="fnote">* Siluria, 4th ed., p. 265.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 443">[ 443 ]</a></p> + +<p>They contain Calamites and other plants which agree generically +with Carboniferous forms.</p> + +<p><b>Middle Old Red Sandstone.</b>—In the northern part of +Scotland there occur a great series of bituminous schists and +flagstones, to the fossil fish of which attention was first called +by the late Hugh Miller. They were afterwards described by Agassiz, +and the rocks containing them were examined by Sir R. Murchison and +Professor Sedgwick, in Caithness, Cromarty, Moray, Nairn, Gamrie in +Banff, and the Orkneys and Shetlands, in which great numbers of +fossil fish have been found. These were at first supposed to be the +oldest known vertebrate animals, as in Cromarty the beds in which +they occur seem to form the base of the Old Red system resting +almost immediately on the crystalline or metamorphic rocks. But in +fact these fish-bearing beds, when they are traced from north to +south, or to the central parts of Scotland, thin out, so that their +relative age to the Lower Old Red Sandstone, presently to be +mentioned, was not at first detected, the two formations not +appearing in superposition in the same district. In Caithness, +however, many hundred feet below the fish-zone of the middle +division, remains of <i>Pteraspis</i> were found by Mr. Peach in +1861. This genus has never yet been found in either of the two +higher divisions of the Old Red Sandstone, and confirms Sir R. +Murchison’s previous suspicion that the rocks in which it +occurs belong to the Lower “Old Red,” or agree in age +with the Arbroath paving-stone.*</p> + +<p><i>Fossil Fish of the Middle Old Red Sandstone.</i>—The +Devonian fish were referred by Agassiz to two of his great orders, +namely, the Placoids and Ganoids. Of the first of these, which in +the Recent period comprise the shark, the dog-fish, and the ray, no +entire skeletons are preserved, but fin-spines, called +ichthyodorulites, and teeth occur. On such remains the genera <i> +Onchus, Odontacanthus,</i> and <i>Ctenodus,</i> a supposed +cestraciont, and some others, have been established.</p> + +<p>By far the greater number of the Old Red Sandstone fishes belong +to a sub-order of Ganoids instituted by Huxley in 1861, and for +which he has proposed the name of <i> +Crossopterygidæ</i>,† or the fringe-finned, in +consideration of the peculiar manner in which the fin-rays of the +paired fins are arranged so as to form a fringe round a central +lobe, as in the Polypterus (see <i>a,</i> Fig. 499), a genus of +which there are several species now inhabiting the Nile and other +African rivers. The reader will at once recognise in <i> +Osteolepis</i> (Fig. 500), one of the common fishes of the Old Red +Sandstone, many points of</p> + +<p class="fnote">* Siluria, 4th ed., p. 258.<br> +† Abridged from <i>crossotos,</i> a fringe, and <i> +pteryx,</i> a fin.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 444">[ 444 ]</a></p> + +<center><img src="../images4/fig499.jpg" width="408" height="159" alt= +"Fig. 499: Polypterus. Living in the Nile and other rivers."> +</center> + +<p>analogy with <i>Polypterus.</i> They not only agree in the +structure of the fin, at first pointed out by Huxley, but also in +the position of the pectoral, ventral, and anal fins, and in having +an elongated body and rhomboidal scales. On the other hand, the +tail is more symmetrical in the recent fish, which has also an +apparatus of dorsal finlets of a very abnormal character, both as +to number and structure. As to the dorsals of <i>Osteolepis,</i> +they are regular in structure and position, having nothing +remarkable about them, except that there are two of them, which is +comparatively unusual in living fish.</p> + +<center><img src="../images4/fig500.jpg" width="373" height="144" alt= +"Fig. 500: Restoration of Osteolepis."