the Ear tit's Contraction from Cooling. 285 



Teneriffe, the centre of the mountain maybe trachytie when the 

 sides around are basaltic. I found a similar fact in a dissected 

 volcanic mountain of Western Oahu*. And at Mauna Loa, 

 where the ejections are almost all basaltic, there are felspathic 

 lavas at the very summit of the dome about the summit crater. 

 A sight of the boiling movements of Kilauea in 1840 led me to 

 explain this association in volcanic mountains on the principle 

 of liquation — the felspathic material being the least fusible, and 

 being therefore left at the centre, while the more fusible iron- 

 bearing lavas were drawn off by the outbreaks through the dif- 

 ferent sides of the cone. The diversity, on this view, is not 

 proof of diversity of origin. 



Again, the two kinds of igneous rocks occur on a majestic 

 scale over the Pacific slope* of the Rocky Mountains. The tra- 

 chytic rocks there appear, as stated by Eichthofen, to be ge- 

 nerally the older f. Clarence King, in his description of the 

 Shoshone falls on Snake River, states that, out of 700 feet in 

 thickness of igneous rocks exposed in the bluff, the lower 300 

 feet are made up of trachyte, while the upper 400 are of basalt, 

 and that a continuous field of igneous rocks, mainly basaltic at 

 surface, stretches over the country of Snake River for 300 miles 

 or more. The outflows are of later date than the Miocene Ter- 

 tiary. The fact of the very wide geographical distribution of 

 the basalt on the Pacific slope appears to be good proof, as in 

 the case of the dolerite of the Atlantic border (but better, because 

 of the wider range of the ejections), that the source of its mate- 

 rial was not local or dependent on the fusion of sedimentary 

 strata. And if this is true of the basaltic rock, it is probably 

 so also of the trachyte. Whatever doubt may exist, the general 

 argument is made a demonstration by the fact explained {, that 

 a vast undercrust fire-sea was a necessity in order that the great 

 heavings and bendings of the earth's crust essential to moun- 

 tain-making on the Pacific border should take place. 



The conclusion arrived at militates not only with the theory 

 of Hunt, but also to some extent with Mallet's, unless the latter 

 is made to appeal to the true crust for the material to be fused 

 by the motion attending mountain-making. The motion in the 

 true crust, even in the catastrophic period of mountain-making, 

 is very much less and slower than that which is experienced by 

 the plicating strata above it, and must therefore be a much 



* See on this subject the author's i Geol. Report, Wilkes's Exp. Exped.' 

 pp. 204, 269, 368, 372. 



f "The Natural System of Volcanic Rocks," Mem. Calif. Acad. Sci. 

 vol. i. pt. 2 (4to, San Francisco, 1868). The trachytes of Auvergne are 

 similarly older than the dolerites of the region. But more recent trachytes 

 occur in Italy, and the rock of modern eruptions in Iceland is trachyte. 



% Supra, page 211. 



