588 J. E. SPURR 



taken for a diorite porphyry, was found to change gradually 

 into a coarsel)' crystalline diorite ; but generally the beds are 

 separate. The coarse-grained rock contains some dark inclu- 

 sions, which appear to be like the finer-grained rocks. The 

 diorite is also associated with alaskite ^ which occurs in many 

 large segregated masses irregularly distributed, and with a few 

 quartz veins, also segregational. The alaskite and quartz veins 

 are found only in the diorite ; never in the fine-grained rocks. 

 At the eastern end of the butte, also associated with the diorite, 

 are considerable masses of hornblendite which grade into normal 

 diorite. 



Although a careful search was made, no intrusive phenomena 

 showing that the andesites are intrusive into the interbedded 

 diorites, or vice versa, were found. Although the whole butte 

 is of rugged rock and entirely free from any vegetation sufificient 

 to obscure exposures, yet the succession of the different beds is 

 as normal and regular as in the case of sediments. The ande- 

 sites are evidently similar to the Tertiary andesites which occur 

 plentifully in the whole region round about. The conclusion 

 reached in the field, therefore, was that Mason's Butte repre- 

 sents the roots of old volcanic flows, that the apparent bedding 

 is a flow structure on a large scale, and that the coarse-grained 

 and fine-grained rocks, the diorites and andesites, are different 

 forms of crystallization from a single magma. Some of the 

 fine-grained rocks are separate layers from the coarse-grained 

 ones, while some are simply variations in them. The diorite 

 naturally incloses portions of an earlier formed crust, which is 

 identical with the finer-grained beds, 



Microscopic evidence. — The structure and composition of 

 typical specimens from this butte will now be described : 



Hornblende-biotite-quartz-diorite'^ [w] N.). — This rock in the 

 hand specimen is medium coarse granular and rather dark, on 



'See Am. GeoL, April, 1900, p. 230. Alaskite is proposed as a general name for 

 roclcs consisting of quartz and alkali feldspars without essential ferromagnesian con- 

 stituents. 



^ These numbers refer to the specimens in the writer's collection, and are given 

 for the purposes of subsequent identification. 



