BASALT AND KHYOL1TE. 285 



to the bottom if their position in the molten mass was mainly a question of 

 specific gravity. The writer can not but regard the lavas as derived from a 

 local reservoir, all the ejected material having had a common source in 

 some primordial magma. The order of succession is governed by far- 

 reaching physical forces which may vary greatly in different volcanic areas, 

 dependent on conditions of heat and pressure. A powerful orographic 

 movement such as frequently happens during a period of volcanic action 

 may be sufficient to affect the entire geological conditions in any eruptive 

 center. In widely separated parts of the world the extravasated products 

 are singularly alike, yet the sequence of lavas within restricted limits show 

 very considerable variation. 



Supposing the products of eruption and order of succession to have 

 been much the same over the geological province of the Great Basin, it 

 does not follow that the same succession of events took place in another 

 region where the geological conditions were obviously different. Within the 

 observations of the writer instances are known outside the Great Basin 

 where such an order of events not only did not take place, but where the 

 mutual relations of nearly identical lavas exhibit a succession strikingly ar 

 variance with the sequence of flow as found at Eureka. The Yellowstone 

 Park may be cited as an instance where the succession of lavas is some- 

 what different. In the latter locality the earliest eruptions were of inter- 

 mediate composition, consisting of hornblende-andesite and homblende- 

 mica-andesite. While the sequence of lavas may vary owing to geological 

 conditions, the laws governing the differentiation of lava hold good 

 everywhere. 



Basalt and Rhyoiite. The writer accepts, with some important modifica- 

 tions, the views of Mr. Clarence King regarding rhyolite and basalt, not 

 only as geologically closely related rocks, but also as extreme members of 

 the same primordial magma. He differs from Mr. King as to the manner 

 in which these extreme products were derived from an earlier molten mass. 

 It is nothing against this view of their common origin that rhyolitic out- 

 bursts frequently occur unaccompanied by basalt, or that basaltic exposures 

 abound without any evidences of the presence of acid lavas. Both rocks 

 break out in the closest proximity and not infrequently through the same 



