124 CAKBONIFEROUS FOBMATIONS AND KAUNAS OF COLOBADO. 



The Molas t'oi'iiuitiDii is the lowest of th(^ three Upper Carboniferous foruuitions 

 distinguished in I he Aninuis Valley and adjacent region. It is distinguished on 

 lithologic grounds, because it records a preceding interval of erosion, by which 

 formations of Mississippian age were in large measure destroyed, and, in a subor- 

 dinate degree, because of certain faunal peculiarities which further collections may 

 or may not affect. The Molas beds were not included in the original definition of 

 the Hermosa formation, although the Hermosa was said to rest on the Ouray lime- 

 stone. In the localities examined before that definition was formulated there was, 

 in fact, a hiatus in the observations, the thin Molas beds being nowhere well 

 exposed. This formation is especially characterized by its deep-red, friable, sandy 

 strata, which are variably calcareous and often shaly. They are seldom very 

 distinctly bedded, and disintegrate so rapidly on weathering that good exposures 

 are rare. In the lower part of the formation dark chert nodules al)ound, often 

 making a large part, of flat lentils or discontinuous la3'ers. The best section of 

 the formation thus far observed is situated on the southwest slope of the Needle 

 Mountains, on the south side of Tank Creek. At that locality there is a continuous 

 section, about 75 feet in thickness, representing the whole formation as there 

 developed between the Ouray limestone and the Hermosa complex. There is no 

 distinct stratification in the section. At the base is a zone of gradation into the 

 Ouray limestone, for the upper zone of the latter is much broken up with the red 

 calcareous mud of the Molas filling the interstices. This filling of the formation is 

 very common. For some feet above this transition zone there is a chaotic mixture 

 of chert and limestone fragments, with not a few of bluish or white cjuartzite, but 

 none of granite or schist. In all the lower part of the section much of the material 

 is an impure limestone, reddish in color, but with the saccharoidal texture of the 

 Ouray. It is probably a limestone sand rock. The matrix in which the fragments 

 of chert, quartzite, and limestone are held is a red marl-like material. There are in 

 places indications that a calcareous mud was broken up before consolidation, and 

 was worked over with the fragments of foreign rocks. Gradually the section 

 becomes more and more sandy, but chert fragments were found almost up to the 

 fossiliferous limestone of the Hermosa. 



The shale contains nodules of chert carrj'ing a Mississippian fauna related to 

 that in the upper portion of the Ouray limestone, and it is believed that they were 

 derived from the Oura}-. Otherwise the shale is almost unfossiliferous; but from 

 some thin limestones in the uppermost part of the formation, in a single locality in 

 the Needle Mountains quadrangle, a few species were obtained by Mr. Cross which, 

 while not of themselves altogether diagnostic, are unlike the Mississippian fauna of 

 the Ouray and similar to that which comes in at the base of the Hermosa, which is 

 clearlj^ Upper Carboniferous. On this account the beds down to the top of the 

 Ouray are regarded as belonging in the Upper Carboniferous. 



