Vol. 52.] AND INTENSIVE IGNEOUS EOCKS. 617 



reach altitudes of 9000 and 12,000 feet. "\Vc are nearly opposite 

 the southern end of the range, the nearest peaks heing 40 miles off, 

 and its breadth about 50 miles. As the eye follows the western front 

 of the range northward, it gradually sinks behind the level-topped 

 plateau, until only the higher peaks are visible. These may be 

 traced westward across the northern boundary of the Yellowstone 

 Park to Electric and Emigrant Peaks, beyond which our imagina- 

 tion alone can follow them to their northern end, 140 miles distant. 

 Remembering how much of this range is extrusive rock, and trying 

 to locate those spots where intruded stocks have been discovered, 

 whose relations have been greatly magnified by the process of 

 detailed study and description, we are impressed with their insigni- 

 ficance when compared quantitatively with the breccias in which 

 they occur. 



The great rhyolite-plateau, with its monotonous covering of pine 

 forest, occupies the foreground northward, stretching from the base 

 of the Teton Mountains to the north-east, north, and north-west, 

 for distances of 50 and 70 miles, and descending in great terraces 

 towards the valley of the Snake River. The whole 2000 square miles 

 lies before us as a map, greatly foreshortened, with boundaries 

 outlined by mountains on the east, north, and west. Turning our 

 faces westward, we look down across the broad open valley of the 

 Snake River, into which the western slopes of the Tetons gradually 

 descend. The farther side is 100 miles away, and beyond our 

 horizon this vast intermontane plain continues monotonously for 

 hundreds of miles — all basalt. 



Surely the depths at which these once molten floods of rhyolite 

 and basalt were differentiated must have been profound, and the 

 processes of their differentiation, when compared with those which 

 produced the small bodies of magma in the conduits of the Electric 

 Peak and of the Crandall volcanoes, must have been the more 

 fundamental. 



There are, then, in this region extrusive rocks, whose volumes are 

 of such magnitude that the evidence drawn from the succession of 

 their eruptions and from their composition is of a higher order than 

 that derived from the lesser and more localized eruptions, whose 

 volume, however, may be estimated in cubic miles. It is upon 

 evidence of this order that I ventured to enunciate the principle, 

 that in a region of eruptive activity the succession of eruptions 

 commences in general with magmas representing a mean composition 

 and ends with those of extreme composition. 



Plate XXIX. 



Geological Sketch-map of portions of Idaho, Montana, and Wyoming, showing 

 in part the distribution of the volcanic rocks ; on the scale of 40 miles to 

 1 inch. 



Discussion. 

 Dr. H. J. Jounston-Lavis and Mr. J. J. H. Teall spoke. 



