614 PROF. J. P. IDDINGS ON EXTRUSIVE [NOV. 1896, 



in Idaho. In this direction it descends in a series of great terraces, 

 and continues at lower altitudes for unknown distances down the 

 broad plain which forms the valley of the Snake River. It has been 

 followed for some distance down the channel of Palls Eiver, which 

 has cut through the capping of basalt, and extends along the western 

 base of the Teton range to Pierre's Hole, where it underlies basalt. 1 

 And similar lava is exposed underlying the same basalt-sheet 150 

 miles to the south-west down the Snake River at Shoshone Falls. 2 

 No proper estimate of the volume of this rhyolitic lava can be made, 

 but, judging from the area exposed, its volume is much more than 

 400 cubic miles. 



This rhyolitic lava is quite uniform in its composition ; the 

 average silica- percentage is about 74. Its eruption may have 

 occupied a long time, but evidences of repeated outbreaks are not 

 numerous, and much of the lava belonged to single flows, since 

 canons 1000 feet deep cut through a continuous body of rock in 

 most cases. 



Closely connected with the eruptions of this rhyolite-lava were 

 others of basalt. Not only is there no evidence of any extensive 

 period of erosion separating the eruptions of one from that of the 

 other, but streams of basalt immediately underlie the rhyolite in 

 places, resting on the greatly eroded surface of the long-extinct 

 volcanoes. Basalt-streams in rare instances lie between rhyolite in 

 such a way as to prove their comparative contemporaneity. But 

 the great bulk of the basalt poured out immediately after the rhyolite. 

 Its eruption was also from fissures, still farther south-west and 

 west. And the result was the flooding of all the great plain that 

 stretches westward through Idaho along the line of the Snake River. 

 The extent and thickness of this enormous sheet of basalt can only 

 be guessed at, so little is known at present of its boundaries. That 

 it is practically continuous as a covering for the country, and that 

 it rests immediately upon the great rhyolite flood, are both well- 

 established facts ; also that it was the last great outburst of lava in 

 this region, and closed the series of Tertiary eruptions in this part 

 of the world. Its eruption undoubtedly took place from many 

 fissures. Any estimate of its volume must be wide of the truth, 

 but when we consider that the area of country known to be covered 

 by it is much more than 300 miles long from east to west, and 50 

 to 60 miles wide, or more than 18,000 square miles, and when we 

 consider what thicknesses have been observed in the canon cut 

 through it, an average thickness of 200 feet must be well within 

 the limits of truth. With this thickness its volume is at least 

 700 cubic miles. 



In this region, then, we have bodies of extrusive rock of great 

 magnitude, to be reckoned in hundreds of cubic miles, and bodies 

 of intrusive rock of much less magnitude, but of no mean propor- 



1 Bradley, F. H., Report in Havden's 6th Annual Rep. U.S. Geol. Surv. 



2 King, 0., U.S. Geol. Expl. 40th Parallel, vol. i. p. 592 (Systematic 

 Territories, pp. 213-215, Washington, 1873. 



Geology), Washington, 1878. 



