T. M. Chatcwd—Urao. 59 



absence of augite is rendered still more striking by the fact 

 that in the Syracuse peridotite, which is no more decomposed 

 than this rock, the augites in the groundmass are apparently as 

 fresh as when they were first formed. The explanation of this 

 probably lies in the fact that the patches of yellow base, some 

 of which is quite isotropic, are much more abundant here 

 than in the other rock described, and, perhaps, the augites 

 did not have a chance to crystallize out, being deposited as 

 a glass. There seems to be no doubt from its general appear- 

 ance that this is a portion of the same original rock mass as 

 that before described, and probably so situated with reference 

 to it at the time of formation, that the now brown rock 

 crystallized more rapidly than the other portion of the erup- 

 tive mass, represented by the rock at the first exposure 

 described. 



The dyke of blue earthy material, spoken of in Part I., 

 has yellow grains scattered through it. The nature of the 

 original rock, which this blue decomposed dyke represents, 

 cannot be definitely determined. A thin slice shows a few 

 fragments of brown mica, and sections composed entirely of 

 serpentine, occurring for the most part in irregular grains, 

 but occasionally showing the form of olivine, imbedded in a 

 green to bluish-green groundmass, which appears to be partly 

 serpentine and partly chlorite. The porphyritic grains are 

 composed of white, yellow and greenish yellow serpentine. 

 The arrangement of the serpentine, the olivine forms still 

 preserved, indicate that all the porphyritic serpentinized sec- 

 tions originally were olivine. It is quite probable that the rock 

 consisted once of olivine with a small quantity of biotite im- 

 bedded in a groundmass made up largely of glassy base con- 

 sisting chiefly of olivine substance which has weathered to 

 serpentine and chlorite. 



Art. VI. — On Urao ; by Thomas M. Chatard.* 



Lsr this Journal for August, 1888, p. 146, 1 gave the analyses 

 of the waters of some American alkali lakes, among them, 

 Owens Lake, Cal. The salts now to be described were obtained 

 by the spontaneous evaporation of the water of the lake, and in 

 connection with the results obtained from other localities, throw 

 much light on the true character of the composition of the na- 

 tive sodium carbonates. 



* Condensed from "Natural Soda, its occurrence and Utilization," a forthcoming 

 bulletin of the U. S. Geological Survey. Published by permission of the Director. 



