CAKBONIFEEOUS UNDIVIDED. 367 



Grand Canyon, at Kanab Creek, and near the Uinkaret Mountains the upper sandstone is soft 

 and produces a slope in the profile, while the lower is hard and unites its steep escarpment with 

 that of the Redwall limestone. The sandstones contain no fossils, but an intercalated limestone 

 below the middle of the series at Canyon Creek bears the familiar Coal Measure shells. 



The RedwaU limestone has, upon fresh fracture, a gray color and shows its red rust only on 

 weather-stained cliffs. In its general character it is heavy bedded to massive. At the top 

 sandstone alternates with the limestone for from 200 to 500 feet. Through its lower half the 

 firm Umestone is interrupted by occasional shaly bands, which serve to break its escarpment 

 into a series of narrow terraces, but above them stands a sheer perpendicular face, of from 800 

 to 1,000 feet. The average total thickness is 2,500 feet. 



Accompanying these stratigraphic notes are lists of fossils, according to which 

 the highest beds were referred to the "Permo-Carboniferous" and the upper half 

 of the Redwall to the "CoaliMeasiires" (Pennsylvanian), with a doubtful determi- 

 nation of "Lower Carboniferous" below the middle. Gilbert regarded it as possible 

 that the Redwall extended down to include Devonian. 



A recent publication on the stratigraphic succession and age of the rocks of 

 northern Arizona is that by Darton,^*^ already cited. 



J 13. COIiORADO. 



The Carboniferous terranes of Colorado have been described, compared, and 

 correlated by Girty ^^^ in a monographic work, to which reference should be made 

 for a general treatment of the whole subject. 



The following extracts present the general relations : 



The lower Carboniferous strata of Colorado are geologically comparatively unimportant 

 by reason of their thinness, but they are widely distributed and form one of the richest metal- 

 liferous horizons in the entire rock series. This horizon is everywhere a limestone or a dolomite, 

 sometimes more or less interstratified with shale, but owing to minor differences of lithologic 

 character and circumstances of distribution several local names have been received by it. In 

 central Colorado this limestone is called the LeadAolle limestone. In the San Juan region it is 

 called the Ouray limestone, and along the Front Range it has received the name of the MiUsap 

 limestone. The lower portion almost everywhere contains a distinctive and umnistakable 

 Devonian fauna. At least this fauna has been found at so many and such widely separated 

 points that it may safely be regarded as generally present over the central part of the State 

 wherever the formation is brought to view. In the few fragments of it which are stiU visible 

 along, the Front Range, however, the Devonian fauna seems to be wanting. 



********* 



Along the eastern margin of the Front Range the Mississippian limestone appears in several 



places. It is in this tract called the MiUsap Umestone. The MiUsap limestone was first 



described in the Pikes Peak foho, from which I abstract the following: It consists only of local 



remnants resting upon the Fremont limestone in Garden Park and along the western line 



toward Canyon. It is represented by about 30 feet of thinly bedded variegated dolomitic 



limestone, with a few thin sandstone layers. Chert nodules in the upper limestone layers 



carry casts of Spirifer rockymontanus and Seminula sultilita. It is divided from both the 



formation which preceded and that which followed it by an erosional unconformity. 



********* 



In the San Juan region Mississippian time is represented by the Ouray limestone, the major 

 portion of the formation, however, containing a Devonian fauna. 



The Ouray Umestone is defined by Cross and Spencer somewhat as follows: It consists of 

 massive beds of limestone separated by thin intercalations of marl or shale. Certain thin 

 bands are frequently quite coarsely crystalline, but the large mass of the formation is a dense 



