NOTES ON 



THE CHEMISTRY OF THE MOUNT DIABLO ROCKS. 



BY W. H. MELVILLE. 



{Presented before the Society Deceynber 31, 1890, as a Supplement to the Me?noir on the 

 Geology of Mount Diablo, by H. W. Turner.) 



Introductory Note. — In the following notes, taken during the study of the 

 altered and unaltered rocks in the Cretaceous series of Mount Diablo, the 

 object was to ascertain the chemical changes by which certain of these rocks 

 were brought into their present highly metamorphic condition, and to ascer- 

 tain also to what chemical agencies the changes could be ascribed. To attain 

 this end, analyses of a number of rocks, both altered and unaltered, and in 

 some cases forming transitions, are compared. 



The varieties of rocks which occur in the Mount Diablo district and of 

 which types were analyzed are the following : Sandstones, shales, phthanite,' 

 glaucophane schists, diabase, pyroxenite, gabbro, and serpentines. The 

 sandstones are grouped by themselves, while those specimens which illustrate 

 transitions are arranged in series with appropriate descriptions, and with 

 discussions of chemical composition. 



Series I. — On the banks of the creek in Bagley canon, and a few feet from 

 the Aiicella-hesiYmg (calcareous) shale, a series of apparent transitions of 

 shale into gabbro was found. Here sulphur waters ooze from the bank, and 

 the odor of sulphuretted hydrogen is well marked. These waters have per- 

 meated the rocks, and the specimens were still wet when taken out. The 

 sulphur was determined as sulphuric acid, the form in which it existed after 

 exposure to the air and drying. 



a. Shale, which exhibits a resinous luster, whereas in almost all shales in 

 the metamorphic area a dull appearance is the rule ; the color is brownish- 

 black. 



b. Shaly gabbro somewhat resembling serpentine; it is friable and con- 

 tains carbonates and sulphates in some quantity. Apart from its being 

 more friable than a as well as its light-green color, it bears considerable re- 

 semblance to that in luster and structure. It represents the exact contact 

 between a and c, which follows. 



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