250 GEOLOGY OF THE EUREKA DISTRICT. 



base until it becomes a characteristic audesitic-pearlite. As the andesites 

 and pearlites become more and more acidic the rock gradually passes over 

 into dacite, the eruptions of which usually occur in ol^scure liills and low 

 ridges, and although covering comparatively restricted areas are clearly 

 seen to overlie the hornblende-mica-andesite in all the local centers of 

 eruption wherever the two rocks are observed together. In the neighbor- 

 hood of South Hill, where the largest exposures of dacite have been 

 observed, they rest superimposed against the andesite, and at Dry Laki , 

 where, however, only a small body of dacite is known, it is evident that 

 a similar sequence of flow was maintained. 



In low hills near the entrance to Sierra Canyon northeast of South 

 Hill instances may be seen of finely banded rhyolite lying in direct super- 

 position upon good exposures of dacite. This dacite, though a moderately 

 conapact rock, possesses in places a pumiceous texture and in a marked 

 degree strongly resembles many forms of rhyolite, but especially the variety 

 with which it is here associated. Both rocks are highly acidic, but the 

 dacite is richer of the two rocks in mineral secretions and is character- 

 ized by a great abundance of laminae of biotite. In the few areas where 

 both rocks occur together in such a way that their relations can be made 

 out, the rhyolite has been the last to reach the surface. 



The district affords abundant and frequent evidence of the relative 

 geological position of andesite to rhyolite. Not only is this shown by 

 the relationship between the rhyolites and dacites, but over much more 

 extended areas the rhyolite encircles and overlies the andesite, filling in and 

 smoothing out the accidented sui-face of the older rock, which in timi may 

 occasionally be seen in isolated exposures rising above a broad expanse of 

 superimposed rhyolite. Further and conclusive evidence is found in the fre- 

 quent dikes of rhyolite penetrating the hornblende-mica-andesite in several 

 places adjoining the Hoosac fissure. The rhyolitic pumices, tufis and allied 

 rocks appear in many instances to have preceded the more highly crystalline 

 compact rhyolites represented by the typical Rescue Canyon and Pinto 

 Peak rocks. 



While it is by no means evident that all the overflows of pumice 

 broke out before the denser rock, yet there is ample proof that long and 



