64 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



ular shape upon the surface. The forms which they assume are sometimes 

 so fantastic as to suggest to the untechnical that they are the fossil remains 

 of some gigantic animal. Their forms, however, are always rounded, and 

 are more commonly that of a sphere or some solid of revolution. In many 

 cases, that they are the filling in of a pre-existing cavity in the limestone 

 is evident from the fact that they are hollow in the center and contain 

 crystals of pyrite or other minerals lining the cavity. 



The series is generally heavily bedded, and the rock is almost always 

 granular, and in the upper part often coarsely crystalline. A character- 

 istic feature, especially of the upper portion of the formation, is a ribbed 

 structure produced by irregular lines and spots of white crystalline mate- 

 rial. In some cases the ribbing is so fine and regular as to produce an ap- 

 pearance resembling that of the Eozoon. 



This typical appearance of the rock is shown in Plate VI, on which 

 are represented two specimens from the Blue Limestone of Iron hill, taken 

 a short distance below the ore body on the Silver Wave claim, which were 

 also subjected to microscopical examination. The upper figure in the plate 

 is a photograph of a specimen polished on one side to show the fine ribbing 

 which is peculiar to this limestone. The lower figure shows a specimen 

 roughly shaped by the hammer, in which tire ribbings or veins of white 

 crystalline spar are coarser and more irregular. These white crystalline 

 veins may be supposed to be produced by the dissolving out of a portion of 

 the limestone and its redeposition in a crystallized form. As bearing on 

 the question of the relative solubility in natural waters of carbonates of 

 lime and magnesia, a partial analysis of the white spar was made, and it 

 was found to have the same proportions of the two salts as the dark granular 

 rock. 



Composition. The composition of the rock, which is remarkably uniform, 

 is that of a normal dolomite, the average of six lime and magnesia deter- 

 minations from different localities giving 



Carbonate of lime 54.695 



Carbonate of magnesia 43.197 



the proportion in normal dolomite being 



Carbonate of lime 54.30 



Carbonate of magnesia 45.70 



