RECAPITULATION OF PALEOZOIC FORMATIONS. 163 



widely separated points that it nia,y safely be regarded as generally present over the 

 central part of the State wherever the formation is brought to view. In the few 

 fragments of it whicli are still visible along the Front Range, however, the Devonian 

 fauna seems to be wanting. This individualitj" in the matter of distribution has led 

 me to suspect that the Devonian and Carboniferous portions of the Leadville lime- 

 stone may not have be^en deposited strictly consecutively. 



Beginning with its typical locality at Leadville, this formation is described as a 

 heavil}' bedded series of dolomitic limestone, iu the upper part often coarselj' crys- 

 talline, having an average thickness of 200 feet. The color is a deep grayish blue, 

 frequently nearly black in the upper portion, while some of the lower beds are lighter 

 in color, approaching a drab. The upper bed is marked by characteristic concretions 

 of black chert, often containing casts of fossils. A list of species of fossils derived 

 from this horizon is apj^ended to the description in the Leadville monograph, and 

 the conclusion drawn that "this horizon represents the Lower Carboniferous of this 

 district." (Page 66.) This conclusion I believe to be correct, though it hardly seems 

 to follow from the faunal list, which, with one exception, contains nothing but Upper 

 Carboniferous species. In the Leadville monograph, from which I have quoted above, 

 this limestone is called the Blue or Ore-bearing limestone, and it was not until later 

 that it received the name by which it is now known. 



Under the title of the Blue or Leadville limestone this formation is redescribed 

 bv the same author in the Tenmile folio. No new facts are brought out, and it will 

 not be necessary to repeat the description. 



It is eas}'^ to recognize this horizon in Peale's Eagle River section, in which 

 bed 26, described as a black, flintj^ limestone, with pieces of pyrite and fragments 

 of Spirifer or Spiriferimi, clearly represents it. Possibly a part of No. 25, which 

 indicates a space probably filled mainly by limestone, also belongs to the Leadville, 

 but it is evident that little besides distribution is added in this instance. 



At Red Cliff, according to Tilden, the Leadville limestone comprises 260 feet of 

 blue, dolomitic limestone below, overlaid by black, siliceous limestone 300 feet thick. 



In the Aspen district the Leadville limestone has an average thickness of about 

 350 feet. It consists of two members, the lower of which is a heavy dolomite from 

 200 to 250 feet in thickness, and the upper a massive blue limestone from 100 to 150 

 feet. Spurr states that there is abundant evidence of two periods of dolomitization, 

 one of which occurred before the deposition of the Blue limestone, while the other 

 was much later, and was closely connected with the ore deposition (page 22). This 

 fact and the circumstance that these two divisions have a well-marked plane of sepa- 

 ration (page 23) afford evidence in support of the hypothesis that an interruption in 

 sedimentation occurred between the Devonian and the Carboniferous portions of the 



