52 CARKONIFEIKHIS FORMATIONS AND FAUNAS OF COLORADO. 



v.'ould thus bocouu' sUuulurtl for the Elk MoLiiitaiii !ire;i, tlio Aspen ;uid Teimiilo 

 sections for the Grand River area, and the Leadville section for the South Parle area. 

 The un'iis of the Aspen nionognqih and the Tciunilc folio are botli properly 

 included in the Grand River region, but they are so marginal as to be geographically 

 nearer related, the one to the Crested Butte, the other to the Leadville district. 

 Nevertheless, I will consider the sections described in these reports, and use them as 

 a basis of comparison with other geologic reports covering the Grand River region. 



The Paleozoic geologic section is remarkabh' uniform over this central Colorado 

 area, and essentialh' the same subdivisions are recognized and essentially the same 

 nomenclature is emplo^'ed both in the four monographic publications just mentioned 

 and in many smaller papers of later issue which it will not be necessarj' specificali}^ to 

 consider. In the case of the publications of the Haj'den survej^ and others of an early 

 date, the system at present in use of discriminating lithologic groups and giving to 

 them local names was not employed, and the strata were divided, if at all, into groups 

 supposed to have a definite time value. As, however, paleontologic evidence was often 

 deficient, and as the discrimination of these groups was frequentlj' made on evidence 

 more or less fanciful, and in a different manner by different geologists, the result is 

 far from satisfactory. It is usualh^ possible in these reports to identify by their 

 faunal and lithologic characters certain horizons, such as the Sawatch quartzite, the 

 Leadville limestone, the Maroon formation, etc., when they are mentioned, but seldom 

 to determine satisfactorily, even "in the few detailed sections which the earl}^ litera- 

 ture contains, what should be the limits in them of the formations now recognized. 

 This is so far true that I have attempted a close correlation of these sections onlj' in 

 a very few instances. Much of the older literature, therefore, adds to our knowledge 

 of the distribution of tiie formations or of geologic structure, with which I am not 

 so much concerned, but not to the details of their lithology, paleontology, or mass. 



The geology about Leadville, especiallj' with regard to its mining industries, 

 was described many times in papers of the briefer sort before the Leadville mono- 

 graph was published, and an almost equal number followed its appearance. In a 

 kimilar manner much careful observation had been recorded, and the geologic section 

 was almost as well known as it is to-day, when the Aspen monograph appeared. 

 Lakes in 1887, Emmons, Siver, and Brunton in 1888, Henrich in 1889, Newberry in 

 1890, and Carljde in 1893, had all written accounts of varying length and excellence 

 before Spurr brought his work to publication. But since his is probably the most 

 authoritative account, as it is certainlj- the longest and most elaborate one, I Avill 

 confine my discussion to it alone and neglect the claims of temporal priorit3\ 



The section at Aspen, condensed from Spurr's monogTaph," is as follows, 

 descending: 



aU. S. Geo]. Surv., Mon., vol. 31, 1898. 



