ROOFING SLATE OF IGNEOUS ORIGIN 23 



1. The structural relations of the green and the black slates 

 seem to prove that the green slates are derived, by dynamic 

 metamorphism, from an igneous rock; further, that this rock 

 was an intrusive massive rock, not an interbedded tuff; and that 

 it was intruded into the Mariposa slates at some period subse- 

 quent to their deposition, but before their assumption of slaty 

 cleavage. 



2. Microscopic evidence, though inconclusive owing to the 

 lack of a sufificient supply of material, proves that the green 

 slates are composed of thoroughly crystalline material. The 

 rock forming one of the nearby dikes is shown to be a gabbro. 



3. Chemical analyses of the green slates show that they are 

 widely different in composition from the black slates, and, 

 indeed, from any normal clay slate. Comparison of the same 

 analyses with those of igneous rocks of the region show striking 

 similarities in composition between the green slates and certain 

 massive, basic, igneous rocks — gabbros and allied rocks. 



There are, of course, no reasons why an igneous rock should 

 not be susceptible of change, under proper conditions, into a 

 roofing slate; and the possibility of such a change occurring 

 would probably have been conceded by most geologists, had the 

 question been brought to their attention, before the foregoing 

 description of an actual occurrence had been published. 'Not- 

 withstanding these facts, the California occurrence seems to be 

 unique. The extensive literature of roofing slate has been 

 examined by the writer, so far as this literature is available, and 

 no similar occurrences of the derivation of roofing slates from 

 massive igneous rocks have been noted. More than this, the 

 possibility of such an occurrence would seem to have been over- 

 looked by most writers, who either expressly or by implication 

 use the term "roofing slate" to include only argillaceous sedi- 

 mentary rocks. This oversight is the more inexcusable, because 

 a large industry has been based for a century or more, in one 

 English district, on roofing slates derived from tuffs. 



Utilizatio7i of the green slate. — While the green slate does not 

 occur in a body of sufficient thickness to be worth quarrying for 

 itself alone, it is a resource of considerable value as it is at 



