CRUST OF THE EARTH—GEOLOGICAL SYSTEMS—EOZOIC, PRIMARY, SECONDARY, TERTIARY, PREHISTORIC FORMATIONS.

The crust of Great Britain has been carefully examined, and from the results of investigations at various periods, the earth has been divided into a series of strata which follow the same order of succession. Sometimes certain strata may not be present, and they may be replaced by others, but the same order of succession will be found. The order is as follows, commencing at the lowest. The illustration is taken in the opposite direction:—

The PalÆzoic Systems.

Laurentian System. It will be perceived from the above list that the Laurentian Rocks are the oldest. The name is derived from the St. Lawrence formations, and was given to the strata by Sir William Logan. They are metamorphosed rocks older than the Cambrian. These rocks are sedimentary, of very old deposition, and of a crystalline nature, consisting of quartz, gneiss, etc. The granite was probably formed by the fusion of its component constituents, quartz, mica, and felspar, which become crystallized by the excessive heat. For a long time no traces of organisms could be detected in this or the Cambrian systems, but modern research has been rewarded with a little success. The original deposits of micaceous gneiss, etc., have been altered, and many true igneous rocks, such as syenite and granite, are found in them. These very old rocks must have been originally deposited in strata converted by heat and pressure into crystalline rocks. These rocks have been divided into two series, under the names of lower and upper Laurentian. They are metamorphic, and consist “mainly of gneiss interstratified with mica-schist, with great beds of quartz, and massive beds of crystalline limestone, of which one varies from 700 to 1,500 feet in thickness. Conglomerates also occur, and there are vast deposits of magnetic and specular iron. Graphite, or blacklead, is disseminated in strings, veins, and beds through hundreds of feet of the lower Laurentian, and its amount is calculated by Dr. Dawson to be equal in quantity to the coal seams of an equal area of the carboniferous rocks” (Nicholson).

Hitherto, no distinctly recognisable fossil has been discovered, with the important exception of the EozoÖn Canadense, which has been pronounced to have been a gigantic foraminifer, growing layer upon layer, and thus forming reefs of limestone; the subject, however, is still a matter of dispute. The eozoÖn was discovered by Mr. J. M’Mullen in 1858 in Canada.

Granite was at one time considered to be the true primitive rock. Gneiss is a word of Saxon origin, and consists of the same materials as granite in different proportions. Mica-schist is made up of two of the same constituents as the granite and gneiss. They are without fossil traces.

The Cambrian system of aqueous origin may be said to contain evidence of the dawn of organic life. It is part of the clay-slate system, and the term “Cambrian” is taken from the ancient name of Wales, where slate is plentiful. Mica-slate is also very important. These Cambrian rocks are of the next oldest formation to the Laurentian, and all the various deposits may be examined in Wales, where also traces of volcanic and ice action may readily be perceived. In the pass of Llanberis one immense ice-borne block is very prominent; no agency but ice could have put it there as it rests. It is estimated that the Cambrian and “Lower” Silurian rocks are from 20,000 to 30,000 feet in thickness, and must embrace a very lengthened period. The fossils of these formations show that zoophytes and certain primitive crustacea lived in the remote ages when these rocks were formed by sedimentary deposition. We have scarce a trace of plant-life. The organic remains include annular worms, the first arrangement of the articulated animals according to Cuvier. The flora and fauna are, of course, very low in the scale of creation, when land and sea were so differently arranged.

The Silurian system was so named by Sir. R. Murchison, after the territory formerly occupied by the Silures, but the system is, of course, universal. We have here sandstones, limestones, and shales, deposits lying upon the Welsh slate. The Upper and Lower Ludlow beds, and the “May Hill” sandstone, then the Lower Silurian, with Caradoc beds, and the Tremadoc slate, etc. In this system volcanic action is observable, and all the organic remains are those of marine animals, such as corals, shell-fish, marine worms, encrinites, molluscs, and other zoophytes in great variety. We find also a number of graptolites, trilobites, echinus (sea-urchin), terebratula, and many other forms.

The Trilobites were amongst the first creatures inhabiting our globe, and it is a curious fact to contemplate, that their eyes (fig. 655) should have been preserved perfect; they present one of those wonderful objects which carry one’s thoughts backwards to the early ages of the world, probably many millions of years, and yet it is found by the peculiar structure of the eyes of these Trilobites that they were placed at the bottom of the sea with perfect power to look upwards at the light of the sun through the transparent waters. The same hand and the same power had then Divine care and solicitude for the well-being of His creatures, as great as He has for those of later ages, and these animals are mentioned in Genesis—“Let the waters bring forth abundantly the moving creature that hath life.”

