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diff --git a/old/3772-h/files/ch26.html b/old/3772-h/files/ch26.html new file mode 100644 index 0000000..502a6df --- /dev/null +++ b/old/3772-h/files/ch26.html @@ -0,0 +1,1357 @@ +<!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 458">[ 458 ]</a></p> + +<p> </p> + +<center><b>Chapter XXVI</b><br> +<br> +SILURIAN GROUP.</center> + +<p class="intro">Classification of the Silurian Rocks. — +Ludlow Formation and Fossils. — Bone-bed of the Upper Ludlow. +— Lower Ludlow Shales with Pentamerus. — Oldest known +Remains of fossil Fish. — Table of the progressive Discovery +of Vertebrata in older Rocks. — Wenlock Formation, Corals, +Cystideans and Trilobites. — Llandovery Group or Beds of +Passage. — Lower Silurian Rocks. — Caradoc and Bala +Beds. — Brachiopoda. — Trilobites. — +Cystideæ. — Graptolites. — Llandeilo Flags. +— Arenig or Stiper-stones Group. — Foreign Silurian +Equivalents in Europe. — Silurian Strata of the United +States. — Canadian Equivalents. — Amount of specific +Agreement of Fossils with those of Europe.</p> + +<p><b>Classification of the Silurian Rocks.</b>—We come next +in descending order to that division of Primary or Palæozoic +rocks which immediately underlie the Devonian group or Old Red +Sandstone. For these strata Sir Roderick Murchison first proposed +the name of Silurian when he had studied and classified them in +that part of Wales and some of the contiguous counties of England +which once constituted the kingdom of the <i>Silures,</i> a tribe +of ancient Britons. The following table will explain the two +principal divisions, Upper and Lower, of the Silurian rocks, and +the minor subdivisions usually adopted, comprehending all the +strata originally embraced in the Silurian system by Sir Roderick +Murchison. The formations below the Arenig or Stiper-stones group +are treated of in the next chapter, when the +“Primordial” or Cambrian group is described.</p> + +<center> +<table border="1" cellspacing="0" cellpadding="4" summary= +"Principal divisions and subdivisions and thickness of each subdivision."> +<tr> +<td align="center" colspan="2">UPPER SILURIAN + ROCKS.</td> +</tr> + +<tr> +<td> </td> +<td align="right"><small>Thickness<br> +in feet</small></td> +</tr> + +<tr> +<td align="left">1. L<small>UDLOW</small> +F<small>ORMATION</small>:<br> + <i>a.</i> Upper Ludlow +beds</td> +<td align="right">780</td> +</tr> + +<tr> +<td align="left"> <i>b.</i> + Lower Ludlow beds:</td> +<td align="right">1,050</td> +</tr> + +<tr> +<td align="left">2. W<small>ENLOCK</small> +F<small>ORMATION</small>:<br> + <i>a.</i> Wenlock limestone and +shale</td> +<td align="right" valign="middle" rowspan="2">above 4,000</td> +</tr> + +<tr> +<td align="left"> <i>b.</i> + Woolhope limestone and shale, and Denbighshire grits:</td> +</tr> + +<tr> +<td align="left">3. L<small>LANDOVERY</small> +F<small>ORMATION</small><br> + (Beds of passage between Upper and +Lower Silurian):<br> + <i>a.</i> Upper Llandovery +(May-Hill beds):</td> +<td align="right">800</td> +</tr> + +<tr> +<td align="left"> <i>b.</i> +Lower Llandovery:</td> +<td align="right">600–1,000</td> +</tr> + +<tr> +<td align="center" colspan="2">LOWER SILURIAN + ROCKS.</td> +</tr> + +<tr> +<td align="left">1. B<small>ALA AND</small> C<small>ARADOC</small> +B<small>EDS</small>, including volcanic rocks:</td> +<td align="right">12,000</td> +</tr> + +<tr> +<td align="left">2. L<small>LANDEILO</small> F<small>LAGS</small>, +including volcanic rocks:</td> +<td align="right">4,500</td> +</tr> + +<tr> +<td align="left">3. A<small>RENIG OR</small> +S<small>TIPER-STONES</small> G<small>ROUP</small>, including +volcanic rocks:</td> +<td align="right">above 10,000</td> +</tr> +</table> +</center> + +<p> </p> + +<hr> +<p class="page"><a name="page 459">[ 459 ]</a></p> + +<center><small>UPPER SILURIAN ROCKS.</small></center> + +<p><b>1. Ludlow Formation.</b>—This member of the Upper +Silurian group, as will be seen by above table, is of great +thickness, and subdivided into two parts—the Upper Ludlow and the +Lower Ludlow. Each of these may be distinguished near the town of +Ludlow, and at other places in Shropshire and Herefordshire, by +peculiar organic remains; but out of more than 500 species found in +the Ludlow formation as a whole, not more than five species per +hundred are common to the overlying Devonian. The student may refer +to the excellent tables given in the last edition of Sir R. +Murchison’s Siluria for a list of the organic remains of all +classes distributed through the different subdivisions of the Upper +and Lower Silurian.</p> + +<p><i>a.</i> <b>Upper Ludlow:</b> <i>Downton +Sandstone.</i>—At the top of this subdivision there occur +beds of fine-grained yellowish sandstone and hard reddish grits +which were formerly referred by Sir R. Murchison to the Old Red +Sandstone, under the name of “Tilestones.” In mineral +character this group forms a transition from the Silurian to the +Old Red Sandstone, the strata of both being conformable; but it is +now ascertained that the fossils agree in great part specifically, +and in general character entirely, with those of the underlying +Upper Ludlow rocks. Among these are <i>Orthoceras bullatum, +Platyschisma helicites, Bellerophon trilobatus, Chonetes lata,</i> +etc., with numerous defenses of fishes.</p> + +<p>These beds, therefore, now generally called the “Downton +Sandstone,” are classed as the newest member of the Upper +Silurian. They are well seen at Downton Castle, near Ludlow, where +they are quarried for building, and at Kington, in Herefordshire. +In the latter place, as well as at Ludlow, crustaceans of the +genera Pterygotus (for genus see <a href="../images4/fig504.jpg">Fig. +504</a>) and Eurypterus are met with.</p> + +<p><i>Bone-bed of the Upper Ludlow.</i>—At the base of the +Downton sandstones there occurs a bone-bed which deserves especial +notice as affording the most ancient example of fossil fish +occurring in any considerable quantity. It usually consists of one +or two thin layers of brown bony fragments near the junction of the +Old Red Sandstone and the Ludlow rocks, and was first observed by +Sir R. Murchison near the town of Ludlow, where it is three or four +inches thick. It has since been traced to a distance of 45 miles +from that point into Gloucestershire and other counties, and is +commonly not more than an inch thick, but varies to nearly a foot. +Near Ludlow two bone-beds are observable, with 14 feet of</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 460">[ 460 ]</a></p> + +<p>intervening strata full of Upper Ludlow fossils.* At that point +immediately above the upper fish-bed numerous small globular bodies +have been found, which were determined by Dr. Hooker to be the +sporangia of a cryptogamic land-plant, probably lycopodiaceous.</p> + +<center><img src="../images4/fig524.jpg" width="407" height="105" alt= +"Fig. 524: Onchus tenuistriatus. Fig. 525: Shagreen-scales of a placoid fish, Thelodus parvidens."> +</center> + +<p>Most of the fish have been referred by Agassiz to his placoid +order, some of them to the genus Onchus, to which the spine (Fig. +524) and the minute scales (Fig. 525) are supposed to belong. It +has been suggested, however, that Onchus may be one of those +Acanthodian fish referred by Agassiz to his Ganoid order, which are +so characteristic of the base of the Old Red Sandstone in +Forfarshire, although the species of the Old Red are all different +from these of the Silurian beds now under consideration.