844 DESCEIPTIVE GEOLOGY. 



conspicuous by the iridescent play of light, but hardly to the same extent 

 as observed in the rhyolites of the Pah-Ute Range. 



Directly west of the highest point of the Berkshire Cation rhyolites 

 occurs a small dike of black, compact basalt. 



There are then represented here in this section the exposures of seven 

 types of eruptive rocks. Of these, melaphyrs are the earliest, followed by 

 small outcrops of propylite, which, however, have a • trachytic affinity ; 

 next the hornblende-andesite and the zone of purple and green dacites. As 

 between the andesites and dacites, the evidence is not conclusive as to age. 

 After the dacites come the trachytes, but the relation between the latter and 

 the rhyolites was not observed here, although the sequence has been noted 

 in too many other localities to leave any reasonable doubt as to their rela- 

 tive positions. The rhyolites, however, are clearly later than the dacites, 

 and the basalts later than the rhyolites. All these rocks, with the exception 

 of basalt, have at some period been subjected to a uniform process of decom- 

 position, in which the feldspars have been more or less replaced by carbon- 

 ate of lime, and, in the case of the melaphyrs, the olivine has b.een converted 

 into a serpentineous product. 



That all these rocks have been subjected to considerable sub-aerial 

 action before the outbursts of basalt would seem highly probable. 



The section C — D, at the base of the geological sheet, crosses the Vir- 

 ginia Range in the region of Berkshire Canon, and the intricate struct- 

 ural relations of the different volcanic outbreaks are indicated as well as it 

 is possible with the scale of the map and the geological data at hand. 



Region west of Pyramid Lake. — Opposite the southern end of Pyramid 

 Lake, where the trachyte flow terminates, a broad sheet of tabular basalt, ris- 

 ing in nearly perpendicular walls, forms the highest peaks, and caps all older 

 volcanic flows, beyond which the hills fall away toward Mullen's Gap. The 

 gap cuts the Virginia Range at right angles to its trend, and, in a measure, 

 isolates, topographically but not geologically, the northern end from the por- 

 tion already described. On both sides of the gap, the hills rise gradually 

 in a broken, irregular mass of ridges, cones, and pinnacles, of an interest- 

 ing volcanic rock, presenting some peculiarities of habit. It is regarded as 

 belonging to the quartz-propylites, and has been represented as such on 



