250 CAWUdNIKEROTTS FOKMATIONS AND FAUNAS OF COLORADO. 



tli;it tlu'sc two 'm(lc|)('ii(lcnt iiitciiicting factors be as fai' as ))riictic:il)l(' differentiated. 

 A inetiiod which would give luatheinatical expression to both tliese forces separately, 

 instead of to their resultant, would be very useful. Qualihed as must lie their 

 acceptation, the tiyures just j>-iv(Mi indicate a less intimate relation Ix'tween the 

 HeruiDsa and Kicd I'oruiations tiuiii between the several incinlirvs of th(> Herraosa. 

 In whatever way the Uvo faunas lie viewed a marked ditl'erence in facies uuist be 

 o-i'anted. As in the Dolores River region at Sinbads Valley the supposed equivalent 

 of the Hermosa greatl}* exceeds in thickness the t^'pical sections, and as its upper 

 beds contain a fauna at once different from that of the Rico formation, and only 

 in a less degree from that of the Hermosa, it would appear as if the faunal break 

 between the Hermosa and Rico formations, and to a certain extent the marked 

 lithologic change which it accompanies, were the expression of a period of 

 nondeposition or erosion represented in other areas by sediments and faunas not 

 found in the San Juan. 



It has been noted that the fauna of the lower division of the Hermosa also has a 

 certain individuality of facies; but it appears from the consideration of correspond- 

 ing horizons in other regions adjacent that the species which our collections indicate 

 might serve to distinguish the lower from the middle division are not as sharply 

 limited in range as in the San Juan. Possibly the rea.son that a faunal break seems 

 to exist in the San Juan region is that there the collections from the lower Hermosa 

 were all made at the very base, some 600 or 700 feet of strata intervening between 

 them and the middle division. 



Ouray. — It would be difficult to say whether the small, isolated Carboniferous 

 area about Ouraj' would more appropriately be included with the San Juan or with 

 the Dolores River region. I have, however, decided to discuss the few collections 

 from Ouray in connection with the former. 



The Ouray limestone is not exposed at Ouray in such a manner that its strati- 

 graphic relation to the Uppei' Carboniferous formations could be satisfactorily made 

 out. I was not sufficiently acquainted with the upper part of the Paleozoic section 

 in the San Juan to determine on lithologic evidence (for the Rico fauna was not found 

 at Ouray) the position there of the Rico horizon. Therefore, while it is safe to say 

 that the Ouraj- collections came from the equivalent of the Hermosa formation, their 

 position relative to either the top or the base of the section was not determined. I 

 am of the opinion, however, that the collection made on the west side of the valley 

 represents a higher horizon than those made on the east side. The latter, while con- 

 taining some of the forms supposed to characterize the lower Hermosa — as, for instance, 

 Productus infiatim and Productus gallatinensis — contain certain* others, such as 

 Spirifer rocJcy)iw7itam(s, rather distinctive of the upper divisions. The same is true 

 of the collection from the west side of the valley, fi-om which it apjiears that the 

 differences distinctive of the lower and middle divisions of the Hermosa formation in 



