HASALT AM) IMIVOLITK. 28") 



to the l)(>ttoin if their ))()siti()ii in the iiiolten muss was iiiainlv a question of 

 specific gravity. Tlie writer can not hut rei^anl tlie hivas as derived from a 

 local reservoir, all the (fject(^d mateiMal having had a coiumon source in 

 some primordial ma^-ma. 'I^lic order of succession is o-overued hv far- 

 reaching physical forces which may \'ar}' greatly in different volcanic areas, 

 dependent on conditions of h(>at and pressure. A powerful orographic 

 movement such as fre(|uent]y happens during a period of volcanic action 

 may be sufficient to affect the entire geological conditions in anv 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 ant)ther 

 region where the geological conditions were obviously different. Within the 

 observations of the writer instances are knowai oiitside the Great Basin 

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

 mutual relations of nearlv identical lavas e.xhibit a succession strikingly at 

 variance with the sequence of flow as found at I^hireka. The Yellow.stone 

 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 hornblende- 

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

 conditions, the laws govenaiug the differentiation of lava hold good 

 everywhere. 



Basalt and Rhyoiite.— The Writer acccpts, witli somc important modifica- 

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

 only as geologically closely related rocks, but also as e.xtreme members of 

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

 in which these extreme ])roducts were derived from an earlier molten ma.ss. 

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

 bursts frequently occur unaccompanied by basalt, or that basaltic expt)sures 

 abound without any e^^dences of the presence of acid lavas, l^oth rocks 

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



