calkins.] Petrography of the John Day Basin. 



111 



A few words may be said here concerning- the plan and scope 

 of the present paper. It is divided into three parts. The first 

 contains a brief sketch of the geological features of the region. 

 Although these features have been treated of at greater length 

 by Dr. Merriam; it is believed that a summary account will be 

 useful as a preface to the petrographical descriptions. These, 

 which form the bulk of the paper, are contained in the second 

 part. In conclusion, some discussion of a few questions of 

 broader geological interest is attempted, in the light of the facts 

 gained by petrographical study. 



The writer's acknowledgements are due to Dr. Merriam for 

 the collection of a large part of the rock specimens, and for 

 much information in regard to their field occurrence. To Professor 

 Andrew C. Lawson he is obliged for much aid and helpful sug- 

 gestion in the laboratory investigation. 



THE GEOLOGICAL SECTION. 



GEOLOGICAL FEATURES OF THE REGION. 



Geographical Limits. — The region with which this paper deals 

 is situated in north central Oregon, well to the east of the Cascade 

 Range. It includes all of the area draining into the John Day 

 River above Clarno's Ferry, with the exception of that portion 

 . drained by the South Fork. The area thus defined is approxi- 

 mately a right-angled triangle. The westernmost and sharpest 

 angle lies near Antelope, the right angle is about twenty miles 

 to the northeast of Ritter, and the southernmost point a few 

 miles east of Canon City. 



The elevations inclosing this drainage basin have apparently 

 a certain geological unity, and collectively are designated as the 

 Blue Mountain Range. They appear to have a foundation of 

 pre-Tertiary rocks, which in the western ridges are largely 

 mantled over by Eocene and Miocene volcauics. To quote 

 Dr. Merriam's general statement, "The rugged eastern ridges, 

 rising to an elevation of over 6000 feet, are composed largely 

 of pre-Tertiary formations which have been much disturbed. 

 To the west the ranges are made up principally of Tertiary lavas 

 forming regular and frequently flat-topped ridges somewhat 