></center> + +<p>Among the “fringe-finned” Ganoids we find some with +rhomboidal scales, such as <i>Osteolepis,</i> Fig. 500; others with +cycloidal scales, as <i>Holoptychius,</i> before mentioned (see +Fig. 498). In the genera <i>Dipterus</i> and <i>Diplopterus,</i> as +Hugh Miller pointed out, and in several other of the fringe-finned +genera, as in <i>Gyroptychius</i> and <i>Glyptolepis,</i> the two +dorsals are placed far backward, or directly over the ventral and +anal fins. The <i>Asterolepis</i> was a ganoid fish of gigantic +dimensions. <i>A. Asmusii,</i> Eichwald, a species characteristic +of the Old Red Sandstone of Russia, as well as that of Scotland, +attained the length of between twenty and thirty feet. It was +clothed with strong bony armour, embossed with star-like tubercles, +but it had only a cartilaginous skeleton. The mouth was furnished +with two rows of teeth, the outer ones small and fish-like, the +inner larger and with a reptilian character. The <i>Asterolepis</i> +occurs also in the Devonian rocks of North America.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 445">[ 445 ]</a></p> + +<p>If we except the Placoids already alluded to, and a few other +families of doubtful affinities, all the Old Red Sandstone fishes +are Ganoids, an order so named by Agassiz from the shining outer +surface of their scales; but Professor Huxley has also called our +attention to the fact that, while a few of the primary and the +great majority of the secondary Ganoids resemble the living bony +pike, <i>Lepidosteus,</i> or the <i>Amia,</i> genera now found in +North American rivers, and one of them, <i>Lepidosteus,</i> +extending as far south as Guatemala, the Crossopterygii, or +fringe-finned Ichthyolites, of the Old Red are closely related to +the African <i>Polypterus,</i> which is represented by five or six +species now inhabiting the Nile and the rivers of Senegal. These +North American and African Ganoids are quite exceptional in the +living creation; they are entirely confined to the northern +hemisphere, unless some species of <i>Polypterus</i> range to the +south of the line in Africa; and, out of about 9000 living species +of fish known to M. Günther, and of which more than 6000 are +now preserved in the British Museum, they probably constitute no +more than nine.</p> + +<img src="../images4/fig501.jpg" width="222" height="282" alt= +"Fig. 501: Pterichthys. Upper side, showing mouth." align="right"> + +<p>If many circumstances favour the theory of the fresh-water +origin of the Old Red Sandstone, this view of its nature is not a +little confirmed by our finding that it is in Llake Superior and +the other inland Canadian seas of fresh water, and in the +Mississippi and African rivers, that we at present find those fish +which have the nearest affinity to the fossil forms of this ancient +formation.</p> + +<p>Among the anomalous forms of Old Red fishes not referable to +Huxley’s Crossopterygii is the <i>Pterichthys,</i> of which +five species have been found in the middle division of the Old Red +of Scotland. Some writers have compared their shelly covering to +that of Crustaceans, with which, however, they have no real +affinity. The wing-like appendages, whence the genus is named, were +first supposed by Hugh Miller to be paddles, like those of the +turtle; and there can now be no doubt that they do really +correspond with the pectoral fins.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 446">[ 446 ]</a></p> + +<p>The number of species of fish already obtained from the middle +division of the Old Red Sandstone in Great Britain is about 70, and +the principal genera, besides <i>Osteolepis</i> and <i> +Pterichthys,</i> already mentioned, are <i>Glyptolepis, +Diplacanthus, Dendrodus, Coccosteus, Cheirancanthus,</i> and <i> +Acanthoides.</i></p> + +<center><img src="../images4/fig502.jpg" width="360" height="268" alt= +"Fig. 502: Cephalapsis Lyellii."></center> + +<p><b>Lower Old Red Sandstone.</b>—The third or lowest +division south of the Grampians consists of grey paving-stone and +roofing-slate, with associated red and grey shales; these strata +underlie a dense mass of conglomerate. In these grey beds several +remarkable fish have been found of the genus named by Agassiz <i> +Cephalaspis,</i> or “buckler-headed,” from the +extraordinary shield which covers the head (see Fig. 502), and +which has often been mistaken for that of a trilobite, such as <i> +Asaphus.