</p> + +<img src="../images4/fig526.jpg" width="147" height="87" alt= +"Fig. 526: Plectrodus mirabilis." align="left"> + +<p>The jaw and teeth of another predaceous genus (Fig. 526) have +also been detected, together with some specimens of <i>Pteraspis +Ludensis.</i> As usual in bone-beds, the teeth and bones are, for +the most part, fragmentary and rolled.</p> + +<img src="../images4/fig527.jpg" width="133" height="123" alt= +"Fig. 527: Orthis elegantula." align="left"><img src= +"../images4/fig528.jpg" width="119" height="104" alt= +"Fig. 528: Rhynchonella navicula." align="right"> + +<p><i>Grey Sandstone and Mudstone, etc.</i>—The next +subdivision of the Upper Ludlow consists of grey calcareous +sandstone, or very commonly a micaceous stone, decomposing into +soft mud, and contains, besides the shells mentioned aon page 459, +<i>Lingula cornea, Orthis orbicularis,</i> a round variety of <i>O. +elegantula, Modiolopsis platyphylla, Grammysia cingulata,</i> all +characteristic of the Upper Ludlow. The lowest or mud-stone beds +contain <i>Rhynchonella navicula</i> (Fig. 528), which is common to +this bed and the Lower Ludlow. As usual in Palæozoic strata +older than the coal, the brachiopodous mollusca greatly outnumber +the lamellibranchiate (see <a href="page 470">p. 470</a>); but the +latter are by no means unrepresented. Among other genera, for +example, we observe <i>Avicula</i> and</p> + +<p class="fnote">* Murchison’s Siluria, p. 140.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 461">[ 461 ]</a></p> + +<p><i>Pterinea, Cardiola, Ctenodonta</i> (sub-genus of <i> +Nucula</i>), <i>Orthonota, Modiolopsis,</i> and <i> +Palæarca.</i></p> + +<p>Some of the Upper Ludlow sandstones are ripple-marked, thus +affording evidence of gradual deposition; and the same may be said +of the accompanying fine argillaceous shales, which are of great +thickness, and have been provincially named +“mud-stones.” In some of these shales stems of +crinoidea are found in an erect position, having evidently become +fossil on the spots where they grew at the bottom of the sea. The +facility with which these rocks, when exposed to the weather, are +resolved into mud, proves that, notwithstanding their antiquity, +they are nearly in the state in which they were first thrown +down.</p> + +<center><img src="../images4/fig529.jpg" width="320" height="230" alt= +"Fig. 529: Pentamerus Knightii."></center> + +<p><i>b.</i> <b>Lower Ludlow Beds.</b>—The chief mass of this +formation consists of a dark grey argillaceous shale with +calcareous concretions, having a maximum thickness of 1000 feet. In +some places, and especially at Aymestry, in Herefordshire, a +subcrystalline and argillaceous limestone, sometimes 50 feet thick, +overlies the shale. Sir R. Murchison therefore classes this +Aymestry limestone as holding an intermediate position between the +Upper and Lower Ludlow, but Mr. Lightbody remarks that at Mocktrie, +near Leintwardine, the Lower Ludlow shales, with their +characteristic fossils, occur both above and below a similar +limestone. This limestone around Aymestry and Sedgeley is +distinguished by the abundance of <i>Pentamerus Knightii,</i> +Sowerby (Fig. 529), also found in the Lower Ludlow and Wenlock +shale. This genus of brachiopoda was first found in Silurian +strata, and is exclusively a palæozoic form. The name was +derived from <i>pente,</i> five, and <i>meros,</i> a part, because +both valves are divided by a central septum, making four chambers, +and in one valve</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 462">[ 462 ]</a></p> + +<img src="../images4/fig530.jpg" width="107" height="199" alt= +"Fig. 530: Lingula Lewisii." align="left"> + +<p>the septum itself contains a small chamber, making five. The +size of these septa is enormous compared with those of any other +brachiopod shell; and they must nearly have divided the animal into +two equal halves; but they are, nevertheless, of the same nature as +the septa or plates which are found in the interior of <i> +Spirifera, Terebratula,</i> and many other shells of this order. +Messrs. Murchison and De Verneuil discovered this species dispersed +in myriads through a white limestone of Upper Silurian age, on the +banks of the Is, on the eastern flank of the Urals in Russia, and a +similar species is frequent in Sweden.</p> + +<p>Three other abundant shells in the Aymestry limestone are, +first, <i>Lingula Lewisii</i> (Fig. 530); second, <i>Rhynchonella +Wilsoni,</i> Sowerby (Fig. 531), which is also common to the Lower +Ludlow and Wenlock limestone; third, <i>Atrypa reticularis,</i> +Linn. (Fig. 532), which has a very wide range, being found in every +part of the Upper Silurian system, and even ranging up into the +Middle Devonian series.</p> + +<center><img src="../images4/fig531.jpg" width="279" height="109" alt= +"Fig. 531: Rhynchonella (Terebratula) Wilsoni."></center> + +<p>The Aymestry Limestone contains many shells, especially +brachiopoda, corals, trilobites, and other fossils, amounting on +the whole to 74 species, all except three or four being common to +the beds either above or below.</p> + +<img src="../images4/fig532.jpg" width="282" height="208" alt= +"Fig. 532: Atrypa reticularis." align="left"> The Lower Ludlow +Shale contains, among other fossils, many large cephalopoda not +known in newer rocks, as the <i>Phragmoceras</i> of Broderip, and +the <i>Lituites</i> of Breynius (see Figs. 533, 534). The latter is +partly straight and partly convoluted in a very flat spire. The<br> +<br> + + +<p> </p> + +<hr> +<p class="page"><a name="page 463">[ 463 ]</a></p> + +<img src="../images4/fig533.jpg" width="157" height="244" alt= +"Fig. 533: Phragmoceras ventricosum." align="right"> + +<p><i>Orthoceras Ludense</i> (Fig. 535), as well as the cephalopod +last mentioned, occurs in this member of the species.</p> + +<p>A species of Graptolite, <i>G. priodon,</i> Bronn (<a href= +"../images4/fig545.jpg">Fig. 545</a>), occurs plentifully in the Lower +Ludlow. This fossil, referred, though somewhat doubtfully, to a +form of hydrozoid or sertularian polyp, has not yet been met with +in strata above the Silurian.</p> + +<p>Star-fish, as Sir R. Murchison points out, are by no means rare +in the Lower Ludlow rock. These fossils, of which six extinct +genera are now known in the Ludlow series, represented by 18 +species, remind us of various living forms now found in our British +seas, both of the families <i>Asteriadæ</i> and <i> +Ophiuridæ.</i></p> + +<center><img src="../images4/fig534.jpg" width="396" height="177" alt= +"Fig. 534: Lituites (Trochoceras) giganteus. Fig. 535: Fragment of Orthoceras Ludense."> +</center> + +<p><b>Oldest known Fossil Fish.</b>—Until 1859 there was no +example of a fossil fish older than the bone-bed of the Upper +Ludlow, but in that year a specimen of Pteraspis was found at +Church Hill, near Leintwardine, in Shropshire, by Mr. J. E. Lee of +Caerleon, <small>F.G.S.