618 DESCRIPTIVE GEOLOGY. 



SECTION VIII. 

 SHOSHONE EANGE AND CAEICO PEAK. 



BY AENOLD HAGUE. 



Shoshone Range. — The Shoshone Range rises from the valley of the 

 Humboldt River in latitude 40° 40', and extends in a southeast direction 

 until it passes south of the southern limit of Map V, Its general course is 

 about north 25° east or south 25° west. Only about 40 miles of the north- 

 ern portion of the range come within the limits of Map IV, embraced in the 

 present chapter. Directly south of the railroad, at the northernmost ex- 

 tremity of the range, there occurs a large field of basalt, separated from the 

 great Shoshone Mesa by the Humboldt Valley, which is here about 6 miles 

 in width. This basalt flow extends south for about 16 miles, and consists 

 of two gently-inclined tables, having a dip to the southeast, and separated 

 from each other by a northeast and southwest valley, called Whirlwind 

 Valley. These fields of basalt consist in general of fine-grained dolerite, 

 made up of plagioclase, augite, olivine, and magnetite, with more or less of 

 a gray globulitic base. Through these doleritic tables break out a great 

 number of vertical dikes of fine-grained anamesitic basalt, containing a high 

 proportion of brown glass, giving a very resinous lustre to the fresh frac- 

 ture. In the gray dolerites, upon weathering, there is a noticeable tend- 

 ency to the development of spherical forms. The ground is in some places 

 strewn with basaltic balls a foot or more in diameter, with concentric shells 

 weathering and peeling off under atmospheric agencies. On the face of the 

 Whirlwind cliffs, overlooking the Reese River plains, these spherical forms 

 are readily seen in the bluffs. There is also noticeable a rude columnar 

 structure, which dies off into irregular jointing. These concentric balls 

 seem to have something to do with the columnar forms, and they may 

 possibly represent the remains of horizontal sections of the rude columns. 

 The basalt here is covered with more or less hyaline incrustations, which 



