90 



LITHOLOGICAL GEOLOGY. 



Fig. 91. 



Fig. 90. 



93, a quartzose sandstone which has irregular cleavage-lines. These last two cases, 



together with that represented in Fig. 88 A, show that the jointed structure and 



cleavage-structure have the same origin. 



Fig. 92. 



Fig. 93. 



Sedgwick first detected the true lines of bedding, and ascertained that the slaty 

 structure was one that had been superinduced upon the clayey strata by some process 

 carried on since they were first deposited. 



Foliation. — The foliated structure (or foliation) of mica schist, gneiss, and related 

 schistose rocks may sometimes be transverse to the bedding, like most slaty cleavage. 

 But it is not generally so. 



(3.) Markings -which result from movements of rocks. — Grooving 



and planing ', and often polishing, of rock surfaces occur in the walls of 

 fractures (as those of veins, for veins occupy fractures), which have 

 resulted from the friction of one wall against the other, usually pro- 

 duced when the fracture was made ; and sometimes on the surfaces of 



Fig. 93 A. 



Drift groovings or scratches. 



layers, from a sliding of one over another through some subterranean 

 movement. They also occur on exposed surfaces of rocks, where they 



