ON A CALIFORNIA ROOFING SLATE OF IGNEOUS 



ORIGIN.^ 



During the field season of 1903 the writer, was enabled to 

 spend several days in the study of the important roofing-slate 

 deposits occurring north of Placerville, El Dorado county, Cali- 

 fornia. A summary of the principal economic results of this 

 investigation will soon appear in a bulletin^ of the United States 

 Geological Survey; while a more detailed description, with maps, 

 will probably be issued later as one chapter in a Survey bulletin 

 on the slate deposits of the United States. 



One result of the study, however, would seem to be of suf- 

 ficient novelty and general geologic interest to be worthy of 

 discussion in this Journal, in considerable detail. This is the 

 determination that a part of the roofing slates of the El Dorado 

 county district have been derived, by dynamic metamorphism, 

 from basic igneous rocks — gabbros or related types, 



THE CALIFORNIA SLATE DEPOSITS IN GENERAL. 



Location and general relations. — Though roofing slate has at 

 different times been quarried, on a small scale, in other parts of 

 the state, the only important slate-producing area in California 

 is located in El Dorado county. The quarries which have been 

 opened in this district are located along a line running about 

 N. 15° W. from Placerville, at distances of from one to six miles 

 from that town. The location and general relations, both geo- 

 graphical and geological, of the slate deposits and quarries, can 

 best be understood by reference to the maps included in the 

 "Placerville Folio" of the United States Geological Survey. 

 The workable roofing-slate deposits of this district occur in a 

 belt of the Mariposa slates, of late Jurassic or early Cretaceous 

 age. The quarries which have been opened are all situated near 

 the western boundary of this belt of Mariposa slates, where it is 



^ Published by permission of the director of the U. S. Geological Survey. 

 ^ Contributions to Ecojiomic Geology, 1903. 



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