
Part I. Szor.iv.§3.] CLEAVAGE. — 311 
the strata, at first in even parallel beds, have been subjected to great 
compression from the directions (4) and (B), in consequence of which 
they have been thrown into folds, while their minute particles have 
been forced to rearrange themselves perpendicularly to the pressure. 
Hence the rocks are both crumpled and cleaved. The fineness of the 
cleavage depends in large measure upon the texture of the original 
rock. Sandstones, consisting as they do of rounded obdurate quartz- 
erains, take either a very rude cleavage or none at all. Fine-grained 
argillaceous rocks, consisting of minute particles or flakes, that can 
adjust their long axes in a new direction, are those in which the 
structure is best developed. In a series of cleaved rocks, therefore, 
cleavage may be perfect in argillaceous beds (0 b, Figs. 75 and 76), 

Fig. 75. Fie. 76. 
DEPENDENCE OF CLEAVAGE UPON THE GRAIN OF THE Rock (B.), 
and imperfect or absent in interstratified beds of sandstone (a a, Fig. 
75) or of limestone (as at Clonea Castle, Waterford, a a, Fig. 76). 
That cleavage has really been produced in this mechanical way by 
lateral pressure has been proved experimentally by Sorby, who effected 
perfect cleavage in pipeclay through which scales of oxide of iron 
had previously been mixed." ‘Tyndall superinduced cleavage on 
bees-wax and other substances by subjecting them to severe pressure. 
Cleavage among rocks occurs on a great scale in countries where the 
strata have been much plicated, that is, where they now occupy much 
less horizontal surface than they once did, having been-subjected to 
powerful lateral pressure, in accommodating themselves to their 
diminished area. The structure of districts with cleaved rocks is 
described in Book IY. Part V. 
(3.) Deformation.—Further evidence of the compression to which 
rocks have been subjected is furnished by the way in which con- 
tiguous pebbles in a conglomerate may be found to have been 
squeezed into each other, and even sometimes to have been elongated 
in a certain general direction. It is doubtless the coarseness of the 
grain of such rocks which permits the effects of compression to be 
so readily seen. Similar effects may take place in fine-grained 
1 din. New Phil. Journ. ly. (1853), p. 1387. The student wili iind recent interesting 
additions to our knowledge.of the microscopic structure and the history of cleaved rocks 
in Mr. Sorby’s address, Q. J. Geol, Soc. xxxvi. p. 72. 