</small>, in shale below the Aymestry +limestone, associated with fossil shells of the Lower Ludlow +formation—shells which differ considerably from those +characterising the Upper Ludlow already described. This discovery +is of no small interest as bearing on the theory of progressive +development, because, according to Professor Huxley, the genus +Pteraspis is allied to the sturgeon, and therefore by no means of +low grade in the piscine class.</p> + +<p>It is a fact well worthy of notice that no remains of vertebrata +have yet been met with in any strata older than the Lower +Ludlow.</p> + +<p>When we reflect on the hundreds of Mollusks, Echinoderms,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 464">[ 464 ]</a></p> + +<p>Trilobites, Corals, and other fossils already obtained from more +ancient Silurian formations, Upper, Middle, and Lower, we may well +ask whether any set of fossiliferous rocks newer in the series were +ever studied with equal diligence, and over so vast an area, +without yielding a single ichthyolite. Yet we must hesitate before +we accept, even on such evidence, so sweeping a conclusion, as that +the globe, for ages after it was inhabited by all the great classes +of invertebrata, remained wholly untenanted by vertebrate +animals.</p> + +<center><i>Dates of the Discovery of different Classes of Fossil +Vertebrata; showing the gradual progress made in tracing them to +rocks of higher antiquity.</i></center> + +<br> +<table border="1" cellspacing="0" cellpadding="4" align="center" +summary= +"Column 1, Fossil; Column 2, Year; Column 3, Formations; Column 4, Geographical localities."> +<tr> +<td> </td> +<td align="left">Year</td> +<td align="left">Formations</td> +<td align="left">Geographical localities</td> +</tr> + +<tr> +<td align="left" valign="middle" rowspan="3">Mammalia</td> +<td align="left">1798</td> +<td align="left">Upper Eocene</td> +<td align="left">Paris (Gypsum of Montmartre).<sup>1</sup></td> +</tr> + +<tr> +<td align="left">1818</td> +<td align="left">Lower Oolite</td> +<td align="left">Stonesfield.<sup>2</sup></td> +</tr> + +<tr> +<td align="left">1847</td> +<td align="left">Upper Trias</td> +<td align="left">Stuttgart.<sup>3</sup></td> +</tr> + +<tr> +<td align="left" valign="middle" rowspan="6">Aves</td> +<td align="left">1782</td> +<td align="left">Upper Eocene</td> +<td align="left">Paris (Gypsum of Montmartre).<sup>4</sup></td> +</tr> + +<tr> +<td align="left">1839</td> +<td align="left">Lower Eocene</td> +<td align="left">Isle of Sheppey (London Clay).<sup>5</sup></td> +</tr> + +<tr> +<td align="left">1854</td> +<td align="left">Lower Eocene</td> +<td align="left">Woolwich Beds.<sup>6</sup></td> +</tr> + +<tr> +<td align="left">1855</td> +<td align="left">Lower Eocene</td> +<td align="left">Mendon (Plastic Clay).<sup>7</sup></td> +</tr> + +<tr> +<td align="left">1858</td> +<td align="left">Chloritic Series, or Upper Greensand</td> +<td align="left">Cambridge.<sup>8</sup></td> +</tr> + +<tr> +<td align="left">1863</td> +<td align="left">Upper Oolite</td> +<td align="left">Solenhofen.<sup>9</sup></td> +</tr> + +<tr> +<td align="left" valign="middle" rowspan="2">Reptilia</td> +<td align="left">1710</td> +<td align="left">Permian (or Zechstein)</td> +<td align="left">Thuringia.<sup>10</sup></td> +</tr> + +<tr> +<td align="left">1844</td> +<td align="left">Carboniferous</td> +<td align="left">Saarbrück, near +Trèves.<sup>11</sup></td> +</tr> + +<tr> +<td align="left" valign="middle" rowspan="5">Pisces</td> +<td align="left">1709</td> +<td align="left">Permian (or Kupferschiefer)</td> +<td align="left">Thuringia.<sup>12</sup></td> +</tr> + +<tr> +<td align="left">1793</td> +<td align="left">Carboniferous (Mountain Limestone)</td> +<td align="left">Glasgow.<sup>13</sup></td> +</tr> + +<tr> +<td align="left">1828</td> +<td align="left">Devonian</td> +<td align="left">Caithness.<sup>14</sup></td> +</tr> + +<tr> +<td align="left">1840</td> +<td align="left">Upper Ludlow</td> +<td align="left">Ludlow.<sup>15</sup></td> +</tr> + +<tr> +<td align="left">1859</td> +<td align="left">Lower Ludlow</td> +<td align="left">Leintwardine.<sup>16</sup></td> +</tr> +</table> + +<p class="fnote">1. George Cuvier, Bulletin Soc. Philom. xx.<br> +2. In 1818, Cuvier, visiting the Museum of Oxford, decided on the +mammalian character of a jaw from Stonesfield. See also <a href= +"ch19.html#page 347">p. 347.</a><br> +3. Prof. Plieninger. See <a href="ch21.html#page 368">p. +368.</a><br> +4. Cuvier, Ossemens Foss. Art. “Oiseaux.”<br> +5. Prof. Owen, Geol. Trans., 2nd series, vol. vi, p. 203, 1839.<br> +6. Upper part of the Woolwich beds. Prestwich, Quart. Geol. Journ., +vol. x, p. 157.<br> +7. <i>Gastornis Parisiensis.</i> Owen, Quart. Geol. Journ., vol. +xii, p. 204, 1856.<br> +8. Coprolitic bed, in the Upper Greensand. See <a href= +"ch17.html#page 299">p. 299.</a><br> +9. The <i>Archæopteryx macrura,</i> Owen. See <a href= +"ch19.html#page 338">p. 338.</a><br> +10. The fossil monitor of Thuringia (<i>Protosaurus Speneri,</i> V. +Meyer) was figured by Spener of Berlin in 1810. (Miscel. +Berlin.)<br> +11. See <a href="ch23.html#page 406">p. 406.</a><br> +12. Memorabilia Saxoniæ Subterr., Leipsic, 1709.<br> +13. History of Rutherglen by Rev. David Ure, 1793.<br> +14. Sedgwick and Murchison, Geol. Trans., 2nd series, vol. ii, p. +141, 1828.<br> +15. Sir R. Murchison. See <a href="#page 459">p. 459.</a><br> +16. See <a href="#page 461">p. 461.</a><br> +<br> +Obs.—The evidence derived from foot-prints, though often to be +relied on, is omitted in the above table, as being less exact than +that founded on bones and teeth.</p> + +<p>In the preceding Table a few dates are set before the reader of +the discovery of different classes of animals in ancient rocks, to +enable him to perceive at a glance how</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 465">[ 465 ]</a></p> + +<p>gradual has been our progress in tracing back the signs of +vertebrata to formations of high antiquity. Such facts may be +useful in warning us not to assume too hastily that the point which +our retrospect may have reached at the present moment can be +regarded as fixing the date of the first introduction of any one +class of beings upon the earth.</p> + +<p><b>2. Wenlock Formation.</b>—We next come to the Wenlock +formation, which has been divided (see Table, <a href="#page 458"> +p. 458</a>) into Wenlock limestone, Wenlock shale, and Woolhope +limestone and Denbighshire grits.</p> + +<img src="../images4/fig536.jpg" width="143" height="254" alt= +"Fig. 536: Halysites catenularius." align="right"> + +<p><i>a. Wenlock Limestone.</i>—This limestone, otherwise +well known to collectors by the name of the Dudley Limestone, forms +a continuous ridge in Shropshire, ranging for about 20 miles from +S.W. to N.E., about a mile distant from the nearly parallel +escarpment of the Aymestry limestone. This ridgy prominence is due +to the solidity of the rock, and to the softness of the shales +above and below it. Near Wenlock it consists of thick masses of +grey subcrystalline limestone, replete with corals, encrinites, and +trilobites. It is essentially of a concretionary nature; and the +concretions, termed “ball-stones” in Shropshire, are +often enormous, even 80 feet in diameter. They are of pure +carbonate of lime, the surrounding rock being more or less +argillaceous* Sometimes in the Malvern Hills this limestone, +according to Professor Phillips, is oolitic.