I.-Metamorphic System.

Description.-Immediately above the granite, and resting directly upon it, lie the most ancient sedimentary rocks in the long series of formations before the human era. These rocks do not exist now as they were laid down. The whole granite surface seems to have retained much of its heat, at least so much as to have changed the character of the mud at the bottom of the seas, and thus formed the hard crystalline rocks of this earliest sedimentary system. It is from being thus changed or metamorphosed by heat that they have received the name of the Metamorphic System.1 They consist of various strata, of which the chief are Gneiss, Quartz-rock, Mica-schist, and Clay-slate.

Gneiss, so named from its thin layers, is a hard crystalline rock, in extremely thin bands, often twisted in a remarkable manner. It consists mainly of the same minerals as granite, from which it was formed, but

Granite.

Gneiss.

Fig. 83.

Mica-schist.

these are broken and confused. Quartz-rock, as its name implies, is composed of fine grains of quartz. Mica-schist obtains its name from the particles of mica that form its chief ingredient. These three kinds of rocks compose one great group, and, though frequently intermingled, occur in the above order. They are extensively developed in Britain, especially in the Highlands of Scotland, where they form the mass of the mountain ranges, and enter into the grand and beautiful scenery of that picturesque region. With them also occurs limestone, which, having been crystallised by heat, forms beautiful marbles, quarried for various purposes.

Clay-slate is that most useful rock, which, split up into thin layers, forms the familiar blue slates for our roofs, and the slate and slate-pencil used every day in school. There is something very interesting in the thought, that the slates you may now see before you, or may be even now using, and the pencil in your hands, are obtained from the earliest of all our rock-formations, and are formed of the fine sand washed from the primitive granite, but lately cooled down!

Organic Remains. The Metamorphic rocks, from being the earliest of our rock-systems, are known also as the Primary rocks, or those first

1 From Greek meta, change, and morphe, form. 2 From Anglo-Saxon gnidan, to rub.

formed. Because they seem to contain no fossils, they have also been called the Non-fossiliferous rocks; but this assertion of the absence of organic remains is not to be taken as final, because in them fossils may yet be discovered, as they have been in the upper beds, which have lately been formed into an independent system. The existence also of plumbago in these rocks, the lead-like mineral used for pencils, which is a kind of coal, and may therefore have been formed of vegetable matter, seems to shew the likelihood of future organic discoveries.

Scenery.-Regarding the scenery of this early epoch, we can form only the dimmest conception. After the intense heat of the primitive granite had subsided, and the once molten mass seems to have become hard and solid as we now see it, a system of things appears to have begun, bearing more likeness to that now existing. Granite mountains reared their heads, great seas rolled their billows, while rivers flowed across the plains, conveying to the ocean-floor the débris of the granite continents. Such organic life as then existed, if it did exist at all, has either been totally destroyed by the great heat of the granite surface, or remains yet to be discovered.

II.-Laurentian System.

Description.—Immediately above the Non-fossiliferous Metamorphic rocks lie the lowest of those that contain fossils. These have received the name of the Laurentian System, from their great development on the shores of the St Lawrence, in Canada. It was only quite lately, in the year 1863, that these rocks were grouped into a distinct system, from the discovery in them of certain fossil remains in Canada, having previously been reckoned Metamorphic. The Laurentian System consists of certain schists, quartzose rocks, and limestones, all very highly crystallised-only little less so than the Metamorphic rocks. They contain no sandstones or shales, that occur so frequently in higher formations, such of these as once existed having been changed by heat. Even the limestone is unlike the limestones that occur higher up, being very highly crystallised. The rocks, however, are all truly sedimentary, deposited under water, and have received their present aspect mainly through the agency of heat. They are found in Canada, the Hebrides, Ireland, and Norway and Sweden.

Organic Remains. The discovery of the fossil remains that caused these rocks to be formed into a separate system, was made in Canada, and excited interest amongst geologists, because belonging to a period when organic existence was thought impossible. The organism discovered received the name of the Canadian Eozöon1 or Dawn-animalcule, and

1 From Greek ēōs, dawn, zōon, an animal.

consists of minute tubes or cells that are visible only under the microscope. Some still deny that this structure is organic, and regard it as merely a mineral appearance; but these are few. The general opinion now is that here we have the earliest life-remains yet discovered on our globe; and this all the more certainly that worm tracks and burrows were found in 1866 in the same formation. The discovery of these evidences of organic life, so long sought in vain, shews that more minute search may result in other remarkable discoveries, and that in all likelihood the name of Eozöon will be found to be premature.

III.-Cambrian System.

Description.-Immediately above the Laurentian lies a series of slates, schists, and crystalline limestones, called the Cambrian System. It is so named from being first most fully described as it is found in North Wales, which in Roman times was called Cambria. The rocks in this series are less changed than the Laurentian, and therefore the remains in them are more numerous and better preserved. They are of great thickness, and are found in Wales, Cumberland, North-west Scotland, Ireland, North America, and elsewhere. Along with the older rocks beneath them, they everywhere form mighty rugged peaked mountains, like those of Wales and the Highlands; and their worn rugged aspect is due to their being so long subjected to wasting influences from their great antiquity. All these earlier rock-formations, from the Metamorphic to the Silurian, are exceedingly rich in mineral wealth, most of the precious metals being obtained from them. Shooting through their hard crystalline masses, we find veins of iron, copper, silver, and gold; and, from the presence of these metals, bare mountain tracts have teeming populations, where even the sheep with difficulty finds its scanty food.

