202 GEOLOGY OF THE COMSTOCK LODE. 



which appears everywhere to lie directly upon quartz-porphyry or still 

 older rocks. If hornblende-andesite proper occurs here, it should show at 

 the contacts; but the nearest area of the hornblende rock is 6,000 feet 

 away. If this is properly to be classed witli the augite-andesites in spite 

 of its mineralogical composition, it is quite jjossible that the three small 

 patches of earlier hornblende-andesite shown on the map, each of them 

 entirely surrounded by augite-andesite, may also be of this character. 



Independence of the augite-andesite eruption. The tWO rOcks arC SO nUlcll alike that 



some lithologists doubt the propriety of classifying them as different species, 

 but in the Wasuge District they are certainly different eruptions. The 

 contacts in the Sutro Tunnel, the Fonnan shaft, and at many points on the 

 surface are well defined, and the mineralogical character is persistent over 

 very large areafe, in spite of a few doubtful localities. It has been seen that 

 there are also points where it is very difficult to say whether the rock is to 

 be regarded as diorite or diabase. The absence of such occurrences would 

 be a matter of surprise, for the character of a rock depends upon combina- 

 tions of chemical and physical conditions, which cannot be identical at any 

 two points. Each so-called rock species represents an endless number of 

 such combinations, and some of these are indistinguishable from those at- 

 tending the formation of allied species. The strange fact is not the occur- 

 rence of transitions, which are after all exceptional, but the persistence of 

 rock types not only within limited areas but throughout the world. 



In determining the succession of the hornblende and augite-andesites 

 position alone can be relied upon, for the two rocks are so closely allied 

 that it would be impossible to distinguish with certainty between an inclu- 

 sion and a local modification in composition. The indications of position, 

 however, all tend to the supposition that the hornblendic rock is the older, 

 as may be seen from an inspection of the sections. 



Occidental lode. — The Occldeutal lode occurs in augite-andesite. Unfortu- 

 nately the principal mines were closed at the period of the investigation, 

 and it could not be studied satisfactorily. The dump of the Occidental mine 

 seems to show that a contact with micaceous diorite is encountered in the 

 workings. This lode is plotted on the map from distinct croppings and 

 mine surveys, and its trace is a further remarkable illustration of the par- 



