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diff --git a/old/3772-h/files/ch27.html b/old/3772-h/files/ch27.html new file mode 100644 index 0000000..81dce06 --- /dev/null +++ b/old/3772-h/files/ch27.html @@ -0,0 +1,663 @@ +<!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 481">[ 481 ]</a></p> + +<p> </p> + +<center><b>Chapter XXVII</b><br> +<br> +CAMBRIAN AND LAURENTIAN GROUPS.</center> + +<p class="intro">Classification of the Cambrian Group, and its +Equivalent in Bohemia. — Upper Cambrian Rocks. — +Tremadoc Slates and their Fossils. — Lingula Flags. — +Lower Cambrian Rocks. — Menevian Beds. — Longmynd +Group. — Harlech Grits with large Trilobites. — +Llanberis Slates. — Cambrian Rocks of Bohemia. — +Primordial Zone of Barrande. — Metamorphosis of Trilobites. +— Cambrian Rocks of Sweden and Norway. — Cambrian Rocks +of the United States and Canada. — Potsdam Sandstone. — +Huronian Series. — Laurentian Group, upper and lower. — +Eozoon Canadense, oldest known Fossil. — Fundamental Gneiss +of Scotland.</p> + +<center><small>CAMBRIAN GROUP.</small></center> + +<p>The characters of the Upper and Lower Silurian rocks were +established so fully, both on stratigraphical and +palæontological data, by Sir Roderick Murchison after five +years’ labour, in 1839, when his “Silurian +System” was published, that these formations could from that +period be recognised and identified in all other parts of Europe +and in North America, even in countries where most of the fossils +differed specifically from those of the classical region in +Britain, where they were first studied.</p> + +<p>While Sir R. I. Murchison was exploring in 1833, in Shropshire +and the borders of Wales, the strata which in 1835 he first called +Silurian, Professor Sedgwick was surveying the rocks of North +Wales, which both these geologists considered at that period as of +older date, and for which in 1836 Sedgwick proposed the name of +Cambrian. It was afterwards found that a large portion of the slaty +rocks of North Wales, which had been considered as more ancient +than the Llandeilo beds and Stiper-Stones before alluded to, were, +in reality, not inferior in position to those Lower Silurian beds +of Murchison, but merely extensive undulations of the same, bearing +fossils identical in species, though these were generally rarer and +less perfectly preserved, owing to the changes which the rocks had +undergone from metamorphic action. To such rocks the term +“Cambrian” was no longer applicable, although it +continued to be appropriate to strata inferior to the +Stiper-Stones, and which were older than those of the Lower +Silurian group as originally defined. It was not till 1846 that +fossils were found in Wales in the Lingula</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 482">[ 482 ]</a></p> + +<p>flags, the place of which will be seen in the table below. By +this time Barrande had already published an account of a rich +collection of fossils which he had discovered in Bohemia, portions +of which he recognised as of corresponding age with +Murchison’s Upper and Lower Silurian, while others were more +ancient, to which he gave the name of “Primordial,” for +the fossils were sufficiently distinct to entitle the rocks to be +referred to a new period. They consisted chiefly of trilobites of +genera distinct from those occurring in the overlying Silurian +formations. These peculiar genera were afterwards found in rocks +holding a corresponding position in Wales, and I shall retain for +them the term Cambrian, as recent discoveries in our own country +seem to carry the first fauna of Barrande, or his primordial type, +even into older strata than any which he found to be fossiliferous +in Bohemia.</p> + +<p>The term primordial was intended to express M. Barrande’s +own belief that the fossils of the rocks so-called afforded +evidence of the first appearance of vital phenomena on this planet, +and that consequently no fossiliferous strata of older date would +or could ever be discovered. The acceptance of such a nomenclature +would seem to imply that we despaired of extending our discoveries +of new and more ancient fossil groups at some future day when vast +portions of the globe, hitherto unexplored, should have been +thoroughly surveyed. Already the discovery of the Laurentian Eozoon +in Canada, presently to be mentioned, discountenances such +views.