</p> + +<img src="../images4/fig537.jpg" width="113" height="283" alt= +"Fig. 537: Favosites Gothlandica." align="left"> + +<p>Among the corals, in which this formation is so rich, 53 species +being known, the “chain-coral,” <i>Halysites +catenularius</i> (Fig. 536), may be pointed out as one very easily +recognised, and widely spread in Europe, ranging through all parts +of the Silurian group, from the Aymestry limestone to near the +bottom of the Llandeilo rocks. Another coral, the <i>Favosites +Gothlandica</i> (Fig. 537), is also met with in profusion in large +hemispherical masses, which break up into columnar and prismatic +fragments, like that here figured (Fig. 537, <i>b</i>). Another +common form in the</p> + +<p class="fnote">* Murchison’s Siluria, chap. vi.</p> + +<br> + + +<p> </p> + +<hr> +<p class="page"><a name="page 466">[ 466 ]</a></p> + +<img src="../images4/fig538.jpg" width="130" height="225" alt= +"Fig. 538: Omphyma turbinatum." align="left"><img src= +"../images4/fig539.jpg" width="107" height="222" alt= +"Fig. 539: Pseudocrinites bifasciatus." align="right"> + +<p>Wenlock limestone is the <i>Omphyma turbinatum</i> (Fig. 538), +which, like many of its modern companions, reminds us of some +cup-corals; but all the Silurian genera belong to the +palæozoic type before mentioned (<a href= +"ch24.html#page 432">p. 432</a>), exhibiting the quadripartite +arrangement of the septalamellæ within the cup.</p> + +<p>Among the numerous Crinoids, several peculiar species of <i> +Cyathocrinus</i> (for genus see <a href="../images3/fig478.jpg">Figs. +478</a>, 479) contribute their calcareous stems, arms, and cups +towards the composition of the Wenlock limestone. Of Cystideans +there are a few very remarkable forms, most of them peculiar to the +Upper Silurian formation, as, for example, the <i> +Pseudocrinites,</i> which was furnished with pinnated fixed arms,* +as represented in Fig. 539.</p> + +<img src="../images4/fig540.jpg" width="152" height="159" alt= +"Fig. 540: Strophomena (Leptæna) depressa." align="left"> + +<p>The Brachiopoda are, many of them, of the same species as those +of the Aymestry limestone; as, for example, <i>Atrypa +reticularis</i> (Fig. <a href="../images4/fig532.jpg">532</a>), and +<i>Strophomena depressa</i> (Fig. 540); but the latter species +ranges also from the Ludlow rocks, through the Wenlock shale, to +the Caradoc Sandstone.</p> + +<img src="../images4/fig541.jpg" width="124" height="138" alt= +"Fig. 541: Calymene Blumenbachii." align="right"> + +<p>The crustaceans are represented almost exclusively by +Trilobites, which are very conspicuous, 22 being peculiar. The <i> +Calymene Blumenbachii</i> (Fig. 541), called the ”Dudley +Trilobite,” was known to collectors long before its true +place in the animal kingdom was ascertained. It is often found +coiled up like the common <i>Oniscus</i> or wood-louse, and this is +so usual a circumstance among certain genera of trilobites as to +lead us to conclude that they must have habitually resorted to this +mode of protecting themselves when alarmed. The other common +species is the <i>Phacops caudatus (Asaphus caudatus),</i> Brong. +(see Fig. 542), and this is conspicuous for its large</p> + +<p class="fnote">* E. Forbes, Mem. Geol. Surv., vol. ii, p. +496.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 467">[ 467 ]</a></p> + +<img src="../images4/fig542.jpg" width="108" height="241" alt= +"Fig. 542: Phacops (Asaphus) caudatus." align="left"><img src= +"../images4/fig543.jpg" width="116" height="167" alt= +"Fig. 543: Sphærexochus mirus." align="right"> + +<p>size and flattened form. <i>Sphærexochus mirus</i> (Fig. +543) is almost a globe when rolled up, the forehead or glabellum of +this species being extremely inflated. The <i>Homalonotus,</i> a +form of Trilobite in which the tripartite division of the dorsal +crust is almost lost (see Fig. 544), is very characteristic of this +division of the Silurian series.</p> + +<p><i>Wenlock Shale.</i>—This, observes Sir R. Murchison, is +infinitely the largest and most persistent member of the Wenlock +formation, for the limestone often thins out and disappears. The +shale, like the Lower Ludlow, often contains elliptical concretions +of impure earthy limestone.</p> + +<img src="../images4/fig544.jpg" width="113" height="255" alt= +"Fig. 544: Homalonotus delphinocephalus." align="right"> + +<p>In the Malvern district it is a mass of finely levigated +argillaceous matter, attaining, according to Professor Phillips, a +thickness of 640 feet, but it is sometimes more than 1000 feet +thick in Wales, and is worked for flag-stones and slates. The +prevailing fossils, besides corals and trilobites, and some +crinoids, are several small species of <i>Orthis, Cardiola,</i> and +numerous thin-shelled species of <i>Orthoceratites.</i></p> + +<p>About six species of <i>Graptolite,</i> a peculiar group of +sertularian fossils before alluded to (p. <a href="#page 463"> +463</a>) as being confined to Silurian rocks, occur in this shale. +Of fossils of this genus, which is very characteristic of the Lower +Silurian, I shall again speak in the sequel (p. <a href= +"#page 474">474</a>).</p> + +<img src="../images4/fig545.jpg" width="204" height="75" alt= +"Fig. 545: Graptolithus priodon." align="left"> + +<p><i>b. Woolhope Beds.</i>—Though not always recognised as a +separate subdivision of the Wenlock, the Woolhope beds, which +underlie the Wenlock shale, are of great importance. Usually they +occur as massive or nodular limestones, underlaid by a fine shale +or flag-stone; and in other cases, as in the noted Denbighshire +sandstones, as a coarse grit of very great thickness. This grit +forms mountain ranges through North and South Wales, and is +generally marked by the great sterility of the soil where it</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 468">[ 468 ]</a></p> + +<p>occurs. It contains the usual Wenlock fossils, but with the +addition of some common in the uppermost Ludlow rock, such as <i> +Chonetes lata</i> and <i>Bellerophon trilobatus.</i> The chief +fossils of the Woolhope limestone are <i>Illænus Barriensis, +Homalonotus delphinocephalus</i> (Fig. 544), <i>Strophomena +imbrex,</i> and <i>Rhynchonella Wilsoni</i> (<a href= +"../images4/fig531.jpg">Fig. 531</a>). The latter attains in the +Woolhope beds an unusual size for the species, the specimens being +sometimes twice as large as those found in the Wenlock +limestone.</p> + +<p>In some places below the Wenlock formation there are shales of a +pale or purple colour, which near Tarannon attain a thickness of +about 1000 feet; they can be traced through Radnor and Montgomery +to North Wales, according to Messrs. Jukes and Aveline. By the +latter geologist they have been identified with certain shales +above the May-Hill Sandstone, near Llandovery, but, owing to the +extreme scarcity of fossils, their exact position remains +doubtful.</p> + +<p><b>3. Llandovery Group—Beds of Passage.