AMERICAN SHALES AND ROOFING SLATES . 29 



the average shale. It contains slightly more of certain readily 

 soluble constituents than does the average shale. This is to be 

 accounted for by the fact that the slate is, on the whole, made 

 up from finer materials than the shale; if it were otherwise, its 

 cleavage would not be so perfect. During the change from shale 

 to slate — or from mud to slate — its composition, omitting water 

 from consideration, was practically unaltered. The only effect 

 of metamorphism was the assumption of slaty cleavage, and this 

 was effected without the introduction of any new constituent. 



Incidentally, the statement in the preceding paper, that the 

 green slate from Slatington, Calif., differs in composition from 

 any normal clay slate, is entirely justified. 



Edwin C. Eckel. 



U. S. Geological Survey, 

 Washington, D. C. 



