148 University of California. [vol. 3 



absent, but ragged patches of iddingsite and of caleite are 

 rather abundant. These rocks are probably more acid than the 

 blue tuffs, and may be classified as either trachytic or rhyolitic 

 tuffs. 



Bocks of the Upper John Bay. — These, so far as the main 

 and lower portion is concerned, are very similar to the rocks of 

 the Middle John Day. They consist of fine-grained tuffs which, 

 although they comprise a small proportion of green strata, are 

 prevailingly of a light buff or light greenish brown color. In 

 the uppermost portion, however, there are at certain localities 

 about 300 to 400 feet of sands and gravels, pointing to conditions 

 of accumulation in contrast to those that controlled the deposition 

 of the underlying portion of the John Day series. There are 

 also in certain exposures of the lower part of this division red 

 beds resembling those of the Lower John Day. A peculiar modi- 

 fication of the tuff-beds by intrusive basalt, consisting in the 

 formation of slender columns perpendicular to the walls of the 

 dykes, has been described by Dr. Merriam.* The heat of the 

 basalt flows poured out upon the uppermost beds has also 

 changed their color to brick-red by oxidation of the ferrous iron. 



The material forming the main part of the Upper John Day, 

 because of its general resemblance to the blue tuff described in 

 detail, will not require extended description. Its macroscopical 

 characters, except usually color, are essentially the same as those 

 of the rocks of the middle division. Under the microscope also 

 it shows the same ashy structure. The original constituents, in 

 order of abundance, are angular and often vesicular glass 

 fragments with a relatively small amount of mierolitic hypocrys- 

 talline rock, plagioclase usually of the andesine or labradorite 

 groups, green augite and accessory iron ore, apatite and zircon. 

 A few grains of apparently original quartz were seen in some 

 slides. The material is generally a little less decomposed than 

 the blue tuff described above, and the secondary filling of the 

 pumice vesicles has taken place to a smaller extent. The 

 secondary materials comprise a little quartz and caleite and some 

 indeterminate zeolites, but the most important is perhaps the 

 substance which imparts to the rock its usual color. This 



* Op. cit., p. 304. 