</b>—We now +come to beds respecting the classification of which there has been +much difference of opinion, and which in fact must be considered as +beds of passage between Upper and Lower Silurian. I formerly +adopted the plan of those who class them as Middle Silurian, but +they are scarcely entitled to this distinction, since after about +1400 Silurian species have been compared the number peculiar to the +group in question only gives them an importance equal to such minor +subdivisions as the Ludlow or Bala groups. I therefore prefer to +regard them as the base of the Upper Silurian, to which group they +are linked by more than twice as many species as to the Lower +Silurian. By this arrangement the line of demarkation between the +two great divisions, though confessedly arbitrary, is less so than +by any other. They are called Llandovery Rocks, from a town in +South Wales, in the neighbourhood of which they are well developed, +and where, especially at a hill called Noeth Grug, in spite of +several faults, their relations to one another can be clearly +seen.</p> + +<p><i>a. Upper Llandovery or May-Hill +Sandstone.</i>—The May-Hill group, which has also been named +”Upper Llandovery,” by Sir R. Murchison, ranges from +the west of the Longmynd to Builth, Llandovery, and Llandeilo, and +to the sea in Marlow’s Bay, where it is seen in the cliffs. +It consists of brownish and yellow sandstones with calcareous +nodules, having sometimes a conglomerate at the base derived from +the waste of the Lower Silurian rocks. These May-Hill beds were +formerly supposed to be part of the Caradoc formation, but their +true position was determined by Professor</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 469">[ 469 ]</a></p> + +<img src="../images4/fig546.jpg" width="263" height="312" alt= +"Fig. 546: Pentamerus oblongus." align="right"> + +<p>Sedgwick* to be at the base of the Upper Silurian proper. The +more calcareous portions of the rock have been called the +Pentamerus limestone, because <i>Pentamerus oblongus</i> (Fig. 546) +is very abundant in them. It is usually accompanied by <i>P. +(Stricklandinia) lirata</i> (Fig. 547); both forms have a wide +geographical range, being also met with in the same part of the +Silurian series in Russia and the United States.</p> + +<p>About 228 species of fossils are known in the May-Hill division, +more than half of which are Wenlock species. They consist of +trilobites of the genera <i>Illænus</i> and <i>Calymene</i>; +Brachiopods of the genera <i>Orthis, Atrypa, Leptæna, +Pentamerus, Strophomena,</i> and others; Gasteropods of the genera +<i>Turbo, Murchisonia</i> (for genus, see <a href= +"../images4/fig567.jpg">Fig. 567</a>), and <i>Bellerophon</i>; and +Pteropods of the genus <i>Conularia.</i> The Brachiopods, of which +there are 66 species, are almost all Upper Silurian.</p> + +<img src="../images4/fig547.jpg" width="137" height="149" alt= +"Fig. 547: Stricklandinia (Pentamerus) lirata." align="left"> <img +src="../images4/fig548.jpg" width="173" height="164" alt= +"Fig. 548: Tentaculites annulatus." align="right"> + +<p>Among the fossils of the May-Hill shelly sandstone at Malvern, +<i>Tentaculites annulatus</i> (Fig. 548), an annelid, probably +allied to Serpula, is found.</p> + +<p><i>Lower Llandovery Rocks.</i>—Below the May-Hill Group +are the Lower Llandovery Rocks, which consist chiefly of hard slaty +rocks, and beds of conglomerate from 600 to 1000 feet in thickness. +The fossils, which are somewhat rare in the lower beds, consist of +128 known species, only eleven of which are peculiar, 83 being</p> + +<p class="fnote">* Quart. Geol. Journ., vol. iv, p. 215, 1853.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 470">[ 470 ]</a></p> + +<p>common to the May-Hill group above, and 93 common to the rocks +below. <i>Stricklandinia (Pentamerus) levis,</i> which is common in +the Lower Llandovery, becomes rare in the Upper, while <i> +Pentamerus oblongus</i> (Fig. 546), which is the characteristic +shell of the Upper Llandovery, occurs but seldom in the Lower.</p> + +<br> + + +<center><small>LOWER SILURIAN ROCKS.</small></center> + +<p>The Lower Silurian has been divided into, first, the Bala Group; +second, the Llandeilo flags; and, third, the Arenig or Lower +Llandeilo formation.</p> + +<p><b>Bala and Caradoc Beds.</b>—The Caradoc sandstone was +originally so named by Sir R. I. Murchison from the mountain called +Caer Caradoc, in Shropshire; it consists of shelly sandstones of +great thickness, and sometimes containing much calcareous matter. +The rock is frequently laden with the beautiful trilobite called by +Murchison <i>Trinucleus Caractaci</i> (see <a href= +"../images4/fig553.jpg">Fig. 553</a>), which ranges from the base to +the summit of the formation, usually accompanied by <i>Strophomena +grandis</i> (see Fig. 551), and <i>Orthis vespertilio</i> (Fig. +550), with many other fossils.</p> + +<center><img src="../images4/fig549.jpg" width="407" height="177" alt= +"Fig. 549: Orthis tricenaria. Fig. 550: Orthis vespertilio. Fig. 551: Orthis (Strophomena) grandis."> +</center> + +<p><i>Brachiopoda.</i>—Nothing is more remarkable in these +beds, and in the Silurian strata generally of all countries, than +the preponderance of brachiopoda over other forms of mollusca. +Their proportional numbers can by no means be explained by +supposing them to have inhabited seas of great depth, for the +contrast between the palæozoic and the present state of +things has not been essentially altered by the late discoveries +made in our deep-sea dredgings. We find the living brachiopoda so +rare as to form about one forty-fourth of the whole bivalve fauna, +whereas in the Lower Silurian rocks of which we are now about to +treat, and where the brachiopoda reach their maximum, they are +represented by more than twice as many species as the +Lamellibranchiate bivalves.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 471">[ 471 ]</a></p> + +<p>There may, indeed, be said to be a continued decrease of the +proportional number of this lower tribe of mollusca as we proceed +from older to newer rocks. In the British Devonian, for example, +the Brachiopoda number 99, the Lamellibranchiata 58; while in the +Carboniferous their proportions are more than reversed, the +Lamellibranchiata numbering 334 species, and the Brachiopoda only +157. In the Secondary or Cainozoic formations the preponderance of +the higher grade of bivalves becomes more and more marked, till in +the tertiary strata it approaches that observed in the living +creation.</p> + +<p>While on this subject, it may be useful to the student to know +that a Brachiopod differs from ordinary bivalves, mussels, cockles, +etc., in being always equal-sided and never quite equi-valved; the +form of each valve being symmetrical, it may be divided into two +equal parts by a line drawn from the apex to the centre of the +margin.</p> + +<p><i>Trilobites.</i>—In the Bala and Caradoc beds the +trilobites reach their maximum, being represented by 111 species +referred to 23 genera.