SEEPENTINE. 281 



The conglomerates which form the Lake-bed deposits are often found 

 to have a calcareous cement, that can be readily separated from the pebbles 

 which it incloses. This conglomerate is of so recent date that at the time 

 of its formation the structural conditions of the range must have been essen- 

 tially those which prevail at the present day, and the waters from which the 

 cementing material was derived were surface waters, which may be sup- 

 posed to have drawn their calcareous constituents from the outcrops of the 

 various dolomitic beds of the lower Paleozoic series. Chemical tests show 

 that the cement is made up almost entirely of carbonate of lime, with little 

 or no cai'bonate of magnesia. It would seem, therefore, that when exposed 

 to the action of surface waters the dolomites of this region have yielded up 

 their carbonate of lime more readily than their carbonate of magnesia. This 

 may be due to a previous disintegration under the action of atmospheric 

 agents which rendered them more attackable or to a superior solvent power 

 of surface waters over underground waters in their action upon the carbonate 

 of lime. 



Serpentine. — The development of serpentine in the Silurian beds of this 

 region is, it is believed, the first observed instance of its occui-rence in the 

 Rocky Mountain region, and therefore deserves some detailed mention. Its 

 principal point of development is in the Red amphitheater in Buckskin gulch, 

 on the south face of Mount Bross, where it is found mainly in the transition 

 beds at the base of the Silurian formation, though extending to a limited 

 extent up as far as the base of the Carboniferous. It was also observed in 

 limited development on the clifiPs at the south base of Mount Lincoln, and 

 specimens were obtained in the Leadville district from the Comstock tunnel 

 in California gulch, where its exact horizon could not be definitely deter- 

 mined. No actual serpentine was found at any other point, but a greeni.sh- 

 colored bed was observed frequently at about the same horizon, which 

 by a microscopical examination of certain specimens was proved to contain 

 amphibole or pyroxene. 



As developed in the Red amphitheater, it occurs generally in limestone, 

 forming a greenish, veined, and clouded rock, like vei-d-antique, the vehis 

 or streaks of serpentine generally running parallel with the stratification, 

 but sometimes crossing it at right angles. It also occurs in a yellow, homo- 