</i> A species of <i>Pteraspis,</i> of the same family, has +also been found by the Reverend Hugh Mitchell in beds of +corresponding age in Perthshire; and Mr. Powrie enumerates no less +than five genera of the family Acanthodidæ, the spines, +scales, and other remains of which have been detected in the grey +flaggy sandstones.*</p> + +<img src="../images4/fig503.jpg" width="190" height="163" alt= +"Fig. 503: Pteygotus anglicus." align="left"> + +<p>In the same formation at Carmylie, in Forfarshire, commonly +known as the Arbroath paving-stone, fragments of a huge crustacean +have been met with from time to time. They are called by the Scotch +quarrymen the “Seraphim,” from the</p> + +<p class="fnote">* Powrie, Geol. Quart. Journ., vol. xx, p. +417.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 447">[ 447 ]</a></p> + +<table border="0" cellspacing="0" cellpadding="0" bgcolor="white" +summary= +"Fig. 504: Pterygotus anglicus. Ventral aspect. Restored by H. Woodward, F.G.S."> +<tr> +<td align="left" valign="top"><img src="../images4/fig504.jpg" width= +"214" height="361" alt= +"Fig. 504: Pterygotus anglicus. Ventral aspect."></td> +<td align="left" valign="bottom"> +<ol class="tab"> +<li>Carapace, showing the large sessile eyes at the anterior +angles.</li> + +<li>The <i>metastoma</i> or post-oral plate (serving the office of +a lower lip).</li> + +<li>Chelate appendages (antennules).</li> + +<li>First pair of simple palpi (antennæ).</li> + +<li>Second pair of simple palpi (mandibles).</li> + +<li>Third pair of simple palpi (first maxillæ).</li> + +<li>Pair of swimming feet with their broad basal joints, whose +serrated edges serve the office of maxillæ.</li> + +<li>Thoracic plate covering the first two thoracic segments, which +are indicated by the figures 1, 2, and a dotted line. 1-6. Thoracic +segments. 7-12. Abdominal segments. 13. Telson, or +tail-plate.)</li> +</ol> +</td> +</tr> +</table> + +<p>wing-like form and feather-like ornament of the thoracic +appendage, the part most usually met with. Agassiz, having +previously referred some of these fragments to the class of fishes, +was the first to recognise their crustacean character, and, +although at the time unable correctly to determine the true +relation of the several parts, he figured the portions on which he +founded his opinion, in the first plate of his “Poissons +Fossiles du Vieux Grès Rouge.”</p> + +<p>A restoration in correct proportion to the size of the fragments +of <i>P. anglicus</i> (Fig. 504), from the Lower Old Red Sandstone +of Perthshire and Forfarshire, would give us a creature measuring +from five to six feet in length, and more than one foot across.</p> + +<p>The largest crustaceans living at the present day are the <i> +Inachus Kaempferi,</i> of De Haan, from Japan (a brachyurous or +short-tailed crab), chiefly remarkable for the extraordinary length +of its limbs; the fore-arm measuring four feet in length, and the +others in proportion, so that it covers about 25 square feet of +ground; and the <i>Limulus Moluccanus,</i> the great King Crab of +China and the Eastern seas, which, when adult, measures 1½ +foot across its carapace, and is three feet in length.</p> + +<p>Besides some species of <i>Pterygotus,</i> several of the allied +genus <i>Eurypterus</i> occur in the Lower Old Red Sandstone, and +with them the remains of grass-like plants so abundant in +Forfarshire and Kincardineshire as to be useful to the geologist by +enabling him to identify the inferior strata at distant points. +Some botanists have suggested that these</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 448">[ 448 ]</a></p> + +<center><img src="../images4/fig505.jpg" width="342" height="184" alt= +"Fig. 505: Parka decipiens. In sandstone of lower beds of Old Red, Ley's Mill, Forfarshire. Fig. 506: Parka decipiens. In shale of Lower Old Red, Park Hill, Fife."> +</center> + +<p>plants may be of the family <i>Fluviales,</i> and of fresh-water +genera. They are accompanied by fossils, called +“berries” by the quarrymen, which they compared to a +compressed blackberry (see Figs. 505, 506), and which were called +“Parka” by Dr. Fleming. They are now considered by Mr. +Powrie to be the eggs of crustaceans, which is highly probable, for +they have not only been found with <i>Pterygotus anglicus</i> in +Forfarshire and Perthshire, but also in the Upper Silurian strata +of England, in which species of the same genus, Pterygotus, +occur.