SILURIAN. 61 



But rarely do the beds have the whiteness of marble, and in such cases it 

 is evidently due to local inetamorpliism. 



The characteristic feature of this limestone is the occurrence at certain 

 horizons of concretions of white, semi-transparent chalcedony or chert. This 

 occurrence is often useful in the mines of Leadville for distinguishing beds 

 of this horizon from locally bleached limestones of the Carboniferous. Chert 

 also occurs in the latter beds, but is always of dark, nearly black color, and 

 the microscope shows in them a very finely granular structure, while those 

 of the Silurian have frequently a radiate structure in the nature of spheru- 

 lites. In neither was it possible to detect any trace of the minute organisms 

 found in similar concretions in many other limestones. 



The average thickness of the White Limestone is from 120 to 160 feet. 

 A small percentage of chlorine can be detected in these, as in all the other 

 limestones from this region which were chemically examined. 



Parting Quartzite. Above the White Limestone occurs a bed of remark- 

 able persistence, but of rather variable thickness, to which the above local 

 name has been given, and which, on somewhat negative evidence, is regarded 

 as constituting the upper limit of the Silurian formation in this region. In 

 the cliff sections it has an average thickness of 40 feet, in one case attaining 

 a maximum of 70 feet. It does not differ lithologically from the numerous 

 white quartzites found at other horizons, but it is of geological importance 

 as determining the dividing line between the Silurian and Carboniferous 

 groups. In the cliff sections a brecciated structure is often observed in the 

 limestone immediately overlying it, and in one case, on the east fork of the 

 Arkansas, evidence of non-conformity by erosion was observed, which ren- 

 ders it possible that the Upper Silurian and Devonian formations may be 

 entirely wanting in this region. 



Fossils. Paleontological evidence as to the age of the above formation 

 is extremely meager. No form was actually found in place. Casts of a 

 Rhynconella, between R. neglecta and R. Indianensis of the Niagara epoch, 

 were found in a prospect shaft in California gulch, not far from the White 

 Limestone quarry, in such a position that they must have been derived from 

 the beds of this horizon at least fifty feet above the base of the formation. 

 Besides this, other specimens were brought in, obtained from talus slopes at 



