DISTEIBDTION OF LAVAS. 243 



many thin sections. Hypersthene is wanting in the normal basalts, and if 

 present is only recognized in the intermediate rocks between typical pyrox- 

 ene-andesite and basalts. An exceptionally fine display of these intermedi- 

 ate rocks makes this group of special geological importance at Eui-eka. A 

 discussion of these transition rocks is reserved till later in the chapter, 

 when treating of the relations of the different groups to each other. 



Manner of Occurrence of Volcanic Lavas. In the Eureka District there are 11O 



grand craters through which the greater part of the lavas reached the surface 

 and from which volcanic energy receding from centers of igneous action 

 gradually decreased in intensity and finally died out altogether. On the 

 contrary, the igneous rocks consist for the most part of extrusive lavas that 

 have poured out through numerous vents scattered over the volcanic area, 

 many of the outbursts being very limited in extent. At first sight it might 

 seem impossible to recognize any order in their distribution, so irregularly 

 do they appear to break out in most unexpected places. Further observa- 

 tion, however, shows how dependent these outbursts are upon the pre- 

 existing orographic structure a knowledge of which is absolutely necessary 

 to a thorough understanding of the volcanic phenomena. 



As regards their mode of occurrence, all the lavas may be classed under 

 four heads: first, and most important, they break out along the three great 

 meridional and approximately parallel lines of displacement, the Hoosac, 

 Pinto, and Rescue faults; second, they border and almost completely 

 encircle the large uplifted masses of sedimentary strata, like the Silverado and 

 County Peak block; third, they occur in numerous narrow dikes penetrat- 

 ing the limestones, but for the most part confined to Prospect Ridge, and, 

 fourth, they occur in one or two relatively large bodies, notably Richmond 

 Mountain and Pinto Peak, along lines of displacement already mentioned. 

 Richmond Mountain is situated at the junction of the Pinto and Rescue 

 faults, while the lava of Pinto Peak has been piled up along an oblique 

 fault, which runs from the Hoosac to the Pinto fault, and which separates 

 Spring Hill from Carbon Ridge. It is along the lines of these latter 

 faults that the most powerful volcanic activity has been displayed. As 

 described elsewhere, these two faults are situated respectively on the east 

 and west sides of the depressed block of Carboniferous rocks lying between 



