222 CARHON'IFKKors KdKMATIONS AND* FAUNAS OF COLORADO. 



'I'lu' (lill'cn'iK'cs hrlwi'cii my list ;iiul tliut in (lie Iv('ii(i\illc inonoiiTiipli inusl in 

 part 1)(' ciiaryod to the ek'inent of personal equation in (lie matter of idcntilication, 

 in wliicli tiio unfortunate condition of preservation in tiie ])re.scnt case alloM's unusual 

 latitude, and in part probably to losses from the collection since the publication of 

 the report. My table shows the present condition of the collection. 



In a previous statement I had reference to the speci(\s J'r<i<Jiiiiii.s nixfritus, Spi- 

 ri'fo'U I'meatii^ and possibly also Sjm'lfera rochyiiumtana^ ^vhich occur in the earlier 

 list but not in mine, for of these species nothing has come to hand which would 

 in any likelihood receive these identifications. My reasons for ditiering from the 

 published identitications arc in most cases given in the description of the species. 



From the faunal list above quoted the conclusion was drawn that the Blue lime- 

 stone is of Mississippian age. "While most of the 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 Spiriferinn do not occur in any of the 

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

 Cai'boniferous of this district."" Although this conclusion is, I believe, a correct 

 one, it hardly seems justified by the faunal list, which is composed chiefly of Upper 

 Carboniferous species, even though there are present a form of Euomplialim like 

 Eu. sjyergenensis and a new species of Spiriferina (which is said to resemble the 

 Coal Measure 8p. hentuchye'iisis) not found in the beds below, unless, indeed, the 

 term Lower Carboniferous is not employed as a division of the time scale equiva- 

 lent to Mississippian, but simplj^ indicates the lower part of the Carboniferous, an 

 equivocal and altogether objectionable usage. 



Of the general age of the Blue limestone as belonging to the Lower rather than 

 to the Upper Carboniferous 1 have little doubt. The fact hinted at in the mono- 

 graph, that there is a distinct faunal break (brought out more strongly in my lists 

 than in the earlier ones) between the Blue limestone and the formation next above, 

 is certainly significant. The presence of a retzioid of the type of Eumetria verneu- 

 iliana would, if the determination were based upon adequate material, be almost 

 conclusive; but as it is, the value of this evidence is somewhat diminished. This 

 occurrence, however, the absence of characteristic and common Upper Carboniferous 

 species, the affinit}' and sometimes identitj^ with Mississippian forms of such as are 

 present, together with the stratigraphic and zoological relations of this fauna with 

 other Mississippian faunas of Colorado, seem to me to be satisfactoiy evidence of its 

 Mississippian age at least. 



I will proceed to consider the fauna, species hy species, in its bearing upon the 

 age of the Blue limestone as Lower rather than Upper Carboniferous. Orthothetes'i 

 sp. more nearly resembles forms occurring in the lower portion of the Mississippian 



nU. g. Geol. Surv., Mon., vol. 12, 1886, p. 66. 



