SHOSHONE MESA. 615 



> 

 Warm Springs is also a small isolated remnant of basalt, which forms the 



foot-hills of this rhj^olite ridge. 



Shoshone Mesa. — The Shoshone Mesa is one of the most sinsrular 

 topographical features of the Fortieth Parallel region. From Rock Creek 

 Valley, the country rises to the southward in broad and even slopes, keeping 

 approximately the same angle for a distance of 15 or 20 miles, and form- 

 ing an elevated plateau, bounded on its southern edge by perpendicular 

 cliffs over 2,000 feet in height, which reach their greatest elevation at its 

 extreme southern end, known as Stony Point. This elevation forms the 

 southern of the three northeast and southwest ridges, or undulations, of the 

 surface, which have -been covered by the volcanic flows of the northern 



region. 



Along the base of the cliffs, the lower thousand feet are seen to be 

 made up of rhyolites, generally of rather dark color, while the surface of 

 the table is composed of flows of dark, compact basalt. These basaltic 

 flows have evidently covered the pre-existing rhyolite ridge, and flowed 

 around the higher hills, which now project above its surface. The rhyolite 

 which forms the base of the cliffs on the southeast, toward Rock Creek, is 

 a dark-purple, thinly-banded rock, containing large crystals of sanidin and 

 rounded quartz-grains in a compact, felsitic groundmass, which, under the 

 microscope, does not show any signs of fibration except around the larger 

 crystals. It constains a little plagioclase and considerable apatite, and the 

 quartz-crystals are full of glass-inclusions, but the groundmass, although 

 showing- microscopically the fluidal structure peculiar to rhyolites, has 

 rather a micro- crystalline development, and contains no micro-felsitic 

 matter. 



Associated with these, at one of the re-entering angles of the cliffs, is a 

 peculiar dark pearlitic rhyolite abounding in large lithophysse an inch in 

 diameter, which are generally hollow, and look like a clayey mass, formerly 

 filled with gaseous matter, which has burst forth, leaving a hollow interior. 

 The rock abounds in large crystals of sanidin and quartz enclosed in an 

 almost black, pearlitic groundmass, which has become whitened in contact 

 with the lithophysae. The centres of some of the lithophysai are still filled 

 by crystals of quartz and feldspar. In some of the larger feldspars, grains 



