Calkins.] 



Petrography of the John Bay Basin. 



169 



Analysis of Recent Ash, John Day Basin. 



Si0 2 



66.64 



AI2O3 



13.93 



Fe-iO:i 



1 ) .95 



FeO 



1.46 



MgO 



1.14 



CaO 



2.61 



Na 2 



5.66 



K 2 



2.64 



H a O at 110° 



1.19 



H 2 above 110° 



3.81 



Ti0 2 



0.18 



P2O5 



0.12 



MnO 



not det. 



Total 



100.33 



The analysis has some curiously anomalous features. The 

 ratio of the alkalies is such as commonly obtains in andesites, 

 but the percentage of alkalies is too high, and lime, magnesia 

 and iron too low to agree with any typical andesite. It is sug- 

 gested that this ash may have come from an andesitic magma, as 

 the crystals would indicate, and that it contains a more than 

 normal proportion of the light pumice grains, the heavier ferro- 

 magnesian minerals having mostly fallen closer to the source of 

 eruption. 



CONCLUSION. 



GENERAL PETROLOGY. 



The John Day region, considered as a petrographical pro- 

 vince or a part of one, is characterized to a certain extent by the 

 fact that its rocks are all derived from what Rosenbusch calls the 

 gabbro-peridotite and granito-diorite magmas. Rocks allied to 

 nepheline syenite are quite unrepresented. The preponderance 

 of soda molecules over potash in all the rocks analyzed is also a 

 significant fact, and the recurrence of anorthoclase-bearing 

 rhyolites of similar type in Eocene. Miocene and Pliocene times 

 is significant of a certain persistency of petrographical character 

 of the region. 



Any comprehensive study of a great series of volcanic rocks 

 should include some attempt to discover whether the succession 

 of chemical types obeyed any definite law. With this end in 



