254 CAKHONIKEROUS FORMATIONS AND KATNAS OK COLOKAIM). 



]i;irino' i(s f;uin;i w illi llial ol' tlic l)us;il Hermo.sa 1 liiid tli;it tlii'cc of its ,si.\ .species do 

 not appear lliei'e. The imist aliuiulant species in (he Molas, however — Sjplrlfer 

 /iiiii/i, .■/.•</.•< and SiiiiiiHilii xithlililti — lire also especially almndaiit in the base of the 

 Ilerniosa. and the somewhat peculiar variations are the same in each case. Not 

 enoiiyh is yet known of llie Molas fauna to assure a sate opinion, hut I expect that 

 it will )u-o\e to be closely related to that of the basal Ilerniosa. Lithologically, 

 however, the Alolas is a distinct thing, and it may be that the fauna will show cor- 

 responding moditications. This is, indeed, already indicated to a certain extent. 



On the other hand, while the Molas fauna shows a distinct relationship with that 

 of the Hermosa formation, it is completely disconnected with the Mississippian 

 fauna of the Ouray limestone, not a single species apparently passing into it from 

 below. In view of this fact, of the unconformity which preceded it, of the inde- 

 pendent significance of its fauna, and of its aflnnitj' with that of the Hermosa, the 

 rennsylvanian age of which is clear, it seems that the Molas formation also can 

 safely be placed in the tapper Carboniferous. 



The fauna of the lower Hermosa is less numerous than that of either of the 

 upper divisions or of the Rico formation. Only L'O species have been obtained from 

 it, while the middle has furnished 42, the upper 49, and the Rico 40. It is also less 

 varied, and is predominantly a brachiopod fauna. In the middle division a number 

 of gasteropod species appear, while the upper, in addition -to brachiopods and gaster- 

 opods, has a considerable force of pelecypods. The numbei' of brachiopod species 

 remains in a general way nearly consta,nt, there being 12 in the lower Hermosa, 17 

 in the middle, and 13 in the upper; but by reason of the accession of other types, 

 especially of the true MoTlusca, they hold a i-egularlj- diminishing ratio to the entire 

 fauna. Thus the}' rate at 60 jjer cent in the lower Hermosa, 40. .5 per cent in the 

 middle, and 26.5 per cent in the upper. In the Rico formation only 5 species have 

 been found, and the}'' represent only 12.5 per cent of the entire fauna. The Mollusca 

 are at the same time increasing in proportion, passing from 10 to 26.2 and 46.9 per 

 cent of the entire fauna in the different divisions of the Hermosa and to 87.5 in the 

 Rico formation. 



The collections from the lower member of the Hermosa formation were all 

 obtained from its basal portion. In areas where the Molas formation appears their 

 occurrence is just above its upper boundary, and where the Molas has not been dis- 

 tinguished the}^ were found within about 50 feet or less of the Ouray limestone, whose 

 upper beds contain, as I have ah'eadj' shown, a fauna of Mississippian age. The 

 fauna exhibited bj' these collections is of Upper Carboniferous age. There is, there- 

 fore, a great faunal break between the base of the Hermosa formation and the Ouray 

 limestone, corresponding to the unconformity by which thej' are known to be sepa- 

 rated. The time interval indicated bj' this faunal break is a long one, representing 

 much of Mississippian time and probably encroaching upon the ensuing Pennsyl- 