</p> + +<p>The following table will show the succession of the strata in +England and Wales which belong to the Cambrian group or the +fossiliferous rocks older than the Arenig or Lower Llandeilo +rocks:</p> + +<br> +<table border="0" cellspacing="0" cellpadding="4" align="center" +summary= +"Succession of strata in England and Wales which belong to the Cambrian group."> +<tr> +<td align="center" colspan="2">UPPER CAMBRIAN</td> +</tr> + +<tr> +<td align="left">T<small>REMADOC</small> S<small>LATES</small></td> +<td><i>(Primordial of Barrande in part)</i></td> +</tr> + +<tr> +<td align="left">L<small>INGULA</small> F<small>LAGS</small></td> +<td align="left"><i>(Primordial of Barrande)</i></td> +</tr> + +<tr> +<td align="center" colspan="2">LOWER CAMBRIAN</td> +</tr> + +<tr> +<td align="left">M<small>ENEVIAN</small> B<small>EDS</small></td> +<td align="left"><i>(Primordial of Barrande)</i></td> +</tr> + +<tr> +<td align="left" valign="middle">L<small>ONGMYND</small> +G<small>ROUP</small></td> +<td align="left"><i>a.</i> Harlech Grits<br> +<i>b.</i> Llanberis Slates</td> +</tr> +</table> + +<br> +<p><b>Tremadoc Slates.</b>—The Tremadoc slates of Sedgwick +are more than 1000 feet in thickness, and consist of dark earthy +slates occurring near the little town of Tremadoc, situated on the +north side of Cardigan Bay, in Carnarvonshire. These</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 483">[ 483 ]</a></p> + +<p>slates were first examined by Sedgwick in 1831, and were +re-examined by him and described in 1846,* after some fossils had +been found in the underlying Lingula flags by Mr. Davis. The +inferiority in position of these Lingula flags to the Tremadoc beds +was at the same time established. The overlying Tremadoc beds were +traced by their pisolitic ore from Tremadoc to Dolgelly. No fossils +proper to the Tremadoc slates were then observed, but subsequently, +thirty-six species of all classes have been found in them, thanks +to the researches of Messrs. Salter, Homfray, and Ash. We have +already seen that in the Arenig or Stiper-Stones group, where the +species are distinct, the genera agree with Silurian types; but in +these Tremadoc slates, where the species are also peculiar, there +is about an equal admixture of Silurian types with those which +Barrande has termed “primordial.” Here, therefore, it +may truly be said that we are entering upon a new domain of life in +our retrospective survey of the past. The trilobites of new +species, but of Lower Silurian genera, belong to <i>Ogygia, +Asaphus,</i> and <i>Cheirurus</i>; whereas those belonging to +primordial types, or Barrande’s first fauna as well as to the +Lingula flags of Wales, comprise <i>Dikelocephalus, Conocoryphe</i> +(for genera see <a href="../images4/fig576.jpg">Fig. 577</a> and +581),† <i>Olenus,</i> and <i>Angelina.</i></p> + +<img src="../images4/fig568.jpg" width="99" height="198" alt= +"Fig. 568: Theca (Cleidotheca operculata." align="right"> + +<p>In the Tremadoc slates are found <i>Bellerophon, Orthoceras,</i> +and <i>Cyrtoceras,</i> all specifically distinct from Lower +Silurian fossils of the same genera: the Pteropods <i>Theca</i> +(Fig. 568) and <i>Conularia</i> range throughout these slates; +there are no Graptolites. The <i>Lingula (Lingulella) Davisii</i> +ranges from the top to the bottom of the formation, and links it +with the zone next to be described. The Tremadoc slates are very +local, and seem to be confined to a small part of North Wales; and +Professor Ramsay supposes them to lie unconformably on the Lingula +flags, and that a long interval of time elapsed between these +formations. Cephalopoda have not yet been found lower than this +group, but it will be observed that they occur here associated with +genera of Trilobites considered by Barrande as characteristically +Primordial, some of which belong to all the divisions of the +British Cambrian about to be mentioned. This renders the absence of +cephalopoda of less importance as bearing on the theory of +development.</p> + +<p class="fnote">* Quart. Geol. Journ., vol. iii, p. 156.<br> +† This genus has been substituted for Barrande’s <i> +Conocephalus,</i> as the latter term had been preoccupied by the +entomologists.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 484">[ 484 ]</a></p> + +<center><img src="../images4/fig569.jpg" width="387" height="180" alt= +"''Lingula Flags'' of Dolgelly, and Ffestiniog; N. Wales."> +</center> + +<p><b>Lingula Flags.