54 



GEOLOGY 



or not, are more or less flattened at right angles to the direction 

 of stress. This may be readily seen where the particles are large 

 (Fig. 29). As a result of the turning (or orientation) and flattening 

 of their particles, rocks so affected split more readily between the 

 elongate and flattened particles, than across them. In other 



Fig. 30. Pre-Cambrian fossiliferous slate. Deep Creek Canyon, 16 miles 

 southeast of Townsend, Mont. (Walcott, U. S. Geol. Surv.) 



words, the rocks cleave along planes normal to the direction 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. 30), and is best illustrated by roof- 

 ing-slate, which was originally a mud, later a shale, and finally 

 assumed the slaty condition under strong compression. Some- 

 times the original bedding may still be seen running across the 



