THE ROCKS OF THE WASHOE DISTRICT. 57 



magma of a different composition. In only one or two cases has anything 

 like a thoroughly granular structure in the groundmass been observed. 

 The greater part of the feldspar microlites are generally well and sharply 

 developed. The same is true in the augite-andesites, and in cases of extreme 

 decomposition the shape of the f6ldspars, large and small, is an important 

 point of distinction l)etween andesites and the older poi-phyritic rocks. 



Field character. — Li thc most Important part of the District lying in the im- 

 mediate neighborhood of the productive portion of the Lode, the hornblende- 

 andesite is dark and fine-grained, and contains only small hornblendes, which 

 are recognizable as such more often by their brilliant surfaces and evidences 

 of cleavage than by their crystal form. The rock breaks easily under the 

 hammer with a somewhat conchoidal fracture, and its luster is more or less 

 glassy. The hornblende-andesites which occur south of Gold Hill are much 

 more porphyritic, and the hornblendes are unusually well developed. 

 Crystals of an inch and a half in length are common, and one decomposed 

 crystal fully four inches long was observed. In none of the varieties are the 

 feldspars visible when fresh except on minute examination, simply because 

 they are transparent, and the dark color is therefore due to the bisilicates 

 and magnetite. Columnar structure is occasionally developed all over the 

 district, but in no great perfection. 



■Weathering. — Ordinarily the hornblende-andesite appears to possess little 

 or no structure in mass, while under the action of the atmosphere it develops 

 considerable fissility in certain directions, so that some croppings present 

 almost the appearance of upturned beds of sedimentary i-ocks with parallel 

 partings at a distance of one or two inches. That the fissile tendency does not 

 extend to an indefinite lamination is evident from the behavior of the sherds. 

 These do not continue to part parallel to their more extended surfaces, but 

 are gradually rounded Ijy the action of frost. By this agency conchoidal 

 fragments are separated from the corners and edges of the loose blocks, 

 and when it is considered through how short a distance the action of the 

 frost can extend, the display of force is quite astonishing. Conchoidal chips 

 of three or four pounds in weight are often found at a distance of two or 

 three feet from the block on which they fit. Large masses of hornblende- 

 andesite breccia also occur, though this form is not so common as with the 



