cVSPE RITES AT STEAMBOAT. 335 



Later andesitea. — Moi'e receiit tliaii tliG liombleade-andesitos described 

 above are other andesitic rocks. These are divisiljle niliierak)g-icall}- into 

 three varieties, and are so hxid do\Tn upon the map. Tliey appear, liow- 

 ever, to have been ejected ahnost simultaneously. They are all so recent 

 that extremely little erosion has taken place and only a few depressions are 

 marked by water- courses. The difference in this respect between tlie areas 

 of later andesites and tlie metamorphic areas is readily seen from the map 

 where the amount of sculpturing corresponds to the geological colors. 



All the later andesites are rough, soft rocks, in which the feldspars are 

 much cracked. Some of them are laminated, the beds averaging perhaps 

 an inch and a half in thickness, and this modification is physically indis- 

 tinguishable from similar occurrences at Clear Lake. One variety of these 

 rocks is highly pyroxenic ; a second is hornblendic, though not free from 

 pyroxene, and sometimes contains mica, while often lacking this constituent. 

 Between the pyroxenic and hornblendic rocks are transitions of the most 

 .unmistakable kind, which I have designated by a separate color and have 

 entitled for the purposes of this one map " transition andesite." In these 

 areas the composition of the rock is curiously variable. Tluis, in a little 

 triangular patch nearly south of the mines and embracing considerably less 

 than two acres, specimens can be collected which in the office might well be 

 supposed to represent three distinct species: one a pyroxerie-andesite, one a 

 simple hornblende-andesite without mica, and the third a micaceous horn- 

 blende-andesite. But the whole area is thoroughly exposed and certainly 

 represents only a simple eruption. The different varieties of rock were 

 found here and elsewhere within a few inches of one another on the same 

 blocks of lava. Such occurrences sliow how hopeless is the attempt to 

 reconstruct the geology of an eruptive district from collections alone. 



The area colored as later hornblende-andesite is covered with rock 

 almost indistinguishable from the later hornblende-andesite near the Com- 

 stock ; indeed, a very largo proportion of the region intervening between 

 the two districts is occupied by this rock and the lava field seems to be 

 continuous from the northern end of the Comstock area to tlie eastern edge 

 of tlio Steamboat Springs map. In both neighborhoods the quantity of 

 mica is variable, but near Steamboat it is exceedingly capricious. Mica is 



