i;8 



University of California. 



LVol. r. 



confirms the conclusion reached by the study of its structure and 

 mineralogical composition, that it is a hypersthene diabase. On the 

 other hand, the analysis of the hornblendic facies is that of a more 

 basic rock than its mineralogical composition would lead us to 

 expect. In view of this fact, and of the close agreement between 

 the silica percentages of the two facies, we are again led to the con- 

 clusion already reached from field and microscopic study that the 

 two rocks are of similar origin, the hornblende rock being derived 

 from the hypersthene diabase by alteration of pyroxene to horn- 

 blende. Hence the hornblendic rocks may be regarded as belong- 

 ing to Gumbel's class of epidiorites. As will be seen by reference 

 to the table, the analyses of the hornblendic rock of the Potrero, 

 No. I, shows a remarkable similarity to that of a typical epidiorite 

 as given by Rosenbusch, seen in No. IV. 



It is interesting to note in this place that a very similar asso- 

 ciation has been described by Mr. Turner* as occurring at Mt. 

 Diablo. He finds there an area of typical diabase which presents 

 undoubted transitions to a well-defined diorite through paramor- 

 phism of the pyroxenic constituent, the two forms having essentially 

 the same chemical composition. The mineralogical composition 

 and alteration of the rocks from the two localities agree closely, 

 and as seen in the analysis quoted above they are closely related in 

 chemical composition. The chief difference noted in the two areas 

 is in the mode of occurrence, the one being a massive outburst, the 

 other a narrow dyke. The hypersthene diabase and epidiorite here 

 described also find a close parallel in the hypersthene gabbro and 

 gabbro-diorite described by Professor Williams f from the vicinity 

 of Baltimore. Slides of typical specimens from that locality were 

 compared with the Potrero rocks and were found to agree closely, 

 particularly with regard to the appearance of the green 'secondary 

 hornblende. The chemical composition is likewise in close agree- 

 ment in the two cases. 



Relations to Other Formations. — The hypersthene diabase has 

 already been shown to be intrusive into the serpentine, occupying 

 one or more dykes which have been subsequently sheared apart. 



*Loc. cit., p. 387. 



tBull. U. S. Geol. Survey, No. 28, 18S6. 