</p> + +<img src="../images4/fig507.jpg" width="231" height="251" alt= +"Fig. 507: Shale of Old Red Sandstone. Forfarshire. With impression of plants and eggs of Crustaceans." + align="left"> + +<p>The grandest exhibitions, says Sir R. Murchison, of the Old Red +Sandstone in England and Wales appear in the escarpments of the +Black Mountains and in the Fans of Brecon and Carmarthen, the one +2862, and the other 2590 feet above the sea. The mass of red and +brown sandstone in these mountains is estimated at not less than +10,000 feet, clearly intercalated between the Carboniferous and +Silurian strata. No shells or corals have ever been found in the +whole series, not even where the beds are calcareous, forming +irregular courses of concretionary lumps called +“corn-stones,” which may be described as mottled red +and green earthy limestones. The fishes of this lowest English Old +Red are <i>Cephalaspis</i> and <i>Pteraspis,</i> specifically +different from species of the same genera which occur in the +uppermost Ludlow or Silurian tilestones. Crustaceans also of the +genus <i>Eurypterus</i> are met with.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 449">[ 449 ]</a></p> + +<p><b>Marine or Devonian Type.</b>—We may now speak of the +marine type of the British strata intermediate between the +Carboniferous and Silurian, in treating of which we shall find it +much more easy to identify the Upper, Middle, and Lower divisions +with strata of the same age in other countries. It was not until +the year 1836 that Sir R. Murchison and Professor Sedgwick +discovered that the culmiferous or anthracitic shales and +sandstones of North Devon, several thousand feet thick, belonged to +the coal, and that the beds below them, which are of still greater +thickness, and which, like the carboniferous strata, had been +confounded under the general name “graywacke,” occupied +a geological position corresponding to that of the Old Red +Sandstone already described. In this reform they were aided by a +suggestion of Mr. Lonsdale, who, after studying the Devonshire +fossils, perceived that they belonged to a peculiar +palæontological type of intermediate character between the +Carboniferous and Silurian.</p> + +<p>It is in the north of Devon that these formations may best be +studied, where they have been divided into an Upper, Middle, and +Lower Group, and where, although much contorted and folded, they +have for the most part escaped being altered by intrusive +trap-rocks and by granite, which in Dartmoor and the more southern +parts of the same county have often reduced them to a crystalline +or metamorphic state.</p> + +<center><small>DEVONIAN SERIES IN NORTH + DEVON.</small></center> + +<table border="1" cellspacing="0" cellpadding="10" summary= +"Left column — Upper, Middle and Lower Devonian Groups; right column — Types of strata found in each group."> +<tr> +<td align="center" valign="middle">U<small>PPER</small> +D<small>EVONIAN OR</small> P<small>ILTON</small> +G<small>ROUP</small></td> +<td align="left" valign="top"> +<b>(a)</b> Sandy slates and +schists with fossils, 36 species out of 110 common to the +Carboniferous group (Pilton, Barnstaple, etc.), resting on soft +schists in which fossils are very abundant (Croyde, etc.), and +which pass down into<br> +<b>(b)</b> Yellow, brown, and red sandstone, with land +plants (<i>Cyclopteris,</i> etc.) and marine shells. One zone, +characterised by the abundance of cucullæa (Baggy Point, +Marwood, Sloly, etc.) resting on hard grey and reddish sandstone +and micaceous flags, no fossils yet found (Dulverton, Pickwell, +Down, etc.)</td> +</tr> + +<tr> +<td align="center" valign="middle">M<small>IDDLE</small> +D<small>EVONIAN OR</small> I<small>LFRACOMBE</small> +G<small>ROUP.</small></td> +<td align="left"><b>(a)</b> Green glossy slates of +considerable thickness, no fossils yet recorded from these beds +(Mortenoe, Lee Bay, etc.).