388 ir. W. TURNER — GEOLOGY OF MOUNT DIABLO. 



blende, irregular portions are nearly colorless, extinguishing, however, 

 simultaneously with the brown parts. 



Professor G. H. Williams^ has described compact greenish hornblende 

 from the Baltimore gabbro area, regarding it as a secondary product of the 

 diallage, it being intimately associated with fibrous hornblende of undoubt- 

 edly secondary character. In another paper f he points out that it has long 

 been recognized and experimentally proved by Mitscherlich and Berthier, G 

 Rose, and Fouque and Levy that pyroxene and hornblende are two crystal - 

 lographic forms for the same molecule, of which the former is stable at high 

 and the latter at low temperatures ; and he gives evidence of the alteration 

 of hypersthene to compact brown hornblende. 



Professor R. D. Irving concludes that the compact brown basaltic horn- 

 blende in certain hornblende-gabbro rocks of the Lake Superior region 

 results from the alteration of pyroxene. 



It may well be, therefore, that all of the hornblende of the diabase-diorite, 

 even when compact and brown in color, is the result of paramorphism from 

 pyroxene. 



All of the slides described above exhibit the same ophitic structure, and 

 the entire area is believed to have been originally diabase. Associated 

 with the diabase-diorite there is an aphanitic black rock, found in small 

 amount only. It has been collected on the southern flank of Black poin^t 

 and'on the southeastern side of Pyramid hill. While one would infer from 

 some hand specimens, showing the fine-grained rock and the diorite in con- 

 tact, that it must occur as dikes in the diorite, a study of an exposure on 

 the southeastern slope of Pyramid hill did not lead to this conclusion ; the 

 rock occurring there in small bunches and, apparently at least, grading into 

 the coarser rock. Number 286 is a specimen of this aphanitic black rock. 

 It is composed of lath-like plagioclase and flecks of fibrous green hornblende 

 imbedded in a fine ground-mass of feldspar microlites and grains of mag- 

 netite. The latter is very abundant, and doubtless gives the black color to 

 the rock. The feldspar phenocrysts are of notable size. 



On page 412 of the supplement to this paper will be found two analyses of 

 specimens from the diabase area by Dr. Melville. Number 44 is a diabase- 

 diorite, and number 43 is one of the diabases above described. It will be 

 observed that the two analyses are remarkably alike, and also that they do 

 not difl^er materially from the analyses of Mr. Becker's pseudodiabase.J It 

 may be noted that the composition of the glaucophane schist (supplement, 

 page 413) is very similar to that of the diabase. 



Peridotite and Pyroxenite. — Another large mass of crystalline rock, ap- 

 parently also eruptive, lies south of the diabase area just described. It is 



* Bulletin 28, U. S. Geol. Surv., 1886, pp. 40-42. 



t Am. Jour. Sci., 3d sen, vol. XXVllI, 1884, p. 262. 



j" Quicksilver Deposits," pp. 98, 99. 



