110 University of California. [vol.:!. 



PAGK 



Rocks of the John Day Miocene 143 



General Classification 143 



Rocks of the Lower John Day ■ 144 



Rocks of the Middle John Day 145 



Rocks of the Upper John Day 148 



Lavas of the John Day Series 151 



The Miocene Basalts 159 



Field Characters and Classification 159 



Olivine Basalts 160 



Olivine-free Basalts in the Lower Mascall 165 



Rocks of the Mascall Formation 166 



General Classification 166 



Canon City Leaf Beds 166 



Tuffs 166 



Rocks of the Rattlesnake Beds 167 



Recent Volcanic Ash . 168 ^ 



Conclusion 169 



General Petrology 169 



Conditions of Accumulation of the Tertiary Deposits 170 



INTRODUCTION. 



Iii the summer of 1899 the writer was a member of an 

 expedition led by Dr. John C. Merriam, whose primary object 

 was the collection of vertebrate fossils from the Tertiary deposits 

 of Eastern Oregon. A considerable number of petrographical 

 specimens was collected at that time. Dr. Merriam 's subse- 

 quent excursions to the region in the summers of 1900 and 1901 

 gave opportunity to add to the collection, and it may now be 

 considered fairly representative. 



The petrography of the region is, as the writer hopes to show, 

 of rather exceptional interest; yet it has been practically 

 untouched by previous writers. The only references found which 

 deal with anything but the broadly general character of the rocks 

 occur in King's report on the Fortieth Parallel Survey. This 

 author compares the John Day beds with his Truckee beds of 

 Nevada, and considers the two formations equivalent. He recog- 

 nizes the volcanic character of the material of the John Day 

 beds, and having examined them petrographieally, classifies them 

 as traehytic tuffs.* The literature bearing on the general geol- 

 ogy is summarized in Dr. John C. Merriam' s "Contribution to 

 the Geology of the John Day Basin. "f 



*Geol. and Geogr. Expl. Fortieth Parallel, Vol. I, Systematic Geology, 1870, pp. 

 ■123 and 454. 



fBiill. Dept. of Geology, Univ. of Cal., Vol. II, No. 9, 1901, p. 275 et si-<j. 



