280 GEOLOGY OF THE COMSTOCK LODE. 



this section with the surface map and with the horizontal section on the 

 Sutro Tunnel level shows that the contact between the diabase and the diorite 

 being- steeper than the dip of the northern branch of the Lode, the fork of 

 the vein is met much farther north on the lower levels than at the surface. 

 The disturbing influence of the sharp bend in the diabase-diorite contact 

 upon the regularity of the faulting action is visible in the larger amount of 

 crushed rock, and the apparently displaced diorite masses on this section. 

 Most of the diorite east of the northerly fissure and nearly all of that on 

 the lower levels is porphyritic. A small ore body occurred near the crop- 

 pings on the northerly branch. Mr. King describes the oi'e there found as 

 "fragmentary masses of blocky quartz, impregnated with native gold, 

 closely resembling the California auriferous quartz." The little ore bodies 

 on the 2300 and 2400-foot levels are more like the ordinary Comstock 

 ores. The evidences of solfataric action are very strong on the lower levels 

 of this section; indeed, the decomposition is so profound as to make litho- 

 logical determinations a matter of the utmost difficulty. 



Cross-section through the Sierra Nevada. The Sierrtt Ncvada SCCtioU shoWS evi- 



dences of very powerful dynamical action, yet of but a small amount of 

 faulting; for the dip of the north fissure is here so irregular that no move- 

 ment whatever could occur in the ordinary dii'ection without extensive frac- 

 turing. The occurrence of limestone on this section has ah'eady been 

 noticed. The diorite beneath it is mainly granular, and that resting upon it 

 is for the most part porphyritic, though no sharp line can be drawn for any 

 considerable distance between these varieties. It appears to me that this in- 

 cluded sheet of stratified rock was largely instrumental, by its weakening 

 efi"ect, in determining the course of the north fissure. Beneath the lime- 

 stone is a small stringer of diabase, no doubt connected somewhere with 

 the main body to the east, but at what point is uncertain. It is accompa- 

 nied by a minute quantity of ore, not unlike that of the Comstock bonanzas, 

 but it would be difficult to gather five pounds of it, and there is no likeli- 

 hood of any ore body of importance being found here. The same stringer 

 of diabase, or a similar one, occurs further north in Utah ground, on the 

 north fissure. The main body of diabase seems to have been struck on the 

 1450 level of the Sierra Nevada by a drill hole, the cores of which were 



