niSTOKY OF VOLCANIC ACTION. 271 



When it ciinies to the pyroxenic magma it is fouiKl to l)reak out aiul 

 follow the sinuous lines of fracture ))reviously followed by rhyolitic lavas. 

 In some instances they present the appearance of actuallv em])loyin<>- the 

 identical conduits used by the feklspathic magma. In this way the ihy- 

 olite Inlays a most important part, not only as a connecting link between 

 the feldspathic and pyroxenic magmas in respect to sequence of flow, but 

 still more as regards geological distribution and mode of occiuTence. Too 

 nnich stress can not be laid upon the fact already mentioned, that the rhy- 

 olite-s were the last to break out along the vents occupied by the hornblende- 

 andesite and the first to reach the surface along the same lines of fracture 

 which were afterward used by the basalts of the pyroxenic magma. That 

 these basic lavas may have occasionally forced open new vents for them- 

 selves is quite possible, but the greater number of outbursts followed the 

 same grand fractures as the earlier higldy acidic magmas which Ijorder the 

 elevated orographic block of Silverado and County Peak. Richmond 

 ilountain, as already pointed out, may have reached the surface through a 

 separate and wholly indej)end('nt vent, but it is so vast and its overflows 

 cover so large an area that it is iin|)()ssible to determine the position of its ' 

 vent or vents and their precise relation to the earlier rhvolite. It must l)e 

 borne in mind, however, that it breaks out at the junction of two grand 

 lines of faulting, coming up from the south on opposite sides of a great 

 uplifted mountain mass. The earliest flows of the pyroxenic magma 

 resembled those of the feldspathic magma, in so far as they carry the same 

 ferro-magnesian silicates as porphyritic secretions. On the other hand, they 

 are sharply contrasted by an andesitic habitus oi' the groundmass, ^\hicli, 

 however, had been slightly foreshadowed by a groundmass carrying pyrox- 

 cut' niicrolites, shown in the basic pearlite fi'om the south end of Carbon 

 Ridgf, where the rock occurs as the earliest eruption at that localit\', fol- 

 lowed l)y a series of feldspathic lavas, closing with rhyolite. 



Following the great body of pyroxene andesite came lavas intermedi- 

 ;ire in composition l)etween them and basalt, breaking through and over- 

 lying the less basic varieties. Some of these are allied to the eaidier flows, 

 while others show a decided tendency to transition into basalt. Most of 

 them are related geologically either with tlie later basaltic eruptions or 