Organic Remains.-The fossil remains are all of the very lowest kinds of life. Sea-weeds, shells of different kinds, and some crustacea, especially one that occurs abundantly in the next formation, called the trilobite, have been discovered. The tracks and burrows of worms, formed in the sand of the ancient seas, may also be seen perforating these hard masses.

Scenery of Period.-During the Cambrian age, quiet seas heaved their waters as now, tenanted with shells and crab-like creatures, while waves rolled on the sandy shores, over which worms crawled, and into which they burrowed; interesting as shewing that creatures had then the same kinds of habits as now, and that what we can see any day along our own shores sends us back to the distant ages when the world was young!

IV.-Silurian System.

Description.-The Silurian System contains rocks less changed by heat than those below, and exhibiting more abundant life. In those already mentioned, the change that has passed over the strata has been so great as to render it difficult to say, with certainty, how the rocks were originally formed, but henceforth all hesitation vanishes. We have slaty sandstone finely laminated, and often exhibiting ripple-marks; conglomerates chiefly of rounded pebbles, clays, and limestones, with corals and other fossil remains, all of great thickness. The system has received the name of Silurian from being very fully developed in a part of South Wales anciently called Siluria, and is therefore named from its chief locality. From these rocks are obtained roofing-slates, freestone for building, flagstones for paving and other purposes, limestones, from which lime is got by burning, and valuable ores of lead, copper, silver, mercury, and gold.

Organic Remains.-Parts of the stems and leaves of water-plants and club-mosses, and a few sea-weeds, are found, but all scarce and much broken. No land animals have yet been obtained, and though it would be rash to say that they do not exist, everything seems to render this very probable. But marine fossils are numerous and well marked, some of them being very beautiful. We find corals of different kinds, named according to their appearance, such as the sun, star, cup, pipe, chain, spider, and honeycomb corals. One of the commonest forms in the Silurian rocks is a very beautiful curved creature like the plume of a goose-quill, called the Graptolite, from looking like a pen on the rock; some single, others double, some

straight, others beautifully spiral. Another very abundant form is the Encrinite, a coral creature more numerous in the Carboniferous System. We find also star-fishes, and numerous shells with single and double valves, some like the periwinkle and cockle being abundant. But the creature that swarmed most in the ancient Silurian seas was the Trilobite, so called from its body consisting of three lobes or divisions, above which was set its head with large double eyes, still to be found entire. It had various forms, and seems to have been very active, and

Coral.

Trilobite.

1 From grapho, I write, and lithos, a stone.

was of all sizes, from mere specks to fine specimens ten or twelve feet long. Creatures like the scorpion, with toothed toes, are also obtained, and in the upper beds fishes.

Scenery of Period. Of the dry land we know little or nothing, except that it did exist, and nourished certain aquatic plants and club-mosses, whose remains were floated down into the great seas. But we can see mighty oceans, in which corals flourished, and encrinites waved their lily stems. Shells were abundant, and numerous creatures gambolled in the bright sun. These seas were fringed by sandy shores, on which worms crawled and left their tracks; gravelly beaches, that have become conglomerates; and great beds of shells, that have given origin to thick limestones. Life gradually assumes more activity, and living forms become more numerous and elevated in the scale of existence as we ascend in the system towards the active period that follows.

V.-Devonian System.

Description.-The Devonian System of rocks has been rendered famous through the writings of several geologists, especially the celebrated Hugh Miller, and is one that in itself possesses the very greatest interest. In early geology, the Coal-measures were considered very important; and as both below and above them a great thickness of red sandstone is found, the rocks above were named the New Red Sandstone; while those below, being of course older, were called the Old Red Sandstone, or, shortly, the Old Red. As these older rocks are extensively developed in Devonshire, this system has also been called the Devonian System, a name now more used than the other. These rocks are found also in other parts of Britain, especially in Forfarshire and Caithness, where they are extensively quarried, and in parts all over the world. The name 'Old Red' indicates that the chief rock is a red sandstone, which is used very extensively for building. This also occurs in fine flags used for pavement, generally of a gray colour-the famous Arbroath and Caithness pavements being from this system. The remarkable rock called Conglomerate or 'Plum-pudding Stone,' which looks as if it consisted of a consolidated sea-beach, is also found extensively in this system.

Organic Remains. There are comparatively few plants found in the Old Red, as compared with the animal remains. We find sea-weeds of different kinds, marsh-plants like our bulrushes, sedges and horsetails, tree-ferns and reeds; but they are neither abundant nor well preserved. Animal remains are numerous, varied, and beautiful. There are many species of corals and shells. The tracks of certain creatures, and deep burrows, sometimes eighteen inches deep and one and a half across, made by large burrowing worms, are frequently found. Many crustaceans are