236 GEOLOGY OF THE EUREKA DISTRICT. 



being tne prevailing rock. They form gentle spurs 200 feet in height, rest- 

 ing against the sedimentary ridges. An interesting transition rock occurs 

 here which unites the characteristic microcrystalline hornblende-mica- 

 andesite with fine examples of pearlite. At Dry Lake, while the petro- 

 graphical features are much the same, with similar transitions and variations, 

 the geological occurrence is somewhat different, the locality being quits 

 remote from the others and lying far to the west of Prospect Ridge and 

 the principal lines of north and south faulting. A suite of specimens from 

 any one of these localities could hardly be distinguished from those of 

 others either in physical features or composition. Identical transition prod- 

 ucts, both as regards degree of crystallization and mineral variation, are 

 recognized from all of them. They show the same order of succession and 

 the same sequence of geological events. The petrographical features of 

 these nearly identical series of lavas will be found described by Mr. Iddings 

 in the chapter already referred to. 



Dacite. This rock is a variety of andesite, in which secretions of quartz 

 play the part of an essential mineral, and is intimately related in mineral 

 composition and structural habit to hornblende-mica-andesite. All the 

 occurrences at Eureka are very similar in physical characteristics, possess- 

 ing a light ash-gray color, a hackly fracture, and a rough, pumice-like 

 texture which relate them closely to certain varieties of rhyolite with which 

 in the field they are frequently associated and from which they are not 

 easily separated, either in their mode of occurrence or lithological appear- 

 ance. The feldspars are nearly all plagioclase and probably largely oligo- 

 clase. Together with the larger secretions of quartz occur numerous thin 

 lamina; of biotite, in amount greatly in excess of that usually found in horn- 

 blende-andesite. Although of less frequent occurrence, occasional hyper- 

 sthene crystals, similar to those found in the andesitic pearlite, may be 

 recognized in the dacites and serve to show the dying out of ferro-mag- 

 nesian minerals in the more acidic lavas. These glassy dacites are closely 

 related to the andesitic pearlites, but occupy much smaller areas, their chief 

 interest lying in the fact that they represent transition products from the 

 andesitic to the rhyolitic lavas, sometimes associated with one, sometimes 

 with the other, and not infreauently as lava flows connecting them 



