1()4 cakhonikkkous fokmations and faunas of colorauo. 



Leadvillc and Oui-iiy limcstoiios. Honip further pvidoiice may possibly l)e'found in 

 the sandstoiio dikos occurriiiy in tlio Inwor doloniitic moiubor, to wiiich Spurr has 

 called adentiiin. In this conneetion. h(i\ve\"er, it remains to bo shown thai the 

 Devonian phase of Leadville deposition is represented in the Aspen beds, and that 

 the stratijiraphie plane noted by Spurr corresponds to the time boimdaries l)etween 

 the Devonian and the Carboniferous. Regarding the former point, the wide distribu- 

 tion of the Devonian fauna and its I'ecognition at points as near as Salida, Glenwood 

 Springs, and the Crested Hutte quadrangle, make its presence at As]:)en a nuitter of 

 great probability. Regarding the second particular, on the other hand, Spurr very 

 naturally seeks to correlate the lower or dolomitio member at AspcMi with the entire 

 Leadville limestone at Leadville, which is only 2(I0 feet thick and doiomitic through- 

 out, and with the lower, doiomitic portion of the Leadville in the Crested Butte 

 quadrangle. This inference would tend to indicate that the line between Devonian 

 and Carboniferous is not coincident with that between the doiomitic and calcareous 

 portions of the Leadville, since I refer fossils from this foi-mation at Leadville to the 

 Carboniferous. It is possible that the difference in facies between the Leadville 

 faunas as we know them, from Leadville on the one hand and Aspen and the Crested 

 Butte region on the other, may be due to a difference of horizon in agreement with 

 the correlation suggested by Spurr. It is noteworthy that the Millsap limestone, 

 whose fauna is similar to that at Leadville and different from those at Aspen and 

 Crested Butte, is also doiomitic. From the dolomite in the Aspen region no fossils 

 are known, but Spurr mentions a number of foraminiferal genera as having been 

 identified by R. M. Bagg in thin sections of the Blue limestone. 



In the Anthracite-Crested Butte folio, where I believe the name Leadville 

 limestone is first mentioned, it is described somewhat as follows: Its thickness varies 

 from 400 to 525 feet and it consists principally of beds of limestone from 5 to 3( > feet 

 thick, sometimes separated b_v bands of quartzite or calcareous shale. At the top of 

 the formation is a massive bluish-black bed 75 to 150 feet thick, known to miners as 

 the "Blue limestone." Below this the limestones are grayer, apparentl}' somewhat 

 doiomitic, and carry a few dark-grav or black chei'ts. The Leadville here seems to be 

 much thicker than at Leadville itself, a condition which can be adequately exi^lained 

 bj^ the erosional unconformit}' by which the formation is followed. 



The character of the Leadville at some of the points where it is best known has 

 been briefly outlined. In the Grand River region it has been studied in more or 

 less detail at AsjDen, at Red Cliff, on Grand River, and in the Tenmile district, and 

 the statement seems justified that it is generally present in that area. In the Elk 

 Mountain region also it may be said to be in all probability continuously present. 

 Its occurrence in the Crested Butte district has already been described. Endlich 

 gives a section made at Fossil Ridge south of the Gunnison, in which the Leadville 

 probably includes 260 feet of light grav and yellowish limestone and shale, 40 feet 



