wBiTE.J CARBONIFEROUS SYSTEM. 25 



exposures of strata at a very few points in Nevada, Colorado, and 

 Idaho, collections of fossils haA^e been made that are not only specifically 

 different from those which are so widely distributed in the Carbon- 

 iferous rocks of that great region, but they are in part specifically 

 identical with some that are found only in the Subcarboniferous strata 

 of the States of the Mississippi Valley. The first of the following 

 groups mentioned, however, is not to be here regarded as distinctively 

 Subcarboniferous, although it may be so in fact, so far as anything is 

 known to the contrary. 



THE LOWER CARBONIFEROUS GROUP. 



In some parts of the great Eocky Mountain region other observers have 

 made out satisfactory lithological distinctions between the Lower and 

 Middle Carboniferous Groups; but within the limits of this district the 

 line of demarcation is so indefinite that I was not able to do more than 

 make out a general separation of the two groups, I nowhere found any 

 well-marked line of separation, and there being, as before stated, no 

 sufficient paleontological distinction between the two groups, very few 

 characteristics can be named as separating them from each other. 



The Lower Carboniferous Group in this district consists of a large pro- 

 portion of compact limestone strata that are usually more or less fossil- 

 iferous, alternating with layers of sandstone that are also usually some- 

 what calcareous. In general aspect this formation is of an indefinite 

 reddishbrownor ferruginous color, often like that of the Weber Quartz- 

 ite. The strata of this group are well exposed along the northern 

 border of the eastern half of the Yampa Plateau, and also in Junction 

 Mountain, of which mountain it forms a large proportion. It is there 

 seen resting upon the Weber Quartzite, and in turn underlying the Mid- 

 dle Carboniferous. In Yampa Mountain, which is largely composed of 

 the Weber Quartzite, the Lower Carboniferous is not satisfactorily sep- 

 arable from the middle group. 



THE MIDDLE CARBONIFEROUS GROUP. 



In this district, as before stated, the Middle Carboniferous Group is 

 separable from the Lower only in aspect, and by some general lithological 

 characteristics. The greater proportion of the bluish-gray limestones 

 which it contains gives it a lighter aspect than that of the Lower, and 

 distinguishes it also from the sandy strata of the overlying Upper (Car- 

 boniferous Group. It is more fossiliferous than either of the other two 

 divisions of the Carboniferous system in this district; but in consequence 

 of the compact and cherty character of the rock, the fossils are seldom 

 well preserved. The limestones of this group, besides being cherty, are 

 often unevenly bedded. As a rule, they lack those marly and carbona- 

 ceous or clayey layers of shale that so commonly separate the lime- 

 stone layers of the equivalent of this group in the Mississippi Valley 

 The Middle Carboniferous Group contains much the greater part of the 

 region, and from which softer layers most of the fossils are there obtained, 

 calcareous strata found in this district, the prevailing material being 

 almost everywhere sandstone and sandy shales. 



THE UPPER CARBONIFEROUS GROUP. 



The Upper Carboniferous Group is quite clearly distinguishablefrom the 

 lower. In this district it is almost wholly composed of a light, yellowish- 

 gray sandstone, often cherty, and containing only a few calcareous layers. 



