246 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



appear from an inspection of the map, for, owing to the greater fluidity of 

 the lava, tlie basalt fields are of less depth than tlie andesitic masses. 

 While, too, some general orographical changes appear to have accompanied 

 the emission of basalt, as was almost inevitable, these were far .smaller 

 in amount than those which closed the Pliocene ^epocli. 



The basalt of the region under discussion is a fairly typical rock, pre- 

 senting the usual structural peculiarities and mineralogical composition on 

 the whole, though the distribution of olivine is irregular. In some occur- 

 rences this mineral forms a large percentage of the whole mass, while in 

 others considerable search with the lens, or even with the microscope, must 

 be made to detect it. Very interesting glassy forms of basalt occur near 

 Borax Lake, of which fdrther mention will be made in tiie next chapter. 

 It does not appear that all the basalt was emitted at the same time, or even 

 approximately so, for the evidences of erosion on some of the areas are very 

 perceptible, while on others there has been no considerable degradation. 

 On the whole, the basalt must be considered as decidedly recent, for 

 only on that supposition can its state of preservation be accounted for. 

 Thus, McPike crater is a rounded mass, unfurrowed b}" rivulets, at the top 

 of which is an extremely regular, basin-like crater about nine hundred feet 

 in diameter and fifty to one hundred feet deep, presenting a surface entirely 

 covered with lapilli. That this basin contains no water is probably due to 

 the porosity of the sides, which seein to be composed of lapilli. The walls 

 ai-e unbroken and vegetation has only begun to find root between the peb- 

 bles. The only sign of age is the fact that the surfaces of the lapilli are 

 reddened with ferric oxide. On the liills directly north of McPike crater 

 the surface is covered with lapilli, which almost entirely conceal the under- 

 lying metamorphic rocks. These pebbles could not possibly have been 

 transported to their present position by water, which, on the contrary, must 

 eventually sweep them down into the valley. In fact, they appear to have 

 fallen as they lie during an eruption, since wliich there has not been suffi- 

 cient rainfall to remove them. The north and south craters at Sulphur 

 Bank are similarly fresh, excepting that in each case one side of the crater 

 is broken down; but there is no evidence that this is a result of erosion, 

 for there is no stream bed or drv wash leadintj into them. Close to the 



