THE WASHOE ROCKS. 103 



gray in the specimens from the mine and a yellowish tint 

 where the rock was exposed to the air. It is manifest and 

 indisputable that the west wall of the 3,000 foot level cooled 

 under a pressure greater than the rock of the west wall of the 

 croppings, tlie difference amounting to that of a vertical 

 column of 3,000 feet of west country rock. The specific 

 gravity of this rock is about 2.80. It is consequently certain 

 that a pressure of above 3,600 pounds per square inch, or 

 about 256 kilos per square cm. has produced no perceptible 

 difference in the mineralogical or physical character of 

 the west country rock. 



Nor that of diabase. — The diabase of the 3,000-foot level 

 is porphyritic and rather finer grained than it usually is on 

 theSutro Tunnel or any higher level. An additional pres- 

 sure, nearly as great as in the case of the diorite, has equally 

 failed to produce a coarser or less porphyritic character in 

 the diabase, or, in short, to induce any approximation be- 

 tween the r^cks. These rocks are so distinct at the 3,000- 

 foot level that no common miner fails to see the difference 

 between them, or to recognize the character of each and the 

 distinction between them as the same which prevailed at 

 higher levels. In short, there is at the lode a very sharp 

 break in the general character of the rock. 



In the effort not to burden my memoir wdth wearisome 

 details, I there perhaps insufiiciently described the distri- 

 bution of the granitoid diabases, though I distinctly as- 

 serted that the commonest variety of the east country diabase 

 is a fine-grained blackish-green rock. The granitoid variety 

 is, in fact, decidedly rare, though circumstances led me to 

 pay particular attention to its occurrences. The granitoid 

 form is not only most likely to be confounded macroscop- 

 ically w4th diorite, but is also least subject to decomposi- 

 tion and best fitted for microscopic study. It forms a very 

 small portion of the mass. 



The two rocks cannot he one eruption. — According to my 