<br> +<b>(b)</b> Slates and schists, with several irregular +courses of limestone containing shells and corals like those of the +Plymouth Limestone (Combe Martin, Ilfracombe, etc.).</td> +</tr> + +<tr> +<td align="center" valign="middle">L<small>OWER</small> +D<small>EVONIAN OR</small> L<small>YNTON</small> +G<small>ROUP.</small></td> +<td align="left"><b>(a)</b> Hard, greenish, red, and purple +sandstone—no fossils yet found (Hangman Hill, etc.).<br> +<b>(b)</b> Soft slates with subordinate +sandstones—fossils numerous at various horizons—Orthis, +Corals, Encrinites, etc. (Valley of Rocks, Lynmouth, etc.).</td> +</tr> +</table> + +<p>The above table exhibits the sequence of the strata or +subdivisions as seen both on the sea-coast of the British Channel +and in the interior of Devon. It will be seen that</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 450">[ 450 ]</a></p> + +<p>in all main points it agrees with the table drawn up in 1864 for +the sixth edition of my “Elements.” Mr. Etheridge* has +since published an excellent account of the different subdivisions +of the rocks and their fossils, and has also pointed out their +relation to the corresponding marine strata of the Continent. The +slight modifications introduced in my table since 1864 are the +result of a tour made in 1870 in company with Mr. T. Mck. Hughes, +when we had the advantage of Mr. Etheridge’s memoir as our +guide.</p> + +<p>The place of the sandstones of the Foreland is not yet clearly +made out, as they are cut off by a great fault and disturbance.</p> + +<img src="../images4/fig508.jpg" width="218" height="437" alt= +"Fig. 508: Spirifera disjuncta. Fig. 509: Phacops latifrons." +align="left"> + +<p><b>Upper Devonian Rocks.</b>—The slates and sandstones of +Barnstaple (<i>a</i> and <i>b</i> of the preceding section) contain +the shell <i>Spirifera disjuncta,</i> Sowerby (S. Verneuilii, +Murch.), (see Fig. 508), which has a very wide range in Europe, +Asia Minor, and even China; also <i>Strophalosia caperata,</i> +together with the large trilobite <i>Phacops latifrons,</i> Bronn. +(See Fig. 509), which is all but world-wide in its distribution. +The fossils are numerous, and comprise about 150 species of +mollusca, a fifth of which pass up into the overlying Carboniferous +rocks. To this Upper Devonian belong a series of limestones and +slates well developed at Petherwyn, in Cornwall, where they have +yielded 75 species of fossils. The genus of Cephalopoda called <i> +Clymenia</i> (Fig. 510) is represented by no less than eleven +species, and strata occupying the same position in Germany are +called Clymenien-Kalk, or sometimes Cypridinen-Schiefer, on account +of the number of minute bivalve shells of the crustacean called <i> +Cypridina serrato-striata</i> (Fig. 511), which is found in these +beds, in the Rhenish provinces, the Harz, Saxony, and Silesia, as +well as in Cornwall and Belgium.</p> + +<p><b>Middle Devonian Rocks.</b>—We come next to the most +typical portion of the Devonian system, including the great +limestones of Plymouth and Torbay, replete with</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 451">[ 451 ]</a></p> + +<center><img src="../images4/fig510.jpg" width="390" height="194" alt= +"Fig. 510: Clymenia linearis. Fig. 511: Cypridina serrato-striata."> +</center> + +<img src="../images4/fig512.jpg" width="203" height="165" alt= +"Fig. 512: Heliolites porosa." align="right"> + +<p>shells, trilobites, and corals. Of the corals 51 species are +enumerated by Mr. Etheridge, none of which pass into the +Carboniferous formation. Among the genera we find <i>Favosites, +Heliolites,</i> and <i>Cyathophyllum.</i> The two former genera are +very frequent in Silurian rocks: some few even of the species are +said to be common to the Devonian and Silurian groups, as, for +example, <i>Favosites cervicornis</i> (Fig. 513), one of the +commonest of all</p> + +<center><img src="../images4/fig513.jpg" width="382" height="316" alt= +"Fig. 513: Favosites cervicornis. Fig. 514: Cyathophyllum cæspitosum."> +</center> + +<p> </p> + +<hr> +<p class="page"><a name="page 452">[ 452 ]</a></p> + +<p>the Devonshire fossils. The <i>Cyathophyllum +cæspitosum</i> (Fig. 514) and <i>Heliolites pyriformis</i> +(Fig. 512) are species peculiar to this formation.