is, then the felspar (which is so truly clay, that it makes the best possible material for the use of the potteries,) and the thin shining plates of mica will be carried farther by the water than the lumps of white quartz or flint sand, which with the other two ingredients made up the granite; and the two former will be deposited in layers, which, by passing a galvanic current through them, would in time become micaschist. If the mica were absent, or if the clay were deposited without it, owing to any cause, then a similar galvanic current would turn the deposit into something like clay-slate. These three mechanically arranged rocks are found abundantly, surrounding and overlying the granite, as if they had been formed from its broken and rough edges, worn away by the waters of the first ocean, and afterwards deposited at the bottom of the sea. In these rocks we have arrived at a second period, still unmarked by life, although apparently better fitted for sustaining it; our earth being then not merely a chaotic mass of cracked and burnt rock, but having had superimposed upon that mass extensive and thick layers of various materials; these contain in their composition most of the elements, both gaseous and solid, by certain combinations of which living animals and vegetables were enabled to perform their functions, and render inanimate matter available for their different wants.

One of the most remarkable facts with regard to these ancient deposited rocks, is their extraordinary thickness in some localities. It is not difficult to understand, that at a time when the granite and granitic rocks were newly formed, and presented

innumerable fractured edges in every direction, the pounding action of moving water, especially if that water was of a high temperature, might grind down the exposed rock with extreme rapidity, and produce extensive deposits, rapidly filling up hollows and depressions. But we can hardly suppose the existence of depressions so considerable as the thickness of the gneiss and clay-slate would require; and it is far more reasonable to assume that a contraction of the crust, the result of gradual cooling, produced a series of wave-like motions in the earth's crust, alternately elevating and depressing portions of the surface, and sometimes producing a succession of elevations or depressions on the same spot. However this may be, it is certain that these old sedimentary rocks have been not unfrequently altered so as to have become crystalline; and they are also very often cracked and broken, the cracks being sometimes filled up with rocks of a different kind, injected apparently in a melted state, and sometimes with other materials, also crystalline, and often containing a greater or less proportion of metallic ore.

Thus do these lowest sedimentary strata, whose vast antiquity is in many cases unquestionable, but which sometimes, like the granite itself, have been elaborated at later periods, occupy a definite place among the rocks of which the earth's crust is made up. They mark, it would appear, a strange and dark passage from that state which we have considered chaotic, to a condition of more regular and quiet deposit; they are, however, with reference to fossiliferous rocks, azoic, or lifeless; and they are also as a class almost as widely spread, and as distinctly

universal, as the granitic rocks themselves. At the end, therefore, of this our first period, we may suppose that there existed a globe, whose surface exhibited alternations of land and water; the land having in some places as distinctly stratified an appearance as it has at present, and the thick masses of strata resting on huge bosses and peaks of granite and other igneous rock-but all was then bare and desolate; not a moss nor a lichen covered the naked skeleton of the globe; not a sea-weed floated in the broad ocean; not a trace existed even of the least highly organized animal or vegetable; everything was still, and with the stillness of absolute death. The earth was indeed prepared, and the fiat of creation had gone forth; but there was as yet no inhabitant, and no being endowed with life had been introduced to perform its part in the great mystery of Creation.

It must, however, be distinctly understood that this view is strictly hypothetical, and is, after all, only one means of explaining certain phenomena. So far as it is an illustration of facts that have been observed, it has its value, and may be received provisionally; but, so far as it is merely a theory of the earth, it is worth neither more nor less than other different theories, many of which were proposed by cosmogonists of ancient date, and some have been put forth in our own time by persons who have as little ground for theorizing. I have chosen in the present case to present it as a sketch, embodying many facts and results of observation, although the cause of the absence of fossils in metamorphic rocks, and of the other appearances that have been observed, may undoubtedly have been very different.

It is not, indeed, till we advance one step further, and consider the condition of the earth, by comparing what we know of its inhabitants with our speculations concerning the position of land then existing above the water, that we can arrive at conclusions at all satisfactory.

The additional facts made known by studying. the remains of animals and vegetables found in the various rocks, give a new aspect even to the form of the speculation; and we shall soon perceive how far this view of the earliest condition of the globe is probable, when we study the first known results of creative power in reference to organic beings.

CHAPTER III.

THE PERIOD OF THE EXISTENCE OF INVERTEBRATED ANIMALS AS THE MOST HIGHLY ORGANIZED INHABITANTS OF THE SEA. THE SILURIAN ROCKS.

WRAPPING round the igneous rocks of Cumberland and the lake district, ranging over a considerable part of the north-east of Ireland, occupying a large portion of South Wales, and present almost everywhere in North Wales, there are found a great number of sedimentary rocks of various kinds, covering the gneiss, mica schist, or clay slate, and covered up in South Wales by a series of coarse red conglomerates or beds of pudding-stone. These sedimentary rocks are expanded sometimes to a thickness of many thousand feet, and they form a remarkable and natural group, which may be conveniently sub-divided into two parts, the lower being by far the most considerable in vertical thickness, but the upper containing a greater number and variety of the fossil remains of animals.

In the British Islands, and very generally in other countries, this lower group of rocks consists of a grayish-coloured sandy stone, often slaty or flaggy, and containing much clayey matter, sometimes including poor bands of limestone, and not unfrequently exhibiting, in the partings between two beds, a number of imperfect remains of shells and other organic sub