252 QUIOKSILVRR DEPOSITS OF THE PACIFIC SLOPE. 



treme members of the crystalline metamorphic series or true eruptive rocks. 

 The region was revisited for the purpose of verifying the structural relations 

 of these occurrences. The questionable rocks were then found to be sui*- 

 rounded by and to pass over into indubitable metamorphie nmtei-ial in such 

 a way as to preclude any separation of them. 



The Sulphur Bank map shows no Chico-Tejon beds or Pliocene fresh- 

 water strata and no andesite. Here, as elsewhere on Clear Lake, it is 

 manifest that the level of the present sheet of water has sunk within no 

 very long period, leaving fertile meadows. The composition, as well as 

 the topographical relations of these meadows, shows that they are drained 

 portions of the lake bed, for they are full of roots of the tule, which grows 

 only near the water's edge and preferably in shallow water. Close to the 

 basalt and in beds continuous with those which underlie the lava these 

 roots are sometimes found petrified. 



Basalt. — Tlie only volcanic rocks on this map are basaltic, but their 

 character and mode of occurrence are rather unusual and therefore interest- 

 ing. They are in part olivinitic and in part free from olivine, but their 

 microstructure is the same in both cases. In the area south of Borax Lake, 

 just beyond the limits of the map, ordinary olivinitic basalt occurs. It 

 adjoins a large field chiefly composed of obsidian and pumice, but contain- 

 ing also rocks which, while manifestly in part glass}', have a thoroughly ba- 

 saltic ap])earance. It is impossible to separate these occurrences in the field, 

 and the more they are studied the more certain it appears that all this ma- 

 terial is substantially from a single eruption. This is confirmed by micro- 

 scopic examination, although the glass is an acid one, containing over 75 per 

 cent, of silica (see pp. 158-162). The glass is usually of a gray color and 

 is transparent even in masses a quarter of an inch or more in thickness. 

 Between it and the pumice there is every conceivable gradation. The 

 glassy forms sometimes include small fragments of cr3'stalline basalt. This 

 area is the only one in which this obsidian appears to be in place, yet the 

 dissemination of chips of the same glass a square inch or less in area is 

 something astonishing. In the immediate neighborhood of the obsidian 

 field these chips are so plentiful that it is diflicult to draw its outline with 

 any accuracy. Thev gradually grow less abundant, ])ut are still to be 



