General Remarks. 



Mount Diablo is an isolated peak of the Coast ranges of California, lying 

 about 27 miles east-by-north of San Francisco. Although less than 4,000 

 feet high, it forms a prominent feature in the landscape, rising as it does 

 from sea level, and is visible from all the main lines of railway leading to 

 San Francisco. Owing to its isolation, the view from the summit is unusu- 

 ally fine, and it is frequently ascended by excursion parties. 



Being easily accessible, it was selected by Professor Whitney as a field for 

 detailed geological work when he first undertook the study of the Coast 

 ranges. He there first obtained evidences of the Cretaceous age of the bulk 

 of the metamorphic rocks of the range, having observed there the unaltered 

 shales containing Aucella mosquensis, Von Buch, passing into silicified shales, 

 or phthanites, about the head-w'aters of Bagley creek.^ Aucella mosquensis f 

 is a fossil common in and characteristic of the Knoxville (Neocomian) beds. 

 These silicified shales are so intermingled with the other metamorphic rocks 

 as to leave no doubt that all of them are of the same age. 



In bulletin number 19 of the United States Geological Survey, Mr. G. F. 

 Becker has further shown that the rocks of the lowest member of the Creta- 

 ceous (the Neocomian) only have been highly silicified, serpentinized, and 

 otherwise altered. 



Through the kindness of Mr. Becker I have been allowed to spend some 

 months in geological field work at Mount Diablo. The work resulted in a 

 geological map covering about 110 square miles, and in a collection of about 

 340 rocks. A reduced copy of this map forms the accompanying plate 15. 



Dr. W. H. Melville, of the chemical division of the Geological Survey, 

 was with me for a time at the mountain, and has made some interesting 

 chemical studies, including quantitative analyses of several of the rocks. 

 The results of his work accompany the present paper as a supplement. 



Mr. Waldemar Lindgren was the first to detect the probably eruptive 

 nature of part of the serpentine (peridotite and pyroxenite) at Mount 

 Diablo, having found remains of olivines in some of it in Bagley canon. 



The Metamorphic Area. 



The area of metamorphic rocks comprises the central mass of the mountain, 

 including the main and northern peaks. It is only about ten square miles 

 in extent ; yet within it are found all or nearly all of the Neocomian meta- 



* Geology of California, vol. 1, 1865, p. 23. 



f The name used by Professor Whitney is Aucella piochii, Gabb (Paleontology of California, vol. 

 II, 1809, p. 194). This was later shown by Dr. White to be a synonym. See his paper on the genus 

 Aucella in Becker's " Quicksilver Deposits," 1888, p. 226. 



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