VARIATIONS OF TEXTURE IN IGNEOUS ROCKS 587 



ANDESITIC ROCKS. 

 mason's BUTTE. 



Field description. — Mason's Butte lies close to Walker River, 

 in Mason Valley, about four miles due south from Wabuska. It 

 is thus situated midway between the northern end of the Walker 

 River range and the northern end of the Smith Valley and 

 Pinenut ranges. The rocks of the butte are related to those of 

 all these ranges, as will be shown later. The butte itself is 

 about a mile and a half long in a northeasterly' direction, and 

 about half a mile wide. It presents from a little distance the 

 appearance of typical volcanic rock, being distinctly thinly 

 bedded, with red and gray zones. On the western face of the 

 butte is a scarp two or three hundred feet high, and from here 

 easterly there are a series of saw teeth, caused by the unequal 

 erosion of the bands of which it is composed ; then the butte 

 sinks gradually into the plain again. The bands of igneous 

 rock dip easterly 15° at the western end of the butte, and the 

 dip increases to the east, so that on the eastern end they dip 

 30°. Here they are locally reversed and dip west, probably 

 from movement subsequent to the eruption, which movement is 

 also evidenced by shearing. The beds strike parallel with the 

 longest extension of the butte (see Fig. i). 



Fig. I. — Cross section of MaSon Butte, showing alternations of course and fine 

 textured dioritic beds. Drawn to scale. Scale, i inch=:850 feet. 



Upon examination, the rocks are found to be diorite and 

 andesite in alternating conformable layers. Fourteen different 

 layers of diorite were found in the half-mile section, with layers 

 of diorite porphyry and andesite between. From a structural 

 point of view, all these rocks grade into one another. Some- 

 times the gradation may be actually seen in a single bed ; for 

 example, a highly porphvritic andesite, which in the field was 