</p> + +<p>Burmeister, in his work on the organisation of trilobites, +supposes that they swam at the surface of the water in the open sea +and near coasts, feeding on smaller marine animals, and to have had +the power of rolling themselves into a ball as a defence against +injury. He was also of opinion that they underwent various +transformations analogous to those of living crustaceans. M. +Barrande, author of an admirable work on the Silurian rocks of +Bohemia, confirms the doctrine of their metamorphosis, having +traced more than twenty species through different stages of growth +from the young state just after its escape from the egg to the +adult form. He has followed some of them from a point in which they +show no eyes, no joints, or body rings, and no distinct tail, up to +the complete form with the full number of segments. This change is +brought about before the animal has attained a tenth part of its +full dimensions, and hence such minute and delicate specimens are +rarely met with. Some of his figures of the metamorphoses of the +common <i>Trinucleus</i> are copied in Figs. 552 and 553. It was +not till 1870 that Mr. Billings was enabled, by means of a specimen +found in Canada, to prove that the trilobite was provided with +eight legs.</p> + +<p>It has been ascertained that a great thickness of slaty and +crystalline rocks of South Wales, as well as those of Snowdon and +Bala, in North Wales, which were first supposed to be of older date +than the Silurian sandstones and mudstones of</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 472">[ 472 ]</a></p> + +<center><img src="../images4/fig552.jpg" width="402" height="254" alt= +"Fig. 552: Young individuals of Trinucleus concentricus. Fig. 553: Trinucleus concentricus."> +</center> + +<img src="../images4/fig554.jpg" width="212" height="168" alt= +"Fig. 554: Palæaster asperimus." align="left"> + +<p>Shropshire, are in fact identical in age, and contain the same +organic remains. At Bala, in Merionethshire, a limestone rich in +fossils occurs, in which two genera of star-fish, <i>Protaster</i> +and <i>Palæaster,</i> are found; the fossil specimen of the +latter (Fig. 554) being almost as uncompressed as if found just +washed up on the sea-beach. Besides the star-fish there occur +abundance of those peculiar bodies called <i>Cystideæ.</i> +They are the <i>Sphæronites</i> of old authors, and were +considered by Professor E. Forbes as intermediate between the +crinoids and echinoderms. The <i>Echinosphæronite</i> here +represented (Fig. 555) is characteristic of the Caradoc beds in +Wales, and of their equivalents in Sweden and Russia.</p> + +<img src="../images4/fig555.jpg" width="188" height="237" alt= +"Fig. 555: Echinosphæronites ballicus." align="right"> + +<p>With it have been found several other genera of the same family, +such as <i>Sphæronites, Hemicosmites,</i> etc. Among the +mollusca are Pteropods of the genus <i>Conularia</i> of large size +(for genus, see <a href="../images4/fig518.jpg">Fig. 518</a>). About +eleven species of Graptolite are reckoned as belonging to this +formation; they are chiefly found in peculiar localities where</p> + +<br> +<br> + + +<p> </p> + +<hr> +<p class="page"><a name="page 473">[ 473 ]</a></p> + +<p>black mud abounded. The formation, when traced into South Wales +and Ireland, assumes a greatly altered mineral aspect, but still +retains its characteristic fossils. The known fauna of the Bala +group comprises 565 species, 352 of which are peculiar, and 93, as +before stated, are common to the overlying Llandovery rocks. It is +worthy of remark that, when it occurs under the form of trappean +tuff (volcanic ashes of De la Beche), as in the crest of Snowdon, +the peculiar species which distinguish it from the Llandeilo beds +are still observable. The formation generally appears to be of +shallow-water origin, and in that respect is contrasted with the +group next to be described. Professor Ramsay estimates the +thickness of the Bala Beds, including the contemporaneous volcanic +rocks, stratified and unstratified, as being from 10,000 to 12,000 +feet.</p> + +<img src="../images4/fig556.jpg" width="148" height="106" alt= +"Fig. 556: Didymograpsus (Graptolites) Murchisonii." align="right"> + + +<p><b>Llandeilo Flags.</b>—The Lower Silurian strata were +originally divided by Sir R. Murchison into the upper group already +described, under the name of Caradoc Sandstone, and a lower one, +called, from a town in Carmarthenshire, the <i>Llandeilo</i> flags. +The last mentioned strata consist of dark-coloured micaceous flags, +frequently calcareous, with a great thickness of shales, generally +black, below them. The same beds are also seen at Builth, in +Radnorshire, where they are interstratified with volcanic +matter.</p> + +<p>A still lower part of the Llandeilo rocks consists of a black +carbonaceous slate of great thickness, frequently containing +sulphate of alumina, and sometimes, as in Dumfriesshire, beds of +anthracite. It has been conjectured that this carbonaceous matter +may be due in great measure to large quantities of imbedded animal +remains, for the number of Graptolites included in these slates was +certainly very great. In</p> + +<center><img src="../images4/fig557.jpg" width="409" height="191" alt= +"Fig. 557: Diplograpsus pristis. Fig. 558: Rastrites peregrinus."> +</center> + +<p> </p> + +<hr> +<p class="page"><a name="page 474">[ 474 ]</a></p> + +<p>Great Britain eleven genera and about 40 species of Graptolites +occur in the Llandeilo flags and underlying Arenig beds. The double +Graptolites, or those with two rows of cells, such as Diplograpsus +(Fig. 557), are conspicuous.</p> + +<img src="../images4/fig559.jpg" width="85" height="153" alt= +"Fig. 559: Diplograpsus folium." align="left"> + +<p>The brachiopoda of the Llandeilo flags, which number 47 species, +are in the main the same as those of the Caradoc Sandstone, but the +other mollusca are in great part of different species.</p> + +<p>In Europe generally, as, for example, in Sweden and Russia, no +shells are so characteristic of this formation as Orthoceratites, +usually of great size, and with a wide siphuncle placed on one side +instead of being central (see Fig. 560).</p> + +<center><img src="../images4/fig560.jpg" width="324" height="151" alt= +"Fig. 560: Orthoceras duplex."></center> + +<p>Among other Cephalopods in the Llandeilo flags is Cyrtoceras; in +the same beds also are found Bellerophon (see <a href= +"../images3/fig488.jpg">Fig. 488</a>) and some Pteropod shells +(<i>Conularia, Theca,</i> etc.), also in spots where sand abounded, +lamellibranchiate bivalves of large size. The Crustaceans were +plentifully represented by the Trilobites, which appear to have +swarmed in the Silurian seas just as crabs and shrimps do in our +own; no less than 263 species have been found in the British +Silurian fauna. The genera <i>Asaphus</i> (Fig. 561), <i>Ogygia</i> +(Fig. 562),</p> + +<center><img src="../images4/fig561.jpg" width="317" height="242" alt= +"Fig. 561: Asaphus tyrannus. Fig. 562: Ogygia Buchii."></center> + +<p> </p> + +<hr> +<p class="page"><a name="page 475">[ 475 ]</a></p> + +<p>and <i>Trinucleus</i> (<a href="../images4/fig552.jpg">Figs. +552</a> and 553) form a marked feature of the rich and varied +Trilobitic fauna of this age.</p> + +<p>Beneath the black slates above described of the Llandeilo +formation, Graptolites are still found in great variety and +abundance, and the characteristic genera of shells and trilobites +of the Lower Silurian rocks are still traceable downward, in +Shropshire, Cumberland, and North and South Wales, through a vast +depth of shaly beds, in some districts interstratified with +trappean formations of contemporaneous origin; these consist of +tuffs and lavas, the tuffs being formed of such materials as are +ejected from craters and deposited immediately on the bed of the +ocean, or washed into it from the land. According to Professor +Ramsay, their thickness is about 3300 feet in North Wales, +including those of the Lower Llandeilo. The lavas are feldspathic, +and of porphyritic structure, and, according to the same authority, +of an aggregate thickness of 2500 feet.