</b>—Next below the Tremadoc slates in +North Wales lie micaceous flagstones and slates, in which, in 1846, +Mr. E. Davis discovered the <i>Lingula (Lingulella),</i> Fig. 570, +named after him, and from which was derived the name of Lingula +flags. These beds, which are palæontologically the +equivalents of Barrande’s primordial zone, are represented by +more than 5000 feet of strata, and have been studied chiefly in the +neighbourhood of Dolgelly, Ffestiniog, and Portmadoc in North +Wales, and at St. David’s in South Wales. They have yielded +about forty species of fossils, of which six only are common to the +overlying Tremadoc rocks, but the two formations are closely allied +by having several characteristic “primordial” genera in +common. <i>Dikelocephalus, Olenus</i> (Fig. 571), and <i>Conocoryphe</i> are prominent forms, as is also <i>Hymenocaris</i> +(Fig. 569), a genus of phyllopod crustacean entirely confined to +the Lingula Flags. According to Mr. Belt, who has devoted much +attention to these beds, there are already palæontological +data for subdividing the Lingula Flags into three sections.*</p> + +<p>In Merionethshire, according to Professor Ramsay, the Lingula +Flags attain their greatest development; in Carnarvonshire they +thin out so as to have lost two-thirds of their thickness in eleven +miles, while in Anglesea and on the Menai Straits both they and the +Tremadoc beds are entirely absent, and the Lower Silurian rests +directly on Lower Cambrian strata.</p> + +<br> + + +<center><small>LOWER CAMBRIAN.</small></center> + +<p><b>Menevian Beds.</b>—Immediately beneath the Lingula +Flags there occurs a series of dark grey and black flags and slates +alternating at the upper part with some beds of sandstone, the +whole reaching a thickness of from 500 to 600 feet. These beds were +formerly classed, on purely lithological</p> + +<p class="fnote">* Geol. Mag., vol iv.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 485">[ 485 ]</a></p> + +<p>grounds, as the base of the Lingula Flags, but Messrs. Hicks and +Salter, to whose exertions we owe almost all our knowledge of the +fossils, have pointed out* that the most characteristic genera +found in them are quite unknown in the Lingula Flags, while they +possess many of the strictly Lower Cambrian genera, such as <i> +Microdiscus</i> and <i>Paradoxides.</i> They therefore proposed to +place them, and it seems to me with good reason, at the top of the +Lower Cambrian under the term “Menevian,” Menevia being +the classical name of St. David’s. The beds are well +exhibited in the neighbourhood of St. David’s in South Wales, +and near Dolgelly and Maentwrog in North Wales. They are the +equivalents of the lowest part of Barrande’s Primordial Zone +(Étage C). More than forty species have been found in them, +and the group is altogether very rich in fossils for so early a +period.</p> + +<img src="../images4/fig572.jpg" width="138" height="281" alt= +"Fig. 572: Paradoxides Davidis." align="right"> + +<p>The trilobites are of large size; <i>Paradoxides Davidis</i> +(see Fig. 572), the largest trilobite known in England, 22 inches +or nearly two feet long, is peculiar to the Menevian Beds. By +referring to the Bohemian trilobite of the same genus (<a href= +"../images4/fig576.jpg">Fig. 576</a>), the reader will at once see how +these fossils (though of such different dimensions) resemble each +other in Bohemia and Wales, and other closely allied species from +the two regions might be added, besides some which are common to +both countries. The Swedish fauna, presently to be mentioned, will +be found to be still more nearly connected with the Welsh Menevian. +In all these countries there is an equally marked difference +between the Cambrian fossils and those of the Upper and Lower +Silurian rocks. The trilobite with the largest number of rings, <i> +Erinnys venulosa,</i> occurs here in conjunction with <i> +Agnostus</i> and <i>Microdiscus,</i> the genera with the smallest +number. Blind trilobites are also found as well as those which have +the largest eyes, such as <i>Microdiscus</i> on the one hand, and +<i>Anoplenus</i> on the other.</p> + +<br> + + +<center><small>LONGMYND GROUP.</small></center> + +<p>Older than the Menevian Beds are a thick series of olive green, +purple, red and grey grits and conglomerates found in North and +South Wales, Shropshire, and parts of Ireland</p> + +<p class="fnote">* British Association Report 1865, 1866, 1868 and +Quart. Geol. Journ., vols. xxi, xxv.