DISTRIBUTION OF LAVAS. 243 



uumy thin sections. Ilypcrstliene is waiitini;' in the nonujil basalts, and it" 

 present is only recooniizt'd in the intermediate nx-ks Ix-tween t\])ical |)\r<(.\- 

 ene-andesite and basalts. An exeeptionall}' fine disjtlav r.t" these intermedi- 

 ate rocks makes this group of special geological importance at Kureka. A 

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

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



Manner of Occurrence of Volcanic Lavas.— In the Ktircka 1 )istl'ict there are llo 



grand craters througli which the greater part of tlie lavas reached the surface 

 and from which volcanic energy receding from centers of igneous action 

 gradually decreased in intensity and finall\- died fmt altogether. On the 

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

 luiA-e poured out through lunnerous vents scattered over the volcanic area, 

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

 seem impossil^le to recognize any order in their distril)ution, so irregularly 

 do they api)ear to break out in most iniexpeeted places. Further observa- 

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

 existing orograj)hic structure a knowdedge of which is al)S(>luteh' 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, thev break out along the three great 

 meridional and apj)roxiniateU- parallel lines of displacement, the Hoosac, 

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

 encircle the large u})lifted masses of sedimentary strata, like the Silverado and 

 County Peak l)lock; third, they occur in lunnerous narrow dikes penetrat- 

 ing the limestones, l)ut for the most i)art confined to Prospect Ridge, and, 

 fourtli, the^■ occiu- in one or two relatively large bodies, notably Richmond 

 ^lountain and Pinto Peak, along lines of displacement already mentioned. 

 Richmond ^lountain is situated at the junction of the Pinto and Rescue 

 faults, wliilc tlir lava of Pinto Peak has lieen ])iled up along an ol)li(pic 

 fault, which nms from the Iloosac to the Pinto fault, and which se})arates 

 Spring Hill from ("arlion Ridge. It is along the lities of these latter 

 faults tluit tile most powerful volcanic acti\il\ lias been disphn'ed. As 

 descril)ed elsewliere, these two faults are situated respectively on the east 

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