</p> + +<center><img src="../images4/fig515.jpg" width="359" height="207" alt= +"Fig. 515: Stringocephalus Burtini. Fig. 516: Uncites Gryphus."> +</center> + +<p>With the above are found no less than eleven genera of +stone-lilies or crinoids, some of them, such as <i> +Cupressocrinites,</i> distinct from any Carboniferous forms. The +mollusks, also, are no less characteristic; of 68 species of +Brachiopoda, ten only are common to the Carboniferous Limestone. +The <i>Stringocephalus Burtini</i> (Fig. 515) and <i>Uncites +Gryphus</i> (Fig. 516) may be mentioned as exclusively Middle +Devonian genera, and extremely characteristic of the same division +in Belgium. The <i>Stringocephalus</i> is also so abundant in the +Middle Devonian of the banks of the Rhine as to have suggested the +name of Stringocephalus Limestone.</p> + +<img src="../images4/fig517.jpg" width="262" height="213" alt= +"Fig. 517: Megalodon cucullatus." align="left"> + +<p>The only two species of Brachiopoda common to the Silurian and +Devonian formations are <i>Atrypa reticularis</i> (Fig. 532), which +seems to have been a cosmopolite species, and <i>Strophomena +rhomboidalis.</i></p> + +<p>Among the peculiar lamellibranchiate bivalves common to the +Plymouth limestone of Devonshire and the Continent, we find the <i> +Megalodon</i> (Fig. 517). There are also twelve genera of +Gasteropods which have yielded 36 species, four of which pass to +the Carboniferous group, namely <i>Macrocheilus,</i></p> + +<p> </p> + +<hr> +<p class="page"><a name="page 453">[ 453 ]</a></p> + +<img src="../images4/fig518.jpg" width="132" height="280" alt= +"Fig. 518: Conularia ornata." align="left"><img src= +"../images4/fig519.jpg" width="132" height="217" alt= +"Fig. 519: Bronteus flabellifer." align="right"> + +<p><i>Acroculia, Euomphalus,</i> and <i>Murchisonia.</i> Pteropods +occur, such as <i>Conularia</i> (Fig. 518), and Cephalopods, such +as <i>Cyrtoceras, Gyroceras, Orthoceras,</i> and others, nearly all +of genera distinct from those prevailing in the Upper Devonian +Limestone, or Clymenien-kalk of the Germans already mentioned. +Although but few species of Trilobites occur, the characteristic +<i>Bronteus flabellifer</i> (Fig. 519) is far from rare, and all +collectors are familiar with its fan-like tail. In this same group, +called, as before stated, the Stringocephalus, or Eifel Limestone, +in Germany, several fish remains have been detected, and among +others the remarkable genus Coccosteus, covered with its +tuberculated bony armour; and these ichthyolites serve, as Sir R. +Murchison observes (Siluria, p. 362), to identify this middle +marine Devonian with the Old Red Sandstone of Britain and +Russia.</p> + +<img src="../images4/fig520.jpg" width="252" height="145" alt= +"Fig. 520: Calceola sandalina." align="right"> + +<p>Beneath the Eifel Limestone (the great central and typical +member of “the Devonian” on the Continent) lie certain +schists called by German writers “Calceola-schiefer,” +because they contain in abundance a fossil body of very curious +structure, <i>Calceola sandalina</i> (Fig. 520), which has been +usually considered a brachiopod, but which some naturalists have +lately referred to a Goniophyllum, supposing it to be an abnormal +form of the order <i>Zoantharia rugosa</i> (see <a href= +"../images3/fig474.jpg">Fig. 474</a>), differing from all other corals +in being furnished with a strong operculum. This is by no means a +rare fossil in the slaty limestone of South Devon, and, like the +Eifel form, is confined to the middle group of this country.</p> + +<p><b>Lower Devonian Rocks.