66 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



The characteristics which may serve in the field to distinguish the rock 

 of the Blue from that of the White Limestone are as follows : 



1 . Color, which is darker. 



2. Composition, the former being almost free from silica, the latter con- 

 taining 10 per cent, and upwards. 



3. Texture, the former being generally crystalline, while the latter is 

 more compact. 



4. Chert secretions, which in the former are always black and in the 

 latter nearly white. 



5. Structure, the Blue Limestone being generally more heavily bedded 

 than the White. 



Fossils. The only fossils obtained from this horizon were found in the 

 extreme upper part of the formation, either in the limestone itself or in 

 chert nodules, which are found scattered over its weathered surface. The 

 following' forms were obtained from five different localities : 



Euomphaliis, closely resembling E. Spergenensis, Hall, from Warsaw limestones 

 of Spergcii hill. 



Spiriferina, which is probably new, though somewhat resembling 8. Kentuckensis. 



Athyris siibtilita. 



Pleurophorux oblongus. 



Products coztatiis. 



Spirifera (Martinia) lincata. 



Spirifera Rockymontana. 



Streptorhynchm crassus (crenistria). 



Cyathopltylloid corals, resembling Zaphrentis. or Cyathaxonia cynodon. 



While most of these forms are common to the Coal Measures of the 

 East, the first-mentioned is there found in the Lower Carboniferous. For 

 this reason and because this form and the Spiriferina do not occur in any 

 of the higher beds, it seems justifiable to assume that this horizon represents 

 the Lower Carboniferous of this district. 



The upper limit of this formation has been fixed at the top of the 

 massive Blue Limestone, which is generally marked by the frequency of 

 chert concretions, and in the mining districts has been followed by prefer- 

 ence by the ore-bearing solutions. Locally, however, limestone formation 

 seems to have continued somewhat intermittingly for some distance above 

 this horizon. 



