are generally very obscure. The Dudley limestone is perhaps the most prolific. A multitude of little animals. or insects, (the trilobite), of a species unlike any existing, or imbedded in other rocks, pervade

this mass. In Wales, between the laminæ of the slate, they are also abundant, and they are, in fact, common to Europe and America. From the myriads of these found together entombed, and

the contracted attitude in which they are frequently observed, it is inferred that their destruction was the effect of sudden catastrophes, which overtook a whole herd, and as suddenly enveloped them in the matter, which subsequently became hard rock. Five hundred and forty-six species of shells, zoophytes, crustaceous animals, and plants, have been identified with this ancient group of rocks, and among them there are several genera, which still occupy a place in the animated creation, having survived all the changes and convulsions, in which so many tribes of beings have perished.

As we approach the primitive rocks all traces of organized beings vanish. From man to the screw-like encrinite-from the caverned bones to the transition rocks we have seen a continuous chain in the types of animated beings, whose petrified relics tell the tale of their existence. Here, however, it is broken off, the light by which we have thus read the history of created beings is withdrawn. We appear to have arrived at the period when the Omniscient fiat first imparted life to inanimate matter: and have now to contemplate "the earth without form and void."

It has been suggested indeed, that the absence of

fossil remains in the early sedimentary strata, is no proof of the absence of life at the period when they were deposited: but that the first organized beings might have been of the class of those gelatinous, flesh-like substances, to which the name of Medusa has been given (from the organs of motion spreading out like the snaky hair of the fabulous Medusa). The ocean it is said might have swarmed with these, and no traces have been left of their existence.

NON-FOSSILIFEROUS ROCKS.

Hitherto, water has been the principal agent in the production of the great physical changes, which we have reviewed. All the sedimentary strata, whether they be clay or hard rock, chalk or marble, sand or sandstone, are vestiges of the dominion of the universal ocean, or of subordinate masses of its world-encircling waters. Everywhere its presence at successive periods may be traced, and so extensive appear to have been its operations, that we might almost have arrived at the conclusion of Thales, that all things had their origin in water. But another powerful agent, which we have seen operating in volcanos, must now be introduced, to account for the phenomena of primitive, and other non-fossiliferous rocks.

Although we cannot consult the archives of nature

These rocks being destitute of organic remains and generally beneath all the other formations, are supposed to have existed antecedent to the creation of animated beings, to be in fact, coeval with the origin of the earth itself, and of the same nature as the interior of the globe-hence the term primitive. Some geologists have conjectured that the primitive rocks are themselves merely the fused wreck of pre-existing masses, but this endless system of mutation, which they would thus establish, lacks both proof and analogy.

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in the organic contents of these masses, they present to us internal evidence of their origin, scarcely less equivocal. Instead however of tracing the progress of the development of animals and vegetables-of beholding in imagination regions now sterile clad with tropical splendour,-groves of umbrageous palms in situations where the degenerate soil now produces only the bramble, of watching with satisfaction the gradual extinction of the hideous monsters, which dwelt in countless herds in pestiferous marshes now transformed into scenes of fertility, it is our business to examine the changes which inorganic matter has undergone;—an investigation not less interesting in its results, but requiring a knowledge of which we have hitherto had no need, of chemistry and mineralogy, without a slight acquaintance with which, the subject is unintelligible.

In the primitive rocks no traces of organized remains have been observed: they present, in fact, a field of enquiry which lies beyond the range in which we have hitherto been confined; we here appear to have reached the fountain head of the long and varying stream of organized being which we have imperfectly and hastily traced, the source from whence animated nature first sprang-the period of creation: a result of science, which those who have indulged in the dream of the eternal duration of things, never expected to witness.

This division of the subject of my "Sketch" well deserves a separate volume, it must, however, be briefly dismissed. In the primitive rocks, which consist partly of stratified, and mainly of unstratified masses, known by the names of granite, gneiss, mica slate, primitive limestone, and other local appellations, the greater part

of our metallic treasures lie concealed. These are usually found in veins or irregular lines of extraneous matter, which traverse in all directions and to an unknown depth, both stratified and unstratified rocks. The ancient notion was, that veins were branches and twigs of an immense trunk, which exists in the interior of the globe, raised in the form of vapours and exhalations through the rents and crevices, as the sap is raised and circulated in trees, and that the substance of which they are formed was elaborated from the materials of the rocks.

In recent times several theories have been adopted to explain these phenomena, but their origin is still involved in obscurity: 1st, that they were open fissures caused by the contraction or subsidence of the rocks, and afterwards filled by aqueous solutions, poured in from above, or by sublimation;-2nd, that these rents were the effect of internal violence, and were filled by fused materials ejected from beneath;-3rd, the theory of Dr. Boase, which assigns their origin to the same period and circumstances as the masses which enclose them.* In some instances however, there seems positive indications of a difference of age, as where metalliferous veins are crossed by others, they are displaced, and a disturbance in the line of direction of the intersecting veins takes place,† significantly termed by the miners a heave.

Mr. Robert Fox was the first to point out the

* Professor Sedgwick suggests, that there are three different sets of veins, which may be referred to each of these hypotheses.

+ See Taylor's Communication to the British Association, on Mineral Veins.