CHAPTER III.

THE STORY OF A PIECE OF SLATE.

"It is a lonely place, and at the side

Rises a mountain rock in rugged pride;

And in that rock are shapes of shells, and forms
Of creatures in old worlds, and nameless worms-
Whole generations lived and died, ere man
A worm of other class, to crawl began."

CRABBE.

WAS not always what you now see me. Far, far back in that almost infinite past, which geology claims before it can explain its phenomena, I was lying along the bottom of a tolerably shallow sea, as part of an extended sheet of fine mud. My birthplace is registered in the heart of the North Welsh mountains, and the formation to which I belong goes by the name of the Cambrian.

Its rocks form some of the grandest scenery in the world. Steep precipices, on which grow rare ferns and wild plants, frequently too tempting to the botanical student, are the result of succeeding dislocations, jointings, and bedding. Mountain streams brawl over them; and waterfalls, whose substance is evaporated into prismatic mists, pitch from the precipices of these Cambrian hills. Fre

quently the rocks are so hard and bare, that even the lichen and moss fail to obtain foothold, and so the naked slate shines in the varying sunlight in coloured shades from pink to deep blue. Here, with the gathering cumuli, ring-like crowning their peaks, the Welsh hills stand forth in all their characteristic grandeur. No wonder that crowds of tourists should strive to forget the cares of business, and endeavour to get a mouthful of purer air, whilst climbing their steep sides!

It requires some faith in geology to carry the mind definitely backwards to the time when these rugged hills were extended sheets of marine mud! But no mathematical deduction is more certain. You never find clay or sandstone rocks so full of fossils as limestones, for the simple reason that the former are of mechanical origin, and the occurrence of organic remains is therefore accidental. Whereas limestones are of vital origin, resulting from organic agencies almost entirely.

You examine the slate rocks of which I am a humble representative. Their colour and generai texture you easily recognize from the too familiar appearance of the London housetops. But, when in position, you are scarcely prepared to find that what you had imagined to be the result of bedding or lamination in the slates is actually due to what is termed cleavage. This is a peculiar feature about thin-bedded, argillaceous or clayey rocks, that they undergo, when subjected to pressure, and perhaps

heat as well, a certain change, which causes every particle to change its position. By virtue of this process, the rock splits not so readily along the lines of stratification as along that of the cleavage, or planes where the material has been re-arranged. In addition to this structure, which is frequently diagonally across the line of stratification, these slate rocks are broken up into large cubic masses, caused by great joints traversing the rocks, irrespective of any previous alterations.

The stratification itself is not horizontal, but frequently pitched up at a very steep angle, and commonly the rocks are contorted into a series of ribbon-like foldings. After all this cleavage, jointing, dislocation, and faulting, the solid rocks have been subjected to thousands of centuries of atmospheric and marine wear-and-tear! Can it be wondered at, therefore, that there should result from all these combined agencies, continued through untold millenniums, all that wildness and grandeur of physical scenery which distinguish these old Cambrian rocks wherever they are met with ?

The old rocks, especially those of an argillaceous character, are nearly always marked by contortions, to which those of a later date are strangers. It is from amidst them also that we have great bosses of granite coming to the surface, the contorted slate rocks surrounding them on every side. How is this? I will endeavour to explain.

My hot-tempered friend, the piece of granite, told you how it was absolutely necessary to his origin that the molten rock of which he was portion should be overtopped by a tremendous thickness of material when it was cooling. This my own

experience will bear out. The contortions which characterize my family equally required an amount of overlying material to be piled upon them, or they could not have arrived at such singular appearances.

A mass of half-hardened rock, if displaced by a foreign body, such as a boss of granite being thrust up, would rise up as one great hill or mountain. But if there was sufficient pressure overlying the formation thus disturbed, then it would be thrown into a series of foldings, in order to make place for the laterally-intruded material. Of course the whole exterior surface would then be elevated; but this elevation would not be in a conical form, but along a large tract of country.

In geological books you will find how, on a small scale, this experiment has been conducted. A series of layers of cloth has been formed; pressure was

applied to the sides, when the surface naturally rose into a sort of mound; but the moment a heavy weight was laid on the top cloth (thus representing the overlying material of which I spoke), then the layers of cloth, when pressed at the sides, became folded up into a series of contortions. My hearers will now see why granite outcrops should frequently be the companions of slaty contortions; for the agency of overlying rock-masses, which originated the former, by their pressing weight caused the latter, when disturbed, to assume the wrinkled, fantastic shapes they now present!

It is not long since the Cambrian formation was deemed the oldest in the world; even its most learned and indefatigable observer called it the Protozoic, imagining its organic remains to be the "first life-forms." This provisional place of honour, however, has since been bestowed on a still older, and of course even a more contorted and metamorphosed class of rocks, termed Laurentian. Whether this in its turn will have to give place to one older still I cannot tell; but this I know, that the more you study the rocks and their contained fossils in the field, the more will you be convinced of the enormous antiquity of the earth, and of the incalculable period during which life has been divinely manifested upon it! Human arithmetic will never be able to compute my own age, and therefore the very attempt would be futile. Seeing that we slate rocks are, as far as England is