CAMBRIAN. 59 



lively a smaller proportion of carbonate of magnesia than any other lime- 

 stone of the range, the specimen analyzed having 



Carbonate of lime 25.43 



Carbonate of magnesia .... 4.03 



The whole series may often be observed to be divided into two equal 

 parts, the lower half consisting of very pure white quartzite, while the 

 upper half weathers brown and is more or less stained by iron oxide and 

 other impurities. 



While the lower series is very persistent in its character, the upper 

 portion or transition series, which has a maximum thickness of 100 feet, is 

 extremely variable, and, though readily recognized in all cliff sections, often 

 seems to be wanting in those afforded by the numerous drill-holes in the 

 neighborhood of Leadville. 



Owing to their similar lithological character and to the general absence 

 of fossil evidence, it is difficult to establish a hard and fast line between this 

 and the succeeding formation above. In practice the line has been drawn 

 at the top of the shaly beds and the commencement of the beds of more 

 massive limestone. The transition beds consist essentially of alternating 

 bands of calcareous quartzite and shales. The name Sandy Limestones is 

 often applied to them for the reason that on weathered surfaces of the cliff 

 faces they appear like sandstones, the carbonate of lime having been entirely 

 washed out and only the fine quartz grains left on the thin surface crust. 



One especially persistent bed of sandy limestone, generally about a 

 foot in thickness, is often very useful in determining the horizon, on account 

 of the striking appearance of its weathered surface. It is a silicious dolo- 

 mite, generally of whitish color on fresh fracture, containing spots of dark 

 brick-red resembling casts of fossils; for which reason the name Red-cast 

 beds has been given to it. Fig. 1, Plate V, the reproduction of a photo- 

 graph of a weathered specimen, shows its characteristic appearance. 



Certain of the shaly beds are found to contain a considerable develop- 

 ment of pyroxene and amphibole, which often give a decided green color 

 to the rock. The microscope shows besides an admixture of fine ore parti- 

 cles, and in some cases there is so large a concentration of pyrites as to 

 constitute veritable ore bodies. 



