﻿30 A. Hague and J. P. Iddings — Volcanic Rocks, etc. 



Those, characterized by large porphyritic crystals of feldspar 

 and quartz with much mica and hornblende, approach the 

 hornblende-mica-andesites, while those poor in the ironmag- 

 nesian silicates closely resemble the rhyolites. In thin section 

 it is seen that the feldspars of those rocks rich in porphyritic 

 crystals are almost wholly triclinic, scarcely an orthotomic 

 feldspar having been recognized among them. The species 

 probably belong to the andesin-oligoclase series. Porphyritic 

 quartz is at the same time abundant in dihexahedral crystals 

 or rounded grains, bearing glass inclusions, frequently with the 

 accompanying phenomena of strain. Biotite occurs in excess 

 of the hornblende, which is often entirely absent. The differ- 

 ent varieties of these dacites have their equivalents throughout 

 the Great Basin of Utah and Nevada, especially at Mount 

 Prometheus in the Toyabe Range, and in the Eureka and 

 Washoe mining districts. No well defined rhyolites with 

 characteristic sanidin are found in this suite of rocks, although 

 •chemical analysis might possibly show that some of the litho- 

 idal varieties without any well defined mineral constituents 

 •more properly belong to rhyolite than dacite. 



An examination of this collection shows clearly that the 

 vast accumulation of material poured forth from the volcanoes 

 of the Cordillera of Salvador bears the closest resemblance in 

 composition to the lavas from the volcanoes of the northern 

 Cordillera in California, Oregon, and Washington Territory, 

 notably Mount Shasta, Lassen's Peak, Mount Hood and Mount 

 Eanier. 



Moreover a rigid comparative study of these rocks proves 

 clearly the closest similarity between the volcanic lavas of 

 Salvador and those of the Great Basin of Utah and Nevada, 

 both as to macroscopical habitus and microscopical detail. In 

 comparing rocks of corresponding types from the two regions 

 they are seen to carry the same constituent minerals with 

 identical characters in thin section, and the same gradation in 

 mineral composition. Transition rocks from one well-defined 

 type into the next more or less basic or acidic type are suffi- 

 ciently well represented in Salvador to show the same laws 

 of mineral development as observed in the lavas of the Great 

 Basin. 



It is interesting to note that nearly every rock in this collec- 

 tion has its counterpart in Nevada, although there occur many 

 varieties in the latter region not found in the limited series 

 from Central America. This is specially true of rhyolite 

 which covers so large areas in Nevada, but is wanting in this 

 collection. It seems probable that many rhyolitic eruptions 

 may have occurred in Salvador which will be found upon fur- 

 ther search. 





