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in quantity to the coal-seams of an equal area of the Carboniferous rocks.

Not only is the Lower Laurentian series of vast thickness and greatly metamorphosed, but it must have been elevated above the sea, and subjected to vast denudation, prior to the deposition of the upper group. This is shown by the fact that the Upper Laurentian lies unconformably upon the truncated edges of the Lower Laurentian. The Upper Laurentian group is about 10,000 feet thick, and consists wholly of stratified crystalline rocks. These consist mainly of gneissic and felspathic rocks, often characterised by the occurrence of lime-felspar or Labradorite. The series is extensively developed in Labrador, and is sometimes spoken of as the "Labrador series."

In Britain it has been conjectured, with great probability, that the "fundamental gneiss" of the Hebrides and the "hypersthene rocks" of the Isle of Skye belong to the Laurentian series.

LIFE OF THE LAURENTIAN PERIOD.-The Laurentian rocks are often spoken of as the Azoic series (Gr. a, without; zoe, life); but the name appears to be inappropriate, because there is good evidence to show that living beings were in existence in the Laurentian period. In the first place, it is certain that the Laurentian rocks, though now highly metamorphic, were originally deposited as ordinary sedimentary beds of sandstone, conglomerate, shale, and limestone. There is, therefore, no reason whatever for supposing that the seas of the Laurentian period differed in any respect from modern seas, so far at any rate as to render the existence of living beings impossible; while we know that one of the results of metamorphic action is the obliteration of the fossils in the rock affected. Secondly, by the researches of Sir William Logan there was discovered in one of the limestones of the Lower Laurentian group, the body which has been described under the name of Eozoön Canadense, and which is believed to be a gigantic Foraminifer. The organic nature of this body was first detected by Dr Dawson of Montreal, and his opinion as to its nature has since been confirmed by the highest authorities. Thirdly, there is good reason to believe that the graphite of the Laurentian rocks is nothing more than metamorphic coal, and that it is derived from vegetables which flourished during the Laurentian period.

HURONIAN PERIOD.

The rocks of the Huronian series rest unconformably upon the denuded edges of the Laurentian rocks on the borders of

Lakes Superior and Huron. They are about 18,000 feet in thickness, and consist of quartzites (altered sandstones), siliceous slates, conglomerates, and limestones. The conglomerates sometimes contain pebbles derived from the subjacent Laurentian rocks. No fossils have hitherto been found in any part of the Huronian series, and its exact age is therefore doubtful. Not improbably it may correspond with the Lower Cambrian rocks of other regions, but it may represent an independent formation to be intercalated in point of time between the Laurentian and Cambrian groups.

CAMBRIAN PERIOD.

ROCKS OF THE PERIOD.-The exact limits of the Cambrian Rocks are as yet not well defined, different authorities taking different views as to the strata which should be considered under this head. The name "Cambrian" is derived from the fact that these strata are the lowest rocks visible in North Wales and its borders (Cambria). The Cambrian rocks are generally divided into a Lower and Upper division, and they are well developed in various parts of Europe and America. The following gives a general idea of the nature, distribution,

and mineral characters of the Cambrian rocks :

I. Cambrian Rocks of Britain.-The Lower Cambrian rocks of Britain are best seen in the Longmynd Hills in Shropshire, and consist of about 25,000 feet of variously-coloured sandstones, grits, and shales, often ripple-marked, and exhibiting rain-prints, but with very few fossils. These are succeeded by a great series of micaceous flagstones, slates, and shales, which vary in thickness from 6000 to 2000 feet, and are, in part at any rate, of Upper Cambrian age. They are known as the Lingula Flags, from the occurrence in them of a Brachiopod belonging to the genus Lingula. In North Wales the Lower Cambrian strata are often highly metamorphosed, and the celebrated Welsh roofing-slates are also derived from this division. Cambrian rocks occur in other parts of Britain, and the following table exhibits their leading members :

1. Lower Cambrian :

a. Longmynd beds (25,000 feet).
b. Llanberis slates (3000 feet).

c. Harlech grits (6000 feet).

d. Oldhamia slates of Ireland.

2. Upper Cambrian :

e. Lingula Flags of Wales (about 6000 feet).

f. Tremadoc Slates of North Wales (2000 feet).
g. Skiddaw Slates of the north of England (7000 feet).

The last-mentioned group of rocks, namely, the Skiddaw Slates of the north of England, is in a doubtful position. They consist of about 7000 feet of dark-coloured shales and slates, and they are most clearly the equivalent of the Quebec group of Canada, containing many of the same fossils. Upon the whole, it seems safer in the meanwhile to regard them as Upper Cambrian.

II. Cambrian Rocks of Bohemia and Sweden.-In Bohemia, M. Barrande has succeeded in demonstrating as underlying the Lower Silurian rocks of that country a zone of rocks, which correspond to the Lingula Flags of Britain, and are therefore of Upper Cambrian age. This zone contains many remarkable and characteristic fossils, and is often spoken of as the "Primordial Zone." In Sweden and Norway the Lower Cambrian rocks are represented by a sandstone containing impressions supposed to be referable to sea-weeds or "fucoids." This "Fucoidal sandstone" is succeeded by beds of so-called "alum-schist," which are of Upper Cambrian age, and correspond with the Lingula Flags of Britain.