</b>—A great series of sandstones +and glossy slates, with Crinoids, Brachiopods, and some corals,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 454">[ 454 ]</a></p> + +<img src="../images4/fig521.jpg" width="266" height="108" alt= +"Fig. 521: Spirifora mucronata." align="left"> + +<p>occurring on the coast at Lynmouth and the neighbourhood, and +called the Lynton Group (see Table <a href="#page 449">p. 449</a>, +form the lowest member of the Devonian in North Devon. Among the 18 +species of all classes enumerated by Mr. Etheridge, two-thirds are +common to the Middle Devonian, but only one, the ubiquitous <i> +Atrypa reticularis,</i> can with certainty be identified with +Silurian species. Among the characteristic forms are <i>Alveolites +suborbicularis,</i> also common to this formation in the Rhine, and +<i>Orthis arcuata,</i> very widely spread in the North Devon +localities. But we may expect a large addition to the number of +fossils whenever these strata shall have been carefully searched. +The Spirifer Sandstone of Sandberger, as exhibited in the rocks +bordering the Rhine between Coblentz and Caub, belong to this Lower +division, and the same broad-winged Spirifers distinguish the +Devonian strata of North America.</p> + +<img src="../images4/fig522.jpg" width="132" height="370" alt= +"Fig. 522: Homalonotus armatus." align="right"> + +<p>Among the Trilobites of this era several large species of <i> +Homalonotus</i> (Fig. 522) are conspicuous. The genus is still +better known as a Silurian form, but the spinose species appear to +belong exclusively to the “Lower Devonian,” and are +found in Britain, Europe, and the Cape of Good Hope.</p> + +<p><b>Devonian of Russia.</b>—The Devonian strata of Russia +extend, according to Sir R. Murchison, over a region more spacious +than the British Isles; and it is remarkable that, where they +consist of sandstone like the “Old Red” of Scotland and +Central England, they are tenanted by fossil fishes often of the +same species and still oftener of the same genera as the British, +whereas when they consist of limestone they contain shells similar +to those of Devonshire, thus confirming, as Sir Roderick has +pointed out, the contemporaneous origin which had been previously +assigned to formations exhibiting two very distinct mineral types +in different parts of Britain.*</p> + +<p class="fnote">* Murchison’s Siluria, p. 329.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 455">[ 455 ]</a></p> + +<p>The calcareous and the arenaceous rocks of Russia above alluded +to alternate in such a manner as to leave no doubt of their having +been deposited in different parts of the same great period.</p> + +<img src="../images4/fig523.jpg" width="238" height="615" alt= +"Fig. 523: Psilophyton princeps." align="right"> + +<p><b>Devonian Strata in the United States and +Canada.</b>—Between the Carboniferous and Silurian strata +there intervenes, in the United States and Canada, a great series +of formations referable to the Devonian group, comprising some +strata of marine origin abounding in shells and corals, and others +of shallow-water and littoral origin in which terrestrial plants +abound. The fossils, both of the deep and shallow water strata, are +very analogous to those of Europe, the species being in some cases +the same. In Eastern Canada Sir W. Logan has pointed out that in +the peninsula of Gaspe, south of the estuary of St. Lawrence, a +mass of sandstone, conglomerate, and shale referable to this period +occurs, rich in vegetable remains, together with some fish-spines. +Far down in the sandstones of Gaspe, Dr. Dawson found, in 1869, an +entire specimen of the genus <i>Cephalaspis,</i> a form so +characteristic, as we have already seen, of the Scotch Lower Old +Red Sandstone. Some of the sandstones are ripple-marked, and +towards the upper part of the whole series a thin seam of coal has +been observed, measuring, together with some associated</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 456">[ 456 ]</a></p> + +<p>carbonaceous shale, about three inches in thickness. It rests on +an under-clay in which are the roots of Psilophyton (see Fig. 523). +At many other levels rootlets of this same plant have been shown by +Principal Dawson to penetrate the clays, and to play the same part +as do the rootlets of Stigmaria in the coal formation.