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 486">[ 486 ]</a></p> + +<p>and Scotland. They have been called by Professor Sedgwick the +Longmynd or Bangor Group, comprising, first, the Harlech and +Barmouth sandstones; and secondly, the Llanberis slates.</p> + +<img src="../images4/fig573.jpg" width="242" height="225" alt= +"Fig. 573: Histioderma Hibernica." align="left"> + +<p><b>Harlech Grits.</b>—The sandstones of this period attain +in the Longmynd hills a thickness of no less than 6000 feet without +any interposition of volcanic matter; in some places in +Merionethshire they are still thicker. Until recently these rocks +possessed but a very scanty fauna.</p> + +<p>With the exception of five species of annelids (see <a href= +"../images3/fig460.jpg">Fig. 460</a>) brought to light by Mr. Salter +in Shropshire, and Dr. Kinahan in Wicklow, and an obscure +crustacean form, <i>Palæopyge Ramsayi,</i> they were supposed +to be barren of organic remains. Now, however, through the labours +of Mr. Hicks, they have yielded at St. David’s a rich +fauna of trilobites, brachiopods, phyllopods, and pteropods, +showing, together with other fossils, a by no means low state of +organisation at this early period. Already the fauna amounts to 20 +species referred to 17 genera.</p> + +<p>A new genus of trilobite called <i>Plutonia Sedgwickii,</i> not +yet figured and described, has been met with in the Harlech grits. +It is comparable in size to the large <i>Paradoxides Davidis</i> +before mentioned, has well-developed eyes, and is covered all over +with tubercles. In the same strata occur other genera of +trilobites, namely, <i>Conocoryphe, Paradoxides, Microdiscus,</i> +and the Pteropod <i>Theca</i> (<a href="../images4/fig568.jpg">Fig. +568</a>), all represented by species peculiar to the Harlech grits. +The sands of this formation are often rippled, and were evidently +left dry at low tides, so that the surface was dried by the sun and +made to shrink and present sun-cracks. There are also distinct +impressions of rain-drops on many surfaces, like those in <a href= +"../images3/fig444.jpg">Fig. 444</a> and 445.</p> + +<p><b>Lanberis Slates.</b>—The slates of Llanberis and +Penrhyn in Carnarvonshire, with their associated sandy strata, +attain a great thickness, sometimes about 3000 feet. They are +perhaps not more ancient than the Harlech and Barmouth beds last +mentioned, for they may represent the deposits of fine</p> + +<p class="fnote">* Brit. Assoc. Report, 1868.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 487">[ 487 ]</a></p> + +<img src="../images4/fig574.jpg" width="214" height="159" alt= +"Fig. 574: Oldhamia radiata." align="right"> + +<p>mud thrown down in the same sea, on the borders of which the +sands above-mentioned were accumulating. In some of these slaty +rocks in Ireland, immediately opposite Anglesea and Carnarvon, two +species of fossils have been found, to which the late Professor E. +Forbes gave the name of <i>Oldhamia.</i> The nature of these +organisms is still a matter of discussion among naturalists.</p> + +<img src="../images4/fig575.jpg" width="117" height="256" alt= +"Fig. 575: Oldhamia antiqua." align="left"> + +<p><b>Cambrian Rocks of Bohemia</b> <i>(Primordial zone of +Barrande).</i>—In the year 1846, as before stated, M. Joachim +Barrande, after ten years’ exploration of Bohemia, and after +collecting more than a thousand species of fossils, had ascertained +the existence in that country of three distinct faunas below the +Devonian. To his first fauna, which was older than any then known +in this country, he gave the name of Étage C; his two first +stages A and B consisting of crystalline and metamorphic rocks and +unfossiliferous schists. This Étage C or primordial zone +proved afterwards to be the equivalent of those subdivisions of the +Cambrian groups which have been above described under the names of +Menevian and Lingula Flags. The second fauna tallies with +Murchison’s Lower Silurian, as originally defined by him when +no fossils had been discovered below the Stiper-Stones. The third +fauna agrees with the Upper Silurian of the same author. Barrande, +without government assistance, had undertaken single-handed the +geological survey of Bohemia, the fossils previously obtained from +that country having scarcely exceeded 20 in number, whereas he had +already acquired, in 1850, no less than 1100 species, namely, 250 +crustaceans (chiefly Trilobites), 250 Cephalopods, 160 gasteropods +and pteropods, 130 acephalous mollusks, 210 brachiopods, and 110 +corals and other fossils. These numbers have since been almost +doubled by subsequent investigations in the same country.