Vol. 52.] AND INTRUSIVE IGNEOUS ROCKS. 613 



gabbros and medium-grained diorites and granite solidified at con- 

 siderable depths below the surface, and might very properly be 

 classed as ' deep-seated ' or 4 abyssal rocks,' if we use that term. The 

 laccolitic rocks are typical, and would seem to belong to what 

 Prof. Brogger calls hypabyssal. But in this region the one 

 kind are just as deeply seated as the other, and in some cases the 

 laccolites are more deeply-seated than the coarse-grained rocks in 

 the cores. So that for these intrusive rocks the terms ' abyssal ' and 

 ' hypabyssal/ if applied with any geological significance, would not 

 fit groups of these rocks, which might be made to accord with 

 textural differences. Moreover, while it is possible to consider the 

 order of eruption of the intrusive rocks apart from that of the 

 extrusive rocks, such a method would not seem a comprehensive 

 one. 



Leaving the discussion of this matter for the present, let us 

 continue our review of the history of volcanic activity in the whole 

 region. After the close of activity along the chain of andesitic 

 volcanoes, denudation set in on a grand scale, and the lofty moun- 

 tains were cut to pieces by the running waters that drained their 

 slopes. A long time must have been consumed in reducing them to 

 nearly their present configuration. How long this was we have 

 no other means of judging than by considering the amount of this 

 erosion, which reduced the height of the Crandall volcano some 

 10,000 feet. 



When volcanic activity again broke out, its seat was shifted west- 

 ward, and its character was changed ; there were no longer scattered 

 explosions from what was probably a line of faulting or fissures, but 

 the gushing forth of gigantic floods of lava, whose composition was 

 quite different from the bulk of those that built up the range of 

 andesitic volcanoes. That this outflow must have been through 

 some great fissure or set of fissures its volume and form, and its 

 relation to the surrounding mountains, clearly attest. Moreover, 

 we have in the fissure-eruption in Iceland in 1783 an indication 

 of what took place in this region. The fissure in Iceland was 

 12 miles long, and the lava that poured out flowed in streams 45 and 

 50 miles long, in places forming lakes 12 and 15 miles wide, and 

 100 feet deep, and in a gorge 200 feet wide reaching a thickness of 

 600 feet. The total volume of the lava has been estimated as 

 greater than that of Mont Blanc. 1 



Some idea of the flood that broke forth west of the volcanic chain 

 in Wyoming may be gained by considering that its present extent is 

 50 miles from east to west, and from north to south it is exposed 

 to view for 90 miles. But its south-western boundary is unknown, 

 since it is covered by still more recent lava. Throughout this vast 

 extent, more than 2000 square miles, it is an unbroken sheet, 

 whose depth in the central portion is unknown, but is more than 

 1000 feet, and in some places more than 2000 feet. It forms the 

 great plateau of the Yellowstone Park, and the region south-west of it 



1 Arch. Geikie, ' Text-book of Geology,' 3rd ed. 1893, p. 222. 



