OCCURRENCE AND SUCCESSION OF ROCKS. 180 



upon their relative age. Difficulties in the deterniination of succession, 

 however, are not peculiar to the geology of massive rocks; for there are 

 many instances of the reversal of sedimentary strata, and with sufficient 

 care the order of succession of eruptives can generally be established with 

 as much certainty as can that of sedimentary rocks in greatly disturbed 



Order of succession. — Tlic ordcr lu whicli the rocks of the Washok District 

 have appeared upon the surface is as nearly as can be ascertained the fol- 

 lowing: 



Granite, 



Metamorphics, 



Granular diorites, 



Porphyritic diorites, 



Metamorphic diorites, 



Quartz-poiphyry, 



Earlier diabase. 



Later diabase ("black dike"), 



Earlier hornblende-andesite, 



Augite-andesite, 



Later hornblende-andesite, 



Basalt. 



It is possible that strata since metamorphosed may have been laid down 

 upon the diorite as well as previous to it. The evidence of the succession 

 of diabase to quartz-porphyry would be more satisfactory if the contact 

 between them were more extensive, and of the age of the basalt there is no 

 direct evidence except that it is later than earlier hornblende-andesite. 

 The other points as to succession are clearly established. One of the most 

 interesting is the occurrence of hornblende-andesite after as well as before 

 augite-andesite, proving a recurrence in the character of eruptions. It thus 

 has a direct bearing upon the general theory of the succession of volcanic 

 rocks. In the following pages some notes are presented on the occurrence 

 and distribution of each of the series. 