</p> + +<p>In the primordial zone C, he discovered trilobites of the genera +<i>Paradoxides, Conocoryphe, Ellipsocephalus, Sao, Arionellus, +Hydrocephalus,</i> and <i>Agnostus.</i> M. Barrande pointed out +that these primordial trilobites have a peculiar facies of</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 488">[ 488 ]</a></p> + +<center><i>Fossils of the lowest Fossiliferous Beds in Bohemia, +or<br> +“Primordial Zone“ of Barrande.</i></center> + +<center><img src="../images4/fig576.jpg" width="379" height="400" alt= +"Fig. 576: Paradoxides Bohemicus. Fig. 577: Conocoryphe striata. Fig. 578: Agnostus integer. Fig. 579: Agnostus Rex. Fig. 580: Sao hirsuta in its various stages of growth."> +</center> + +<p>their own dependent on the multiplication of their thoracic +segments and the diminution of their caudal shield or pygidium.</p> + +<p>One of the “primordial” or Upper Cambrian Trilobites +of the genus <i>Sao,</i> a form not found as yet elsewhere in the +world, afforded M. Barrande a fine illustration of the +metamorphosis of these creatures, for he traced them through no +less than twenty stages of their development. A few of these +changes have been selected for representation in Figure 580, that +the reader may learn the gradual manner in which different segments +of the body and the eyes make their appearance.</p> + +<p>In Bohemia the primordial fauna of Barrande derived its +importance exclusively from its numerous and peculiar trilobites. +Besides these, however, the same ancient schists have yielded two +genera of brachiopods, <i>Orthis</i> and <i>Orbicula,</i> a +Pteropod of the genus <i>Theca,</i> and four echinoderms of the +cystidean family.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 489">[ 489 ]</a></p> + +<p><b>Cambrian of Sweden and Norway.</b>—The Cambrian beds of +Wales are represented in Sweden by strata the fossils of which have +been described by a most able naturalist, M. Angelin, in his +“Palæontologica Suecica” (1852-4). The +“alum-schists,” as they are called in Sweden, are +horizontal argillaceous rocks which underlie conformably certain +Lower Silurian strata in the mountain called Kinnekulle, south of +the great Wener Lake in Sweden. These schists contain trilobites +belonging to the genera <i>Paradoxides, Olenus, Agnostus,</i> and +others, some of which present rudimentary forms, like the genus +last mentioned, without eyes, and with the body segments scarcely +developed, and others, again, have the number of segments +excessively multiplied, as in <i>Paradoxides.</i> Such +peculiarities agree with the characters of the crustaceans met with +in the Cambrian strata of Wales; and Dr. Torell has recently found +in Sweden the <i>Paradoxides Hicksii,</i> a well-known Lower +Cambrian fossil.</p> + +<p>At the base of the Cambrian strata in Sweden, which in the +neighbourhood of Lake Wener are perfectly horizontal, lie +ripple-marked quartzose sandstones with worm-tracks and annelid +borings, like some of those found in the Harlech grits of the +Longmynd. Among these are some which have been referred doubtfully +to plants. These sandstones have been called in Sweden +“fucoid sandstones.” The whole thickness of the +Cambrian rocks of Sweden does not exceed 300 feet from the +equivalents of the Tremadoc beds to these sandstones, which last +seem to correspond with the Longmynd, and are regarded by Torell as +older than any fossiliferous primordial rocks in Bohemia.</p> + +<p><b>Cambrian of the United States and Canada</b> <i>(Potsdam +Sandstone).</i>—This formation, as we learn from Sir W. +Logan, is 700 feet thick in Canada; the upper part consists of +sandstone containing fucoids, and perforated by small vertical +holes, which are very characteristic of the rock, and appear to +have been made by annelids <i>(Scolithus linearis).