PIFFEKENTIATION OF LAVAS. 289 



This liypcitlit'sisot' the progressive (htf'ereiitiatioii 1)V iiioh-euhir chauj^es 

 in a fluid or a inolteu mass under varying- couditious dt' teniperatui-e and 

 pressure is offered to explain the variations in chemical and mineralog-ical 

 composition of lavas. It is offered tentatively and with much hesitation, the 

 writer knowing- the many difliculties involved in the i)r()l)lem. It is ])ased 

 upon and is in accord with the facts seen at Phu-eka and coniirmed by 

 observations in many volcanic areas elsewhere. It at least has the merit of 

 accounting for nearly all variations in the sequence of lavas \\hi(h have 

 from time to time been noted in the Great Basin. It offers a rational 

 explanation for the recurrence of lavas in certain localities and accounts for 

 their absence in others. The pyroxene-andesite furnishes a marked instance 

 of such a recurrence. Occurrences of lava which have been regarded as 

 exceptional and difficult to explain by any general law are now seen to 

 fall within the prescribed limits of variation as laid down here. Nothing 

 seems more clear than that there are certain laws determining the sequence 

 of flow that govern the extravasation of lavas in every great volcanic cen- 

 ter, notwithstanding the fact that we ma}' still be a long ^vay from the cor- 

 rect interpretation in all its details. 



Summary.— The Eureka District presents a most in.structive volcanic 

 region standing quite apart from all other centers of similar eruption, yet 

 typical in the nature of its extravasated material of many localities in the 

 Great Basin. 



The region offers no direct proof of the age of volcanic energy, yet 

 all evidence points to the conclusion that the eruptions belong to the Ter- 

 tiary period and for the most part to the Pliocene epoch. They may have 

 extended well on into Quaternary time, although there is no reason to sup- 

 pose that eruptions took place within historic periods. 



As regards their mode of occurrence the principal eru])tious may he 

 classed under four heads: First, they broke out through profound fissures 

 along the three great meridional lines of displacement, the Hoosac, Pinto, 

 and Rescue faults, and to some extent along the lesser parallel faults; 

 second, following the lines of orographic fractm-e, they border and almost 

 completely encircle the large uplifted masses of sedimentary strata like the 

 HON XX 1!» 



