ANDKSITES. 35 



between hills of rhyolite and dacite, because it is less resistant to erosion than 

 these rocks. 



Relation to earlier andesite. The later andesite directly overlies the earlier 

 andesite, and though in many underground workings and probably at every outcrop 

 the contact is a fault contact, caused by movements subsequent to the eruption of 

 the later andesite, yet in several shafts one andesite has been found apparently 

 lying undisturbed in its normal position upon the other. Such was the case in the 

 Midway, the West End, and the Tonopah Extension shafts. In these places the 

 contact was marked by a band of decomposed breccia, or even clay, yet there was 

 no good evidence of faulting. The quartz veins of the earlier andesite extend up to 

 this contact in full strength and then abruptly disappear. Most likely the earlier 

 andesite was deeply eroded and the veins were exposed before the later andesite was 

 poured out, and possibly the decomposed clay or breccia zone represents the result 

 of surface decomposition and disintegration before the later-andesite period. 



Distinction from earlier andesite. The earlier andesite and the later andesite 

 are usually sufficiently distinct in appearance to permit identification in the field. 

 The later andesite is generally darker; on account of the greater amount of iron 

 present it has the characteristic strong coloration mentioned above. The earlier 

 andesite is characteristically finer grained than the later, and contains smaller 

 and less abundant porphyritic crystals. The porphyritic feldspars in the earlier 

 andesite are usually slim, of simple form, and almost rectangular, while those 

 of the later andesite are apt to be stout and complex as a result of twinning. 

 In the later andesite crystals of fresh or bleached biotite can usually be seen; 

 in the earlier andesite they occur more rarelv. 



Similar characteristics serve, as a rule, for the microscopic determination. 

 The phenocrysts of ferromagnesian silicates augite, biotite, and hornblende and 

 their pseudomorphs or decomposition products are usually more abundant in the 

 later andesite. The typical alteration of the earlier andesite is to quartz, sericite, 

 and a little pyrite; that of the later andesite is to chlorite, quartz, calcite, siderite, 

 and pyrite. While the character of the alteration is a valuable help in diagnosis, 

 it is not by any means a sure test, for in some cases the processes of alteration 

 have been apparently almost exchanged." 



On account of the similarity in the original composition of the earlier and 

 later andesites it is frequently very difficult, either from field or from microscopic 

 study, to refer a specimen to the proper age. Often this economically important 

 question is decided by tracing the doubtful phase into some decided phase in 

 the same rock body. 



nit is probable, however, that the sericite-like aggregates in the altered later andesite are composed largely of 

 minerals like hydrargillite, talc, kaolin, etc., rather than of sericite. See pp. 240-241. 