</i> The lower +portion is a conglomerate with quartz pebbles. I have seen the +Potsdam sandstone on the banks of the St. Lawrence, and on the +borders of Lake Champlain, where, as at Keesville, it is a white +quartzose fine-grained grit, almost passing into quartzite. It is +divided into horizontal ripple-marked beds, very like those of the +Lingula Flags of Britain, and replete with a small round-shaped <i> +Obolella,</i> in such numbers as to divide the rock into parallel +planes, in the same manner as do the scales of mica in some +micaceous sandstones. Among the shells of this formation in +Wisconsin are species of <i>Lingula</i> and <i>Orthis,</i> and +several trilobites</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 490">[ 490 ]</a></p> + +<img src="../images4/fig581.jpg" width="156" height="273" alt= +"Fig. 581: Dikelocephalus Minnesotensis." align="left"> + +<p>of the primordial genus <i>Dikelocephalus</i> (Fig. 581). On the +banks of the St. Lawrence, near Beauharnois and elsewhere, many +fossil footprints have been observed on the surface of the rippled +layers. They are supposed by Professor Owen to be the trails of +more than one species of articulate animal, probably allied to the +King Crab, or <i>Limulus.</i></p> + +<p>Recent investigations by the naturalists of the Canadian survey +have rendered it certain that below the level of the Potsdam +Sandstone there are slates and schists extending from New York to +Newfoundland, occupied by a series of trilobitic forms similar in +genera, though not in species, to those found in the European Upper +Cambrian strata.</p> + +<p><b>Huronian Series.</b>—Next below the Upper Cambrian +occur strata called the Huronian by Sir W. Logan, which are of vast +thickness, consisting chiefly of quartzite, with great masses of +greenish chloritic slate, which sometimes include pebbles of +crystalline rocks derived from the Laurentian formation, next to be +described. Limestones are rare in this series, but one band of 300 +feet in thickness has been traced for considerable distances to the +north of Lake Huron. Beds of greenstone are intercalated +conformably with the quartzose and argillaceous members of this +series. No organic remains have yet been found in any of the beds, +which are about 18,000 feet thick, and rest unconformably on the +Laurentian rocks.</p> + +<br> + + +<center><small>LAURENTIAN GROUP.</small></center> + +<p>In the course of the geological survey carried on under the +direction of Sir W.E. Logan, it has been shown that, northward of +the river St. Lawrence, there is a vast series of crystalline rocks +of gneiss, mica-schist, quartzite, and limestone, more than 30,000 +feet in thickness, which have been called Laurentian, and which are +already known to occupy an area of about 200,000 square miles. They +are not only more ancient than the fossiliferous Cambrian +formations above described, but are older than the Huronian last +mentioned, and had undergone great disturbing movements before the +Potsdam sandstone and the other “primordial” or +Cambrian rocks were formed. The older half of this Laurentian +series is unconformable to the newer portion of the same.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 491">[ 491 ]</a></p> + +<p><b>Upper Laurentian or Labrador Series.</b>—The Upper +Group, more than 10,000 feet thick, consists of stratified +crystalline rocks in which no organic remains have yet been found. +They consist in great part of feldspars, which vary in composition +from anorthite to andesine, or from those kinds in which there is +less than one per cent of potash and soda to those in which there +is more than seven per cent of these alkalies, the soda +preponderating greatly. These feldsparites sometimes form mountain +masses almost without any admixture of other minerals; but at other +times they include augite, which passes into hypersthene. They are +often granitoid in structure. One of the varieties is the same as +the apolescent labradorite rock of Labrador. The Adirondack +Mountains in the State of New York are referred to the same series, +and it is conjectured that the hypersthene rocks of Skye, which +resemble this formation in mineral character, may be of the same +geological age.</p> + +<p><b>Lower Laurentian.</b>—This series, about 20,000 feet in +thickness, is, as before stated, unconformable to that last +mentioned; it consists in great part of gneiss of a reddish tint +with orthoclase feldspar. Beds of nearly pure quartz, from 400 to +600 feet thick, occur in some places. Hornblendic and micaceous +schists are often interstratified, and beds of limestone, usually +crystalline. Beds of plumbago also occur. That this pure carbon may +have been of organic origin before metamorphism has naturally been +conjectured.</p> + +<p>There are several of these limestones which have been traced to +great distances, and one of them is from 700 to 1500 feet thick. In +the most massive of them Sir W. Logan observed, in 1859, what he +considered to be an organic body much resembling the Silurian +fossil called <i>Stromatopora rugosa.