</p> + +<img src="../images4/fig563.jpg" width="179" height="215" alt= +"Fig. 563: Arenicolites linearis." align="right"> + +<p><b>Arenig or Stiper-Stones Group</b> <i>(Lower Llandeilo of +Murchison).</i>—Next in the descending order are the shales +and sandstones in which the quartzose rocks called Stiper-Stones in +Shropshire occur. Originally these Stiper-Stones were only known as +arenaceous quartzose strata in which no organic remains were +conspicuous, except the tubular burrows of annelids (see Fig. 563, +<i>Arenicolites linearis</i>), which are remarkably common in the +Lowest Silurian in Shropshire, and in the State of New York, in +America. They have already been alluded to as occurring by +thousands in the Silurian strata unconformably overlying the +Cambrian, in the mountain of Queenaig, in Sutherlandshire (<a href= +"../images/fig82.jpg">Fig. 82</a>). I have seen similar burrows now +made on the retiring of the tides in the sands of the Bristol +Channel, near Minehead, by lob-worms which are dug out by fishermen +and used as bait. When the term Silurian was given by Sir R. +Murchison, in 1835, to the whole series, he considered the +Stiper-Stones as the base of the Silurian system, but no fossil +fauna had then been obtained, such as could alone enable the +geologist to draw a line between this member of the series and the +Llandeilo flags above, or a vast thickness of rock below, which was +seen to form the Longmynd hills, and was called +”unfossiliferous graywacke.” Professor Sedgwick had +described, in</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 476">[ 476 ]</a></p> + +<p>1843, strata now ascertained to be of the same age as largely +developed in the Arenig mountain, in Merionethshire; and the +Skiddaw slates in the Lake-District of Cumberland, studied by the +same author, were of corresponding date, though the number of +fossils was, in both cases, too few for the determination of their +true chronological relations. The subsequent researches of Messrs. +Sedgwick and Harkness, in Cumberland, and of Sir R. I. Murchison +and the Government surveyors in Shropshire, have increased the +species to more than sixty. These were examined by Mr. Salter, and +shown in the third edition of ”Siluria” (p. 52, 1859) +to be quite distinct from the fossils of the overlying Llandeilo +flags. Among these the <i>Obolella plumbea, Æglina binodosa, +Ogygia Selwynii,</i> and <i>Didymograpsus geminus</i> (Fig. 564), +and <i>D. Hirundo,</i> are characteristic.</p> + +<img src="../images4/fig564.jpg" width="230" height="76" alt= +"Fig. 564: Didymograpsus geminus." align="left"> + +<p>But, although the species are distinct, the genera are the same +as those which characterise the Silurian rocks above, and none of +the characteristic primordial or Cambrian forms, presently to be +mentioned, are intermixed. The same may be said of a set of beds +underlying the Arenig rocks at Ramsay Island and other places in +the neighbourhood of St. David’s. These beds, which have only +lately become known to us through the labours of Dr. Hicks,* +present already twenty new species, the greater part of them allied +generically to the Arenig rocks. This Arenig group may therefore be +conveniently regarded as the base of the great Silurian system, a +system which, by the thickness of its strata and the changes in +animal life of which it contains the record, is more than equal in +value to the Devonian, or Carboniferous, or other principal +divisions, whether of primary or secondary date.</p> + +<p>It would be unsafe to rely on the mere thickness of the strata, +considered apart from the great fluctuations in organic life which +took place between the era of the Llandeilo and that of the Ludlow +formation, especially as the enormous pile of Silurian rocks +observed in Great Britain (in Wales more particularly) is derived +in great part from igneous action, and is not confined to the +ordinary deposition of sediment from rivers or the waste of +cliffs.</p> + +<p>In volcanic archipelagoes, such as the Canaries, we see the most +active of all known causes, aqueous and igneous, simultaneously at +work to produce great results in a</p> + +<p class="fnote">* Trans. Brit. Assoc., 1866. Proc. Liverpool Geol. +Soc., 1869.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 477">[ 477 ]</a></p> + +<p>comparatively moderate lapse of time. The outpouring of repeated +streams of lava—the showering down upon land and sea of volcanic +ashes—the sweeping seaward of loose sand and cinders, or of rocks +ground down to pebbles and sand, by rivers and torrents descending +steeply inclined channels—the undermining and eating away of long +lines of sea-cliff exposed to the swell of a deep and open +ocean—these operations combine to produce a considerable volume of +superimposed matter, without there being time for any extensive +change of species. Nevertheless, there would seem to be a limit to +the thickness of stony masses formed even under such favourable +circumstances, for the analogy of tertiary volcanic regions lends +no countenance to the notion that sedimentary and igneous rocks +25,000, much less 45,000 feet thick, like those of Wales, could +originate while one and the same fauna should continue to people +the earth. If, then, we allow that about 25,000 feet of matter may +be ascribed to one system, such as the Silurian, as above +described, we may be prepared to discover in the next series of +subjacent rocks a distinct assemblage of species, or even in great +part of genera, of organic remains. Such appears to be the fact, +and I shall therefore conclude with the Arenig beds my enumeration +of the Silurian formations in Great Britain, and proceed to say +something of their foreign equivalents, before treating of rocks +older than the Silurian.</p> + +<p><b>Silurian Strata of the Continent of Europe.</b>—When we +turn to the continent of Europe, we discover the same ancient +series occupying a wide area, but in no region as yet has it been +observed to attain great thickness. Thus, in Norway and Sweden, the +total thickness of strata of Silurian age is considerably less than +1000 feet, although the representatives both of the Upper and Lower +Silurian of England are not wanting there. In Russia the Silurian +strata, so far as they are yet known, seem to be even of smaller +vertical dimensions than in Scandinavia, and they appear to consist +chiefly of the Llandovery group, or of a limestone containing <i> +Pentamerus oblongus,</i> below which are strata with fossils +corresponding to those of the Llandeilo beds of England. The lowest +rock with organic remains yet discovered is ”the Ungulite or +Obolus grit” of St. Petersburg, probably coeval with the +Llandeilo flags of Wales.