III. Cambrian Rocks of North America. The Cambrian rocks are represented in North America by the Potsdam sandstone and the Calciferous series. The Potsdam Sandstone is mostly a laminated sandstone, or grit in the State of New York, but limestones are present in addition in the Mississippi basin, and it consists of a great thickness (2000 to 7000 feet) of slates, sandstones, and limestones, along the Appalachian chain. It contains a good many fossils, among which are Trilobites resembling those of the "Primordial Zone" in Bohemia.

The Calciferous series consists of a hard calcareous sandstone, or "sand-rock" in the State of New York; but it consists of sandstone with well-developed magnesian limestone in the basin of the Mississippi; and along the Appalachian chain. it consists of sandstones and limestones, subordinated to great masses of shale. In their last-mentioned development the Calciferous rocks have been termed the "Quebec group," and, as before said, they are undoubtedly the equivalent of the Skiddaw Slates of Britain. They attain a thickness of from 5000 to 7000 feet; but it is not clear whether they are truly referable to the Upper Cambrian or to the base of the Silurian system. Most probably they are transition beds between the two formations.

LIFE OF THE CAMBRIAN PERIOD.-In the Lower Cambrian Rocks fossils have hitherto proved extremely scarce. The commonest organic remains are the burrows and tracks of

Annelides (Arenicolites, Histioderma, &c.) Besides these, the Longmynd beds of Shropshire have yielded a supposed Trilobite (Palæopyge). The green and purple slaty rocks of Wicklow have yielded two species of the singular fossil Oldhamia (fig. 27), which may be a Hydroid Zoophyte, but which is more probably a calcareous sea-weed. Lastly, the "Fucoidal Sandstone" of Sweden, besides the obscure remains which are known as "fucoids," has yielded the remarkable fossil known as Eophyton (fig. 378), which is most probably a plant, along with a small Lingula.

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In the Upper Cambrian Rocks, fossils become tolerably abundant, and belong to varied types. The most characteristic forms are Trilobites, the characters of which are so peculiar as to have gained them the name of "primordial," applied also to the strata in which they are found. The chief genera are Paradoxides (fig. 114), Conocephalus, Sao, Conocoryphe, Ellipsocephalus, Microdiscus, Agnostus, &c. The "Primordial Zone of Bohemia has also yielded a few Pteropods, Brachiopods, and Echinoderms. The Potsdam Sandstone of North America contains various primordial Trilobites (especially those of the genus Dikelocephalus), Brachiopods of the genera Obolus, Obolella, and Lingula, Gasteropods of the genera Pleurotomaria and Ophileta, Annelide-burrows (Scolithus), and numerous so-called "fucoids." Lastly, in the Potsdam Sandstone have been detected the earliest footprints, if they may be so termed, which have been as yet discovered. These have been described under the names of Protichnites and Climactichnites, and they have probably been produced by large Crustaceans.

The Lingula Flags of Britain owe their name to the occurrence in them of a large satchel-shaped Lingula (L. Davisii). Trilobites of the genera Olenus, Agnostus, Paradoxides, &c., occur, and remains of Phyllopodous Crustaceans (Hymeno caris) are by no means rare.

The Skiddaw and Quebec groups, as already mentioned, are in a doubtful position, and are often regarded as being referable to the Lower Silurian. They are chiefly noticeable for the great abundance of Graptolites which they have been shown to contain. Many of these belong to genera which pass up into the Silurian rocks (Didymograpsus, Diplograpsus, and Climacograpsus); but others belong to types which are exclusively confined to this horizon, and which are remarkably complex as compared with later forms. Amongst these may be mentioned the genera Tetragrapsus (fig. 32), Dichograpsus (fig. 33), Loganograpsus, and Phyllograpsus.

CHAPTER XLVIII.

SILURIAN PERIOD.

ROCKS OF THE PERIOD.

FOLLOWING the Cambrian comes the great Silurian series of rocks, first clearly established and definitely worked out by Sir Roderick Murchison, the founder of the Silurian system. The exact limit between the Cambrian and Silurian formations is one which is not clearly defined, since there does not appear to be any general physical break between the two groups. The line of demarcation between them is, in the present state of our knowledge, an arbitrary line, and is derived chiefly from the characters of the Trilobites. There are rocks, however, such as the Tremadoc Slates, the Skiddaw Slates, and the Calciferous and Quebec groups, in which there is an intermixture of Cambrian with true Lower Silurian types. These rocks, therefore, might be regarded as Upper Cambrian or as Lower Silurian, or as passage-beds between the two. It is to be remembered, also, that the Tremadoc Slates and Lingula Flags are regarded by Sir Roderick Murchison as being the basement-beds of the Lower Silurian.

The name "Silurian" was proposed by Sir R. Murchison for a great series of strata lying below the Old Red Sandstone, and occupying those parts of Wales and England which were at one time occupied by the "Silures," a tribe of ancient Britons. The Silurian rocks are largely developed in Wales, the north of England, Scotland, and Ireland, in various parts of Europe, especially Bohemia, Saxony, Russia, and Sweden, and in the North American Continent. The entire series is divisible into the two sections of the Lower and Upper Silurian rocks, each in turn split up into smaller subdivisions, the names of which have usually been taken from localities where they are unusually well developed, or where they were first studied. In Britain the Silurian Rocks are divided into the following groups from below upwards :—

a. Lower Llandeilo group,
b. Upper Llandeilo group,

c. Bala, Caradoc, or Coniston group,
d. Lower Llandovery group,

e. Upper Llandovery group,

f. Wenlock group,

g. Ludlow group,

Lower Silurian.

Upper Silurian.

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