442 



GEOLOGY. 



may be readily seen where the particles are large (Fig. 362). As a result 

 of the orientation and flattening of their particles, rocks so affected spHt 



Fig. 364. — Slaty structure and its relation to bedding planes. Two miles south of 

 Walland, Tenn. (Keith, U. S. Geol. Surv.) 



more readily between the elongate and flattened particles than across 

 them. In other words, the rocks cleave along planes normal to the direc- 

 tion of compression, and break with difficulty and with rough fracture 

 across the planes of cleavage. The condition thus induced is known as 

 slaty structure (Fig. 363), and is best illustrated by roofing-slate, which 

 was orignally a mud, later a shale, and finally assumed the slaty con- 

 dition under strong compression. Sometimes the original bedding may 

 stiU be seen running across the induced cleavage planes (Fig. 364). As 

 the original mud beds were horizontal or nearly so, and as the thrust is 



