366 GEOLOGY OF THE YELLOWSTONE NATIONAL PABK. 



somewhat more steeply than does the present ridge. Similar columnar 

 structure occurs in the purple lithoidal rhyolite (1815) on Winter Creek, 2 

 miles west of this locality. 



CANYONS OF GIBBON RIVER AND MADISON RIVER. 



The lava forming the high bluff back of the Paint Pots and just east of 

 the Gibbon River as it leaves the Geyser Meadow is a variety of rhyolite 

 somewhat different from that forming the plateau country roundabout. It 

 is lithoidal and porphyritic, like the occurrences already described, but is 

 dull steel-gray in color, and full of minute, irregularly shaped cavities, 

 producing a very rough fracture. The phenocrysts are mostly small white 

 feldspars, which are plagioclase, and much fewer transparent sanidines, and 

 very rarely quartz; small augites may be seen with a lens. The rock 

 appears more like a basalt than a rhyolite. It passes southward along the 

 east side of the river into light-purple lithoidal rhyolite with similar plagio- 

 clases, but much more quartz and sanidine. 



The Gibbon River after leaving Geyser Meadow cuts a narrow canyon, 

 500 to 600 feet deep, through rhyolite, which is almost entirely lithoidal, 

 but of variable character. On the west side it is exposed in high vertical 

 cliffs of dark lithoidal rock with pronounced banding or flow structure, 

 which is greatly contorted. Back of the hot springs on the east side there 

 are a number of angular fragments of reddish-brown glassy rhyolite full 

 of small phenocrysts of quartz and feldspar (1852). These are probably 

 not in place. The west wall near the bridge consists of reddish-purple 

 lithoidal rhyolite, full of irregular pores, and crumbling. It is rich in 

 phenocrysts of quartz and feldspar (1851, 1853), and is a nevadite, strictly 

 speaking. The great massive cliff weathers into columnar pinnacles, which 

 are very characteristic of crumbling rhyolite. The denser variety of the 

 rock weathers into fragments with smooth surfaces and sharp edges. 



At the falls of the Gibbon the rhyolite is reddish and lithoidal, passing 

 into glassy rock, that at the base of the falls being black obsidian full of 

 porphyritical crystals, which is so thoroughly cracked that it is difficult to 

 obtain a compact hand specimen of it (1855, 1856). 



The canyon made by the Madison River below the junction of the 

 Gibbon and Firehole rivers has been cut 1,700 feet through the rhyolite 

 mass without reaching the underlying rocks. The canyon presents fine 



