74 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



shoTTS palfeontological affinities to tlie Devonian in its lowest mem- 

 bers. As the above appellation is liere used, it implies a slight separa- 

 tion, both lithological and faunal. We have withiii the region under dis- 

 cussion a succession of strata which can apjiropriately be separated into 

 two divisions. The older one of these I shall quote as " Subcarbonifer- 

 ous." This term is synonjmious, in this instance, with the "Lower Car- 

 boniferous" of some other writers, and the analogies will be apparent 

 upon comx)aring descrij)tion8. 



Subcarhomferous. 



Dr. Peale characterizes the Subcarboniferous group of Colorado as 

 " mainly massive limestones, grading below into Devonian ( ?)." This, 

 in a great measure, will express the constitution and relation of tlie 

 parallel division near the Wind Eiver Mountains. The third chain of 

 the range, which has in jirevious pages been regarded as analogous in 

 structure to hog-backs, is composed of Carboniferous strata. West of 

 Camp BroAvn this chaia enters our district and continues well defined 

 along the base of the range until it is lost in the scattering hills east of 

 Miner's Delight. If we examine the western bases of the isolated hills 

 composing the chain, we will find there the exposures of Lower Carbon- 

 iferous strata. They are comi)osed mainly of massive beds of limestone, 

 slightly dolomitic in i)art. Interstrata of dolomite and some dolomitic 

 shales separate them. Accumulations of arenaceous material within 

 certain strata imi^art to them the character of calcareous sandstones. 

 Wherever they rest ux)on members of the calciferous series it becomes a 

 difficult matter to draw the line betv/een the two. It can be accom- 

 plished, however, if the latter are followed in a southeasterly direction 

 until, finally, the Lower Carboniferous is found to rest directly on the red 

 Potsdam quartzites. 



Conformable with over and underlying beds, this series dij^s to the 

 northeast. About 38° may be regarded as the dip angle. Locally this 

 is increased somewhat, more i^articularly near the ends of the hiUs com- 

 posing the outlying chain. For some distance ui^wards along the west- 

 ern slope of the hills the characteristic, heavy betls, either of gray or 

 yellowish color, may be distingTushed. The various interstrata are thin, 

 and upon weathering show prominently. North of our district, in the 

 regions of greater Palaeozoic development. Dr. Hayden and others have 

 recognized mainly two di^dsions in the Carboniferous formation. There, 

 as here, the line of separation between the two hns been difficult to 

 establish. 'No satisfactory division can be obtained m the Wind Eiver 

 region by the occurrence of fossils, and I have therefore employed, in a 

 measure, the lithological character to furnish it. We find, upon exam- 

 ination, that the gray and yellow limestones, as well as the included 

 dolomites, disappear some distance above the base of the bluffs. They 

 are covered and succeeded by massive beds of blue and some almost 

 white limestone. At the junction of these two series I place the divid- 

 ing Mne. Besides being thus marked, it is indicated by a change in the 

 occurrence of fossils. This change is not so much one of species as one 

 of specimens. In the lower group fossils are found but sparingly, but 

 higher up they occur more abundantly. Eegardmg the termination of 

 the Lower Carboniferous as existing where it has just been i^laced, we will 

 have a thickness of 600 to 700 feet for it. This is subject to some varia- 

 tions, dependent upon local widening of certain strata. 



Most of the fossils contained in this series are very poorly preserved, 

 in rare instances, only, admitting of more than generic determination. 