Thus we see that ages of comparative quiet succeeded the first great contraction of the earth’s crust, probably millions of years, during which time the tides and currents of the ocean had to wash and wear down all the thousands of projecting rocks or inequalities, and dissolve (as before described) all the lime, depositing the sand and clay in those immense strata which form the old-named “transition series”; this appears to have taken place over nearly the whole world at that time, and ages upon ages must have elapsed to form such deposits as the sandstone, claystone, and limestone, in alternation, forming the “Llandilo,” “Caradoc,” and “Wenlock” strata, more than a mile in thickness; these are by some geologists reckoned among the primary series (by some called the “transition rocks”), and in England form the “Cambrian” and “Silurian” systems which are so rich in minerals and metallic veins.

The Old Red Sandstone, or Devonian System, is the next of the series of layers which built up our earth, but a great gap of years separates it from the Silurian. We find the “Old Red” in the Mendips and in Scotland. The rocks in the West of England are apparently of later deposition and of marine origin, while the “Old Red” is apparently a fresh-water deposit. It is very “arenaceous,” and owes its tint chiefly to iron, although there are circumstances in which it appears neither as a sandstone nor with a red colour; but red sandstone describes the true formation very accurately.

In this water-deposited system, whether in lakes or by the sea, we find a considerable advance upon the Silurian. We have flora in more variety—seaweeds and ferns. The remains are all aquatic. We have nothing higher in the scale of creation than the fish, the first vertebrates; and judging by varieties a very considerable time must have elapsed during which the Old Red Sandstone was deposited. The dipterus—or double-winged—is herewith shown as an example of the fossil fish of the Old Red Sandstone period.

It is curious that no remains of any land-inhabiting animals have been discovered in this system—whether in the Old Red or the Devonian formations (the Lower and Upper Red Sandstones). We can only distinguish the remains of aquatic animals or plants. We may picture the great cuttle-fish, the nautilus, and the dipterus, with various orders of mollusca, and the gradual approach to the crustacea, but no terrestrial animals have been discovered. We may take it for granted then that the Old Red Sandstone and Devonian systems are different—the former being found in Scotland and parts of England, and formed of deposits in fresh or brackish water, while in the Devonian system the marine deposits are corals, and all the indications of ocean life, separated from the great inland lakes by a range of hills. Neither of the terms (Old Red Sandstone or Devonian) limit geographically or descriptively the formations of this system. All the rocks are clearly distributed between the Silurian and the Carboniferous. The invertebrates in this last system have not developed very much, but corals are very abundant, and fish of some armoured species are plentiful and curious, while the crustacea were enormous.

We now arrive at the most important of all the rock formations, the one to which we owe our national prosperity—we mean the Coal System.

The Carboniferous Formation.

While the foregoing depositions were being made the earth was still undergoing changes. The sandstones were deposited, and the corals making use of the lime carried into the waters began to build and form masses of limestone under the sea, pushing back the water and changing the forms and positions of land and water. All this went on apparently very quietly—volcanic action was not very frequent—the water was warm. But sometimes earthquakes would heave up the submarine formations into mountains, and therefore we find the fossils of the tiny sea-animals on the hills. Extensive swamps were formed by partially retreating sea-water, and their vegetation became luxuriant. Tree ferns and all the floral appearance of the tropics grew up and formed dense forests, far thicker than any we know of at the present time. It will readily be understood that the condition of the atmosphere must have been particularly favourable to the growth of plants, and therefore not suitable for air-breathing animals. Heat and carbonic acid must have been greatly developed.

We can now perceive how the gradual filling up of the earth for man’s reception was taking place. The rain was taking carbonic acid from the air, for the dead plants gave it out in enormous volumes. The carbonate of lime dissolved in the water-springs, etc., was carried to the sea for polypi to build shells from. The trees were absorbing carbonic acid, too, and while purifying the air, were retaining the carbon in their stems and leaves and branches, which (when they decayed) remained untouched, and accumulated in thick layers to sink down, and by pressure be turned into coal. This great effect was carried out several times; and it is a remarkable fact that we find coal, limestone, and ironstone so near together, all useful to us and to each other in the course of the working of the minerals—so we come to the Carboniferous system.

Coal we have already treated of in Mineralogy, and coal looks at times very different from our preconceived ideas of a sedimentary rock, which we know is regularly deposited in layers. But when we split or break the coal we find its cleavage in a certain direction. Coal is wood squeezed and petrified by ages between enormous layers of sedimentary rocks, and the coal-seam rests upon the soil in which the plants once grew—perhaps more than six hundred thousand years ago!

Of course coal, as we burn it, was not all made at once. We can trace it from the swamp as Peat, on to “Lignite,” or woody coal, through the Tertiary and New Red Sandstone to the coal measures themselves. Even lower down we find the remains in more or less pure carbon forms—the anthracite and the graphite of the primary formations.

