undergo those great depressions which eventually brought them so much under the influence of the earth's internal heat. Masses of granite, trap, porphyry, &c., have been intruded through them, and thus they have been squeezed and contorted in the most fantastic manner. The sandstones, some of them five hundred feet in thickness, have been so affected by heat as to become quartz, or quartzite. Here, then, you have the secret of my origin-the whole history of the changes which brought about my present appearance! The limestones that were contemporaneous with myself were altered so as to resemble loaf-sugar, and had all, or nearly all, their organic remains obliterated. The shales and slates became transformed by heat, chemical change, and pressure, into mica-schists, gneiss, felstones, &c. So that the very peculiarity in dip, contortion, absence of fossils, and mineralogical changes, which mark all the rocks of the Laurentian age, tell of their vast antiquity; whilst the similarity in composition of these rocks in all parts of the world,-in Ireland, Scotland, and North America, as well as the prevalence of similar lowly-organized fossils in their limestones, indicate they have passed through the same transformations since they were contemporaneously deposited as limy muds, sands, and clays along the floors of the primeval seas!

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 general 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 is in reality a sort of rude, massive crystallization. By virtue of this process, the rock splits not so readily along the lines of stratification or bedding as along that of the cleavage, or planes of sub-crystallization.

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 ap

pearances.

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