</p> + +<p>We had already learnt from the works of Göppert, Unger, and +Bronn that the European plants of the Devonian epoch resemble +generically, with few exceptions, those already known as +Carboniferous; and Dr. Dawson, in 1859, enumerated 32 genera and 69 +species which he had then obtained from the State of New York and +Canada. A perusal of his catalogue,* comprising <i>Coniferæ, +Sigillariæ, Calamites, Asterophyllites, Lepidodendra,</i> and +ferns of the genera <i>Cyclopteris, Neuropteris, Sphenopteris,</i> +and others, together with fruits, such as <i>Cardiocarpum</i> and +<i>Trigonocarpum,</i> might dispose geologists to believe that they +were presented with a list of Carboniferous fossils, the difference +of the species from those of the coal-measures, and even a slight +admixture of genera unknown in Europe, being naturally ascribed to +geographical distribution and the distance of the New from the Old +World. But fortunately the coal formation is fully developed on the +other side of the Atlantic, and is singularly like that of Europe, +both lithologically and in the species of its fossil plants. There +is also the most unequivocal evidence of relative age afforded by +superposition, for the Devonian strata in the United States are +seen to crop out from beneath the Carboniferous on the borders of +Pennsylvania and New York, where both formations are of great +thickness.</p> + +<p>The number of American Devonian plants has now been raised by +Dr. Dawson to 120, to which we may add about 80 from the European +flora of the same age, so that already the vegetation of this +period is beginning to be nearly half as rich as that of the +coal-measures which have been studied for so much longer a time and +over so much wider an area. The Psilophyton above alluded to is +believed by Dr. Dawson to be a lycopodiaceous plant, branching +dichotomously (see <i>P. princeps,</i> Fig. 523), with stems +springing from a rhizome, which last has circular areoles, much +resembling those of Stigmaria, and like it sending forth +cylindrical rootlets. The extreme points of some of the branchlets +are rolled up so as to resemble the croziers or circinate vernation +of ferns; the leaves or bracts, <i>a,</i> supposed to belong to the +same plant, are described by Dawson as having inclosed the +fructification. The remains of <i>Psilophyton princeps</i> have +been traced through</p> + +<p class="fnote">* Quart. Geol. Journ., vol. xv, p. 477, 1859; also +vol. xviii, p. 296, 1862.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 457">[ 457 ]</a></p> + +<p>all the members of the Devonian series in America, and Dr. +Dawson has lately recognised it in specimens of Old Red Sandstone +from the north of Scotland.</p> + +<p>The monotonous character of the Carboniferous flora might be +explained by imagining that we have only the vegetation handed down +to us of one set of stations, consisting of wide swampy flats. But +Dr. Dawson supposes that the geographical conditions under which +the Devonian plants grew were more varied, and had more of an +upland character. If so, the limitation of this more ancient flora, +represented by so many genera and species, to the gymnospermous and +cryptogamous orders, and the absence or extreme rarity of plants of +higher grade, lead us naturally to speculate on the theory of +progressive development, however difficult it may be to avail +ourselves of this explanation, so long as we meet with even a few +exceptional cases of what may seem to be monocotyledonous or +dicotyledonous exogens.</p> + +<p><b>Devonian Insects of Canada.</b>—The earliest known +insects were brought to light in 1865 in the Devonian strata of St. +John’s, New Brunswick, and are referred by Mr. Scudder to +four species of <i>Neuroptera.</i> One of them is a gigantic +Ephemera, and measured five inches in expanse of wing.</p> + +<p>Like many other ancient animals, says Dr. Dawson, they show a +remarkable union of characters now found in distinct orders of +insects, or constitute what have been named “synthetic +types.” Of this kind is a stridulating or musical apparatus +like that of the cricket in an insect otherwise allied to the <i> +Neuroptera.</i> This structure, as Dr. Dawson observes, if rightly +interpreted by Mr. Scudder, introduces us to the sounds of the +Devonian woods, bringing before our imagination the trill and hum +of insect life that enlivened the solitudes of these strange old +forests.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch24.html"> +Chapter XXIV</a> / <a href="ch26.html">Chapter XXVI</a></small> +</body> +</html> + |