Outlines of Geology. 243 



enigmatical family of the grauwackes, while the slate of Tintageli 

 and of Camelford is a distinctly unadulterated and admirably- 

 defined clay-slate, and esteemed of an excellent quality as a 

 roofing-material. 



To the rocks which we have just considered, and to some others 

 that will hereafter demand attention, belongs a remarkable feature, 

 of which Cornwall furnishes some admirable and unrivalled in- 

 stances, and which is truly perplexing to every person who looks 

 at and examines the phenomenon unblinded by the dust of books 

 and the fog of theory. The slate is remarkably contorted in some 

 places ; its strata are waved and curved for greater or less extent, 

 and sometimes an entire bed affects a serpentine irregularity, which 

 gradually merges into a straight line above and below. These 

 contortions of the Cornish slate are sometimes independent of 

 any other kind of rock— sometimes they are apparently caused 

 by veins of elvan, to use a term of the country, by veins of a dis- 

 tinct species of rock which is of a porphyritic character, and 

 which invades the slate, as it were, from below, bending it into 

 a thousand shapes, or sometimes merely dislocating and giving a 

 new inclination to its strata, seeming as if, in the one instance, it 

 had acted upon the soft, yielding, and unconsolidated matter that 

 surrounds it ; in the other, as if it had violently protruded itself 

 into the already-hardened mass. On another occasion, this phe- 

 nomenon will come more decidedly under our notice ; the con- 

 comitant effects must be studied, as well as the main ones, and 

 thence we must endeavour to deduce an efficient cause ; — at the 

 same time, I must here remark, lest I should be thought to adopt 

 the principles of those very theories which I am verbally contend- 

 ing with, that such contortions, although best observed in the 

 slaty rocks, are not peculiar to them ; that they are not character- 

 istic of the transition-series of the Wernerians, but that they may 

 be seen, by those who choose to see them, in gneiss, in mica-slate, 

 in clay-slate, grauwacke, limestone, basalt, sandstone, and shale — 

 nay, even in the newest of the new rocks, as at Purbeck in Dor- 

 setshire, in the freestone ; in the chalk at Handfast Point in the 

 Isle of Wight ; in that of Montmartre at Paris ; in the pulverulent 



