604 



SLATE EOCK OF NORTH DEVON. [Ch. XXXVL 



Fig. 708 



and the same bed exhibits cleavage- 

 planes in the direction of the great- 

 est movement, although they are 

 much fewer than in the slaty strata 

 above and below. 



Above the sandy bed d /, the 

 stratum c is somewhat disturbed, 

 while the next bed b is much less 

 so, and a not at all ; yet all these 

 beds, r, 6, and «, must have under- 

 gone an equal amount of pressure 

 with c?, the points a and g having 

 approximated as much towards 

 each other as have d and /. The 

 same phenomena are also repeated 

 in the beds below c?, and might 

 have been shown, had the section 

 been extending downwards. Hence 

 it appears that the finer beds have 

 been squeezed into a fourth of the 

 space they previously occupied, 

 partly by condensation, or the closer 

 packing of their ultimate particles 

 (which has given rise to the great 

 specific gravity of such slates), and 

 partly by elongation in the line of 



the dip of the cleavao-e, of which Vertical section of slate rock in the cliffs 

 ^ , . . ^ near Ilfracombe, North Devon. 



the general direction is perpendicu- 

 lar to that of the pressure. " These 

 and numerous other cases in North 

 Devon are analogous," says Mr. 

 Sorby, " to what would occur if a 

 strip of paper were included in a 

 mass of some soft plastic material which would readily change its di- 

 mensions. If the whole were then compressed in the direction of the 

 length of the strip of paper, it would be bent and puckered up 

 into contortions, whilst the plastic material would readily change its 

 <;Umensions without undergoing such contortions; and the difference 

 in distance of the ends of the paper, as measured in a direct line or 

 along it, would indicate the change in the dimensions of the plastic 

 material." 



The student will readily conceive that, when the shape of a fossil or 

 of a crystal of some mineral, or of a spheroidal concretion, has been 

 altered by lateral pressure, the new forms which they assume respect- 

 ively v>'ill vary according to whether they have yielded in one or more 

 directions. They may have been drawn out solely in the direction of the 



(Drawn by H. C. Sorby.) 



Scale one inch to one foot. 



a, h. c, p. Finc-<rralned slates, the stratifica- 

 tion beinj^ shown partly by lighter or dark- 

 er colors, and partly by different degrees of 

 fineness in the grain. 



d^f. A coarser-grained light-colored sandy 

 slate Avifi. less perfect cleavage. 