</i> It had been obtained the +year before by Mr. J. MacMullen at the Grand Calumet, on the river +Ottawa. This fossil was examined in 1864 by Dr. Dawson of Montreal, +who detected in it, by aid of the microscope, the distinct +structure of a Rhizopod or Foraminifer. Dr. Carpenter and Professor +T. Rupert Jones have since confirmed this opinion, comparing the +structure to that of the well-known nummulite. It appears to have +grown one layer over another, and to have formed reefs of limestone +as do the living coral-building polyp animals. Parts of the +original skeleton, consisting of carbonate of lime, are still +preserved; while certain inter-spaces in the calcareous fossil have +been filled up with serpentine and white augite. On this oldest of +known organic remains Dr. Dawson has conferred the name of <i> +Eozoon</i></p> + +<p> </p> + +<hr> +<p class="page"><a name="page 492">[ 492 ]</a></p> + +<p><i>Canadense</i> (see Figs. 582, 583); its antiquity is such +that the distance of time which separated it from the Upper +Cambrian period, or that of the Potsdam sandstone, may, says Sir W. +Logan, be equal to the time which elapsed between the Potsdam +sandstone and the nummulitic limestones of the Tertiary period. The +Laurentian and Huronian rocks united are about 50,000 feet in +thickness, and the Lower Laurentian was disturbed before the newer +series was deposited. We may naturally expect the other proofs of +unconformability will hereafter be detected at more than one point +in so vast a succession of strata.</p> + +<center><img src="../images4/fig582.jpg" width="340" height="161" alt= +"Fig. 582 and 583: Eozoon Canadense."></center> + +<p class="capt2">Fig. 582. <i>a.</i> Chambers of lower tier +communicating at +, and separated from adjoining chambers at O by +an intervening septum, traversed by passages. <i>b.</i> Chambers of +an upper tier. <i>c.</i> Walls of the chambers traversed by fine +tubules. (These tubules pass with uniform parallelism from the +inner to the outer surface, opening at regular distances from each +other.) <i>d.</i> Intermediate skeleton, composed of homogeneous +shell substance, traversed by <i>f.</i> Stoloniferous passages +connecting the chambers of the two tiers. <i>e.</i> Canal system in +intermediate skeleton, showing the arborescent saceodic +prolongations. (Fig. 583 shows these bodies in a decalcified +state.) <i>f.</i> Stoloniferous passages.<br> +Fig. 583. Decalcified portion of natural rock, showing <i>canal +system</i> and the several layers; the acuteness of the planes +prevents more than one or two parallel tiers being observed.</p> + +<p>The mineral character of the Upper Laurentian differs, as we +have seen, from that of the Lower, and the pebbles of gneiss in the +Huronian conglomerates are thought to prove that the Laurentian +strata were already in a metamorphic state before they were broken +up to supply materials for the Huronian. Even if we had not +discovered the Eozoon, we might fairly have inferred from analogy +that as the quartzites were once beds of sand, and the gneiss and +mica-schist derived from shales and argillaceous sandstones, so the +calcareous masses, from 400 to 1000 feet and more in thickness, +were originally of organic origin. This is now generally believed +to have been the case with the Silurian, Devonian, Carboniferous, +Oolitic, and Cretaceous limestones and those nummulitic rocks of +tertiary date which bear the closest affinity to the Eozoon reefs +of the Lower Laurentian. The oldest stratified rock in Scotland is +that called by Sir R. Murchison</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 493">[ 493 ]</a></p> + +<p>“the fundamental gneiss,” which is found in the +north-west of Ross-shire, and in Sutherlandshire (see <a href= +"../images/fig82.jpg">Fig. 82</a>), and forms the whole of the +adjoining island of Lewis, in the Hebrides. It has a strike from +north-west to south-east, nearly at right angles to the metamorphic +strata of the Grampians. On this Laurentian gneiss, in parts of the +western Highlands, the Lower Cambrian and various metamorphic rocks +rest unconformably. It seems highly probable that this ancient +gneiss of Scotland may correspond in date with part of the great +Laurentian group of North America.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch26.html"> +Chapter XXVI</a> / <a href="ch28.html">Chapter XXVIII</a></small> +</body> +</html> + |