RHYOLITE NEAR THE LAMAR RIVER. 391 



NORTHEASTERN CORNER OF YELLOWSTONE PARK. 



Toward the northeast the rhyolite sheet thins out and overlies the 

 andesitic and older rocks, which had previously been greatly eroded. East 

 of the Yellowstone Canyon the plateau top consists of rhyolite, into which 

 have been cut the canyons of Broad, Deep, and Agate creeks. The rhyolite 

 reaches an altitude of 9,000 feet where it is found in contact with the 

 basalt of Mirror Plateau, and also on the peak northwest of the headwaters 

 of Broad Creek. At the latter place the rock is red, glassy, and somewhat 

 pumiceous (2058), with abundant phenocrysts and included fragments of 

 obsidian Down the drainage channels from the summit of this point the 

 rhyolite is denser and vesicular and contains large lithophysse, while still 

 lower down it is dense, banded, and lithoidal. In the cliff east of Mirror 

 Lake the rock is dense, has small phenocrysts, and exhibits characters 

 that are found in the parts of the sheet near its bottom contact. There is 

 rhyolite along the west base of Amethyst Mountain and Specimen Ridge, 

 where it constitutes the margin of the great plateau. Beyond this to the 

 north and east it occurs only as disconnected remnants, which lie at all 

 altitudes on the slopes and summits of mountains and in the bottoms of 

 valleys, in almost every instance forming a bench or table of greater or less 

 prominence. 



Southeast of Mirror Plateau rhyolite forms a fiat-topped spur between 

 Mist and Cold creeks, lying between the altitudes of 8,750 and 8,300 feet. 

 East of the mouth of Cold Creek it forms a flat spur, and stretches from 

 8,000 to 8,600 feet, reaching to within 200 feet of the bottom of Lamar 

 River, while on the spur east of this it lies between 8,200 and 8,650 feet 

 elevation In each of these occurrences the rhyolite is lithoidal, purple, 

 and porphyritic. 



The flat spurs and ridg-es on the east side of Lamar River from the 

 base of Saddle Mountain to Cache Creek are rhyolite, which also occurs 

 in isolated patches within 200 feet of the river at Opal Creek and at the 

 south base of Bison Peak, and also within a short distance of Amethyst 

 Creek. The character of the rhyolite in these places is quite the same, 

 being purple and lithoidal, with small phenocrysts (2135, 2155, 2161). This 

 is also the character of remnants of the rhyolite sheet that occur on the 

 north side of the northwest end of Specimen Ridge (2158), and- in a bench 

 north of the mouth of Lamar River, 600 feet above the stream (2157), and 



