BECAPITDLATION OF PALEOZOIC FORMATIONS. 193 



for within these limits the Carboniferous fauna is known to occur. Thus defined it 

 is made up of sandstones and conglomerates, with intercalated shales and sandy, 

 fossiliferous limestones. Lithologically the Rico formation resembles the Dolores 

 formation, which immediately overlies it, and which contains the median unfossilif- 

 erous portion of the Red Beds, as well as the upper portion, which carries Triassic 

 fossils, "The g-eneral characters of the Rico formation in the vicinity of Rico are, 

 first, its calcareous nature, in which it resembles the strata above and below; second, 

 the very arkose character and the coarseness of its sandstones, in which respect it 

 differs from the Hermosa and resembles the Dolores; and, third, its chocolate or 

 dark-maroon color, which contrasts sharply with the gray or green of the Hermosa, 

 and which is more or less distinct from the bright vermilion of the Dolores. ' ' The 

 bulk of the formation is made up of sandstones and sand}' shales, composed of such 

 materials as are derived from the disintegration of granite. The sandstones are 

 mostly coarse or conglomeratic." Intercalated with the sandstones and shales, 

 which are for the most part very calcareous throughout, there are several beds of 

 impure limestone, some an earthy gray and others sandy and red in color. 



In the Engineer Mountain quadrangle the Hei'mosa formation is underlain by 

 beds of no considerable thickness but of rather marked lithology which Cross has 

 differentiated and named the Molas formation. His description of this series can be 

 condensed as follows: 



The Molas beds were not included in the original definition of the Hermosa 

 formation, although the Hermosa was said to rest on the Ouray limestone. 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. 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 localitj' 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 Ijroken up with the red 

 calcareous mud of the Molas tilling 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 quartzite, but 

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

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

 of the Ouray. It is probablj' 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 consoli- 

 dation, and was worked over with the fragments of foreign rocks. Gradually the 

 1436i— No. 16—03 13 



