4 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



alternate with one another, the coarser varieties being generally light- 

 colored mica-gneisses rich in feldspar and quartz. The finer-grained, 

 dark-colored varieties are for the most part mica-gneisses richer in biotite. 

 Mica-schists, sometimes highly garnetiferous, occur in smaller quantities, 

 and amphibolites are also found. The pronounced lamination or schistositv 

 of the whole body of these rocks is quite uniform in its position, the layers 

 standing at high angles or nearly vertical, with a general north-south 

 .trend. The microscopical study of these rocks shows them to be normal 

 crystalline schists, having the microstructure of highly metamorphosed 

 rocks and exhibiting no traces of their previous character. Their study in 

 the field was not thorough enough to throw any light on the question of 

 their possible origin. 



South and west of The Crags the crystalline schists are directly 

 overlain by volcanic breccia and tuffs of andesites, whose subaerial 

 accumulation is beyond question. These rocks are generally dark colored, 

 and occur in rugged outcrops and rough, angular talus blocks. In general, 

 the easterly slopes are smooth, covered with soil, and less steep than the 

 western sides of the hills. The andesites are variegated in color and are 

 chiefly hornblende-andesite, carrying some pyroxene and a little biotite. 

 The occurrence of these subaerial breccias shows that at the time of their 

 eruption the crystalline schists were exposed surface rocks which had 

 undergone extensive erosion, by which they had acquired a pronounced 

 mountainous topography. 



On the west and south the schists pass under massive rhyolitic lava, 

 which is part of the great plateau lavas farther south, and whose position 

 with respect to the crystalline schists and andesitic breccias is such as to 

 show that the rhyolitic lava flooded the lower levels of this gneissic region 

 after the andesitic material had been accumulated and had been partly 

 removed by erosion. That the rhyolite overlies the andesitic breccia is 

 clearly shown in the walls of Maple Creek Canyon. The rhyolite also fills 

 the valley bottom between two ridges of andesitic breccia in this vicinity, 

 indicating- the extent to which the andesite had been previously eroded. 



There is marked contrast in the scenery and topography of the 

 gneissic areas and of the country formed by the rhyolite, the former being 

 essentially rugged and broken, while the latter is as yet comparatively 

 little affected by erosion, the streams flowing in trenches and canyons cut 