Coal appears not to have been formed equally in all places during the period in which it originated. The remains of plants found in these strata lead us to infer, that, during that period there existed an exceedingly vigorous and crowded vegetation, consisting principally of tree ferns and equisetaceÆ, of which the Sphenopteris Hoeninghausii (fig. 662), Pecopteris aquilina (fig. 663), and Neuropteris Loshii (fig. 664), are amongst the most beautiful that have been found, and the flora and fauna of this period were of a more or less primitive kind or low order, but very luxurious. The former display a decided advance, and reptiles of aquatic forms appear with large and predaceous fishes. Mountain limestone, which is usually found in the coal formations, includes metallic deposits, and organic remains are very abundant in it. The following are specimens of the fossils—

The Carboniferous system is a very important one, as may be seen. In these beds we have coal, limestone, sandstone, and shale. The Coal Measures consist of grit and sandstone and shale, with coal seams. The Carboniferous Limestone, or “Mountain” Limestone, has no coal in it. The sandstone has been termed “Millstone Grit,” because millstones are made from it. We then have limestone shales, and the sandstone beds. Each coal seam indicates a subsidence of the land and a regular series of underclay or soil in which the plants grew,—the plants themselves,—iron, coal, and then the shale, and so on again, indicating frequent changes and a long lapse of ages.

These “Coal Measures” occupy an area of five hundred square miles in Great Britain alone; and as we have already said, the period which elapsed while these deposits were being laid down represents hundreds of thousands of years. The deposit would increase at the rate of about three feet in a thousand years. And this is only one period of the many changes to which our world has been subjected since it first began its revolution in space around the sun.

Coal is usually found in “basins” or depressions—a sort of trough owing to the upheaval of surrounding strata which became in time denuded (or washed away) with any coal that was there. So it is in places where it is concealed by overlying beds that protect it, that we now find the coal saved from disturbance. When we search and come upon red sandstone and grey-wacke, we may be almost certain that we are near coal, particularly if the surrounding rocks form a “basin.”

We have now briefly sketched the Carboniferous system, for in our recreations we can do little more. We shall find in many places in England tree trunks in the sandy shore, and ample evidence that a forest has been at one time submerged in that spot. So inland the land sank down again and again in successive periods—water, mud, soil, vegetable growth succeeded, to be again submerged and form a new coal seam for the use of man, who was destined to appear after the lapse of ages.

The Permian Period.

At length the land remained undisturbed. It sank no more, and the trees waved luxuriously over the buried forests of past ages, and another cycle set in called the Permian, from the ancient kingdom of Permia, where all the features of that period are exhibited on a very grand scale, and which extended for several hundred miles between the Ural mountains and the Volga. In the Permian period we have the progression of animal life more distinctly developed than in the Carboniferous system, which immediately preceded it. It is true there are indications in the latter that curious animals, called the Cheirotherium and the Labyrinthodon, were alive then, and the remains of numerous insects, such as beetles and crickets, have been found; but the Permian developed them, and reptiles, saurians, and lizards have been traced; but as Sir R. Murchison states, throughout the whole extent the animals are of a single type. We have the hand-like impressions of the feet of the Cheirotherium, so called from the Greek “cheir,” a hand. The soil appears to have been very soft, and peculiarly adapted to receive impressions, and which having been in many places covered over with a stratum of fine sand, and then abandoned by the sea, the whole has hardened into stone, and being now separated, the one contains their footprints, and the other perfect casts of them! Nor are these footmarks all that these sandstones have to tell us of their day; for the ripples of the waves, and even the little pits made by drops of rain as they fell, are in this most marvellous manner preserved, forming objects of wonder and admiration.

The organic remains during this period are not very abundant, and many of the fauna of the previous systems appear to have died out, while others appeared to meet with fuller development in succeeding ages. The Permian is also known as the New Red Sandstone, or Magnesian Limestone group. “Dias” has also been suggested with reference to the “Trias” group, “the Upper New Red Sandstone,” which comes next. The Permian rocks are very varied, and contain minerals, such as copper and sulphur.

As the strata below the new sandstone formation was called the “Carboniferous” system, from its containing much carbon in the form both of coal and carbonic acid, so this has been called the “Saliferous” system, from the occurrence in many places of strata of “rock-salt,” or crystallised chloride of sodium, and (where the rain finds its way down and dissolves it) of brine springs; these (in England) exist chiefly in Cheshire and Warwickshire, but in Poland and Hungary they exist on a much larger scale, the rock-salt being nearly a thousand feet thick. It has been said that these strata of salt were formed by the evaporation of salt lakes, but it is much more probable that salt is one of the natural materials of the earth, and that both salt lakes and oceans have become salt from dissolving out these strata wherever they have come into contact.

It is supposed that during the Permian period the greater portion of the continent of Europe was raised above the ocean, and the deposits were formed in salt lakes, for the appearance of the organic remains tends to establish the fact that the creatures of that period were not far from dry land even in their watery existence, and the reptiles found confirm this view. We have now to examine the Mesozoic, or Secondary System.