HISTORY OF VOLCANIC ACTION. 269 



out gradually underwent changes in mineral composition offering a great 

 variety of volcanic products of which the relative age and order of succes- 

 sion of typical lava flows have been clearly established. It has also been 

 demonstrated that throughout this entire series of lavas the range in silica 

 amounts to about 25 per cent, a range which is quite as wide as is ordinarily 

 found in most centers of eruption, even where the volume of lavas thrown 

 out has been vastly greater and the duration of volcanic energy far longer. 

 The succession of events throughout the volcanic period presents a con- 

 tinuous chapter of geological history complete in itself with the rise, cul- 

 mination and dying out of eruptive energy. So far as ultimate chemical 

 composition of both acid and basic rocks is concerned it furnishes a com- 

 plete cycle of volcanic products. 



Probably the feldspathic and pyroxenic lavas do not approach each 

 other in their tenure of silica within 2 - 25 per cent, at least no body of rock 

 or lava stream is known which indicates a closer coming together of the 

 two magmas. In chemical composition and mineral development the earli- 

 est eruptions of both magmas resemble each other closest, but from this 

 common ground they differentiate steadily until the feldspathic lavas reach 

 the extreme acidic and the pyroxenic the extreme basic end of their respec- 

 ive series. The former and earlier magma exhibits in the overflows a con- 

 stantly increasing acidity through a range of 11 per cent of silica, and the 

 latter an increasing basicity with a falling away in silica of 13 per cent, the 

 point of separation of the two magmas being nearly midway between the 

 extremes in composition. 



Exceptional lavas in other localities may carry somewhat more silica 

 than those thrown out at Eureka, but it is doubtful if flows of any consid- 

 erable size exceed those of Rescue Canyon in acidity by more than 2 per 

 cent unless accompanied by secondary alterations or infiltration products. 

 Obsidians are reported as carrying 78 per cent of silica, but for the most 

 part these highly acidic glasses fall within the limits assigned to normal 

 rhyolites. Basalts somewhat richer in oliviue and magnetic iron are by 

 no means uncommon elsewhere, but these extreme basic varieties have not 

 as yet been recognized within the Great Basin. Not only as regards the 

 range in silica, but for all other essential elements entering into the original 



