Sept. 1, 1870.] 



HARWDICKE'S SCIENCE GOSSIP. 



205 



to catch and fix the process half completed. But, 

 alas, I was not quick enough. My Macrothrix lies 

 side by side with its cast-off exuviae. Will the 

 editor, or one of his accomplished readers, kindly 

 inform me whether my capture is really a novelty 

 or not ? 



THE STORY OF A PIECE OF SLATE. 

 By J. E. Taylok. 



I 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 

 beloug 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 summits of these Cambrian hills. Fre- 

 quently the rocks are so hard and bare, that even 

 the lichen aud 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 cha- 

 racteristic grandeur. No wonder that crowds of 

 tourists should strive to forget the cares of busi- 

 ness, 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. Where- 

 as 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 arc broken up into large cubic masses, 

 caused by great joints traversing the rocks, irre- 

 spective 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 convolutions. 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 ? 



These 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 por- 

 tion should be overtopped by a tremendous thick- 

 ness 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 

 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 readers 

 will now see why granite outcrops should frequently 

 be the companions of slaty contortions; for the 



