338 ON THE APPARENT DISLOCATIONS [Ch. XV 



any of our theories, it is impossible at present to foresee."* 

 The same indefatigable geologist has furnished us with 

 another anomaly in the exhibition of curvatures on the surface 

 of an artificial cliff in the slate, at the back of the gun-wharf, 

 Plymouth ; these curvatures are formed by a number of dove- 

 coloured stripes of unequal thickness, which traverse a faint 

 brown-red ground in very irregular curved lines, bearing a 

 sort of parallelism to each other ; in short, resembling strongly 

 a piece of marbled paper. " The continuity of the lines of 

 colour precludes all possibility of a succession of deposited 

 layers otherwise than in those very lines, and affords at the 

 same time a proof, if any were wanting, that the fissile pro- 

 perty of the slate has not been the result of stratification."! 

 And appearances of the same nature are very common on the 

 surface of the laminae of several kinds of Cornish slates, and 

 more particularly of those which belong to the intermediate 

 or transition series, and which abound in veins and deposits 

 of manganese ore. 



Some varieties of shorl-rock (both in the granite and in the 

 slate), of actynolite-rock, and of other unnamed rocks belong- 

 ing to the slate group of Cornwall, abound in contorted 

 appearances which somewhat resemble the preceding, but are 

 not perfectly analogous, depending on the manner in which 

 their constituent minerals are respectively combined : thus, in 

 the shorl-rock, the quartz and shorl intimately united, ex- 

 hibiting various shades of black and dark blue, alternate with 

 portions in which the shorl is wanting, and consequently con- 

 sist only of white quartz ; and these differently coloured parts 

 present, throughout the whole rock, and in every direction, 

 the most intricate contortions, perfectly resembling those so 

 common in mica-slate. These rocks are generally compact 

 or massive, and their cleavage may be effected with equal 

 facility either across or in the direction of the curves : but 



* System of Geology, vol. i. p. 11 9. 

 f Geol. Trans., vol. iv. p. 399. 



