70 GEOLOGY OF THE DENVER BASIN. 



to this structure. The limestones are irregularly distributed throughout 

 the whole body nt" shales, one horizon, about two-thirds the distance from 

 base to summit, showing a specially large uumber. 



The limestones are the chief source of the fossils, but, except at the 

 horizon just mentioned, no part of the group is specially marked by either 

 abundance of tonus or the development of particular species. 



sandy zone. — This is an almost continuous band of soft, friable, yellow- 

 ish-gray, fine-grained sandstone, composed chiefly of quartz, with local 

 admixtures of clay either in small seams of shale or disseminated through 

 the mass. It raivh tonus an outcrop except in deeply eroded gulches or 

 in ditches, vet its presence is easily detected in the soil, and it constitutes 

 an excellent datum for stratigraphical reference. Besides the beds of thill 

 clay, two or three hands of impure limestone are locally present. 



The most peculiar development of this sandstone occurs about three- 

 fourths of a mile north of the northern boundary of the map