IIISTOK'Y OF VOLCANIC ACTION. 269 



out ji^riuluallv underwent (•liau<»'es in inin(M'al conipositiou offeriug a great 

 vai-iety of volcanic j)roducts of which the reUitive a<i-(' and order of succes- 

 sion (if typical lava flows have heeu clearly established. It has also been 

 demonstrated that throuf^-liout this entire series of lavas tlie ran^-e in silica 

 amounts to al)out 2.") per cent, a ranjje which is quite as wide as is ordinarily 

 found in most centers of eruption, even wlu're 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 cha]iter of geological history complete in itself with the rise, cul- 

 minatiiin and dying out of eruptive energy. So far as idtimate chemical 

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

 plete cycle of volcanic products. 



Prol)ably the feldspathic and pyroxenic lavas do not aj)proach each 

 other in their tenure of silica within 2'25 per cent, at least nc^ body of rock 

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

 two magmas. In chemical composition and miiieral develo})ment the earli- 

 est eruj)tions of both magmas resemble each other closest, but from this 

 common ground they tlift'erentiate steadily luitil 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 1)eing nearly midwa}- 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 7<S ])er cent of silica, biit for the most 

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

 rhvolites. Basalts somewhat richer in olivine and magnetic iron are by 

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

 as yet been recognized within the Great Basin. Xot only as regards the 

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