</p> + +<p>The shales and grits near St. Petersburg, above alluded to, +contain green grains in their sandy layers, and are in a singularly +unaltered state, taking into account their high antiquity. The +prevailing Brachiopods consist of the <i>Obolus</i></p> + +<p> </p> + +<hr> +<p class="page"><a name="page 478">[ 478 ]</a></p> + +<center><i>Shells of the lowest known Fossiliferous Beds in +Russia.</i></center> + +<center><img src="../images4/fig565.jpg" width="394" height="192" alt= +"Fig. 565: Siphonotreta unguiculata. Fig. 566: Obolus Apollinis."> +</center> + +<p>or Ungulite of Pander, and a <i>Siphonotreta</i> (Figs. 565, +566). Notwithstanding the antiquity of this Russian formation, it +should be stated that both of these genera of brachiopods have been +also found in the Upper Silurian of England, i.e., in the Wenlock +limestone.</p> + +<p>Among the green grains of the sandy strata above-mentioned, +Professor Ehrenberg announced in 1854 his discovery of remains of +foraminifera. These are casts of the cells; and among five or six +forms three are considered by him as referable to existing genera +(e.g., <i>Textularia, Rotalia,</i> and <i>Guttulina</i>).</p> + +<p><b>Silurian Strata of the United States.</b>—The Silurian +formations can be advantageously studied in the States of New York, +Ohio, and other regions north and south of the great Canadian +lakes. Here they are often found, as in Russia, nearly in +horizontal position, and are more rich in well-preserved fossils +than in almost any spot in Europe. In the State of New York, where +the succession of the beds and their fossils have been most +carefully worked out by the Government surveyors, the subdivisions +given in the first column of the table below have been adopted.</p> + +<center><i>Subdivisions of the Silurian Strata of New York.<br> +(Strata below the Oriskany sandstone or base of the +Devonian.)</i></center> + +<br> + + +<table border="1" cellpadding="4" cellspacing="0" align="center" +summary= +"Left column, New York names; right column, British equivalents."> +<tr> +<td align="center">New York Names</td> +<td align="center">British equivalents</td> +</tr> + +<tr> +<td align="left"> 1. Upper Pentamerus +Limestone<br> + 2. Encrinal Limestone<br> + 3. Delthyris Shaly Limestone<br> + 4. Pentamerus and Tentaculite Limestones<br> + 5. Water Lime Group<br> + 6. Onondaga Salt Group<br> + 7. Niagara Group</td> +<td align="left" valign="middle">Upper Silurian (or Ludlow<br> +and Wenlock formations</td> +</tr> + +<tr> +<td align="left"> 8. Clinton Group<br> + 9. Medina Sandstone<br> +10. Oneida Conglomerate<br> +11. Gray Sandstone</td> +<td align="left" valign="middle">Beds of Passage, Llandovery +Group.</td> +</tr> + +<tr> +<td align="left">12. Hudson River Group<br> +13. Trenton Limestone<br> +14. Black-River Limestone<br> +15. Bird’s-eye Limestone<br> +16. Chazy Limestone<br> +17. Calciferous Sandstone</td> +<td align="left" valign="middle">Lower Silurian (or Caradoc and +Bala,<br> +Llandeilo and Arenig Formations).</td> +</tr> +</table> + +<p> </p> + +<hr> +<p class="page"><a name="page 479">[ 479 ]</a></p> + +<p>In the second column of the same table I have added the supposed +British equivalents. All Palæontologists, European and +American, such as MM. De Verneuil, D. Sharpe, Professor Hall, E. +Billings, and others, who have entered upon this comparison, admit +that there is a marked general correspondence in the succession of +fossil forms, and even species, as we trace the organic remains +downward from the highest to the lowest beds; but it is impossible +to parallel each minor subdivision.</p> + +<p>That the Niagara Limestone, over which the river of that name is +precipitated at the great cataract, together with its underlying +shales, corresponds to the Wenlock limestone and shale of England +there can be no doubt. Among the species common to this formation +in America and Europe are <i>Calymene Blumenbachii, Homalonotus +delphinocephalus</i> (<a href="../images4/fig544.jpg">Fig. 544</a>), +with several other trilobites; <i>Rhynchonella Wilsoni,</i> <a +href="../images4/fig531.jpg">Fig. 531</a>, and <i>Retzia cuneata; +Orthis elegantula, Pentamerus galeatus,</i> with many more +brachiopods; <i>Orthoceras annulatum,</i> among the cephalopodous +shells; and <i>Favosites gothlandica,</i> with other large +corals.</p> + +<img src="../images4/fig567.jpg" width="117" height="204" alt= +"Fig. 567: Murchisonia gracilis." align="right"> + +<p>The Clinton Group, containing <i>Pentamerus oblongus</i> and <i> +Stricklandinia,</i> and related more nearly by its fossil species +with the beds above than with those below, is the equivalent of the +Llandovery Group or beds of passage.</p> + +<p>The Hudson River Group, and the Trenton Limestone, agree +palæontologically with the Caradoc or Bala group, containing +in common with them several species of trilobites, such as <i> +Asaphus (Isotelus) gigas, Trinucleus concentricus</i> (<a href= +"../images4/fig552.jpg">Fig. 553</a>); and various shells, such as <i> +Orthis striatula, Orthis biforata</i> (or <i>O. lynx</i>), <i>O. +porcata</i> (<i>O. occidentalis</i> of Hall), and <i>Bellerophon +bilobatus.</i> In the Trenton limestone occurs <i>Murchisonia +gracilis,</i> Fig. 567, a fossil also common to the Llandeilo beds +in England.</p> + +<p>Mr. D. Sharpe, in his report on the mollusca collected by me +from these strata in North America,* has concluded that the number +of species common to the Silurian rocks</p> + +<p class="fnote">* Quart. Geol. Journ., vol. iv.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 580">[ 580 ]</a></p> + +<p>on both sides of the Atlantic is between 30 and 40 per cent; a +result which, although no doubt liable to future modification, when +a larger comparison shall have been made, proves, nevertheless, +that many of the species had a wide geographical range. It seems +that comparatively few of the gasteropods and lamellibranchiate +bivalves of North America can be identified specifically with +European fossils, while no less than two-fifths of the brachiopoda, +of which my collection chiefly consisted, are the same. In +explanation of these facts, it is suggested that most of the recent +brachiopoda (especially the orthidiform ones) are inhabitants of +deep water, and that they may have had a wider geographical range +than shells living near shore. The predominance of bivalve mollusca +of this peculiar class has caused the Silurian period to be +sometimes styled ”the age of brachiopods.”</p> + +<p>In Canada, as in the State of New York, the Potsdam Sandstone +underlies the above-mentioned calcareous rocks, but contains a +different suite of fossils, as will be hereafter explained. In +parts of the globe still more remote from Europe the Silurian +strata have also been recognised, as in South America, Australia, +and India. In all these regions the facies of the fauna, or the +types of organic life, enable us to recognise the contemporaneous +origin of the rocks; but the fossil species are distinct, showing +that the old notion of a universal diffusion throughout the +”primæval seas” of one uniform specific fauna was +quite unfounded, geographical provinces having evidently existed in +the oldest as in the most modern times.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch25.html"> +Chapter XXV</a> / <a href="ch27.html">Chapter XXVII</a></small> +</body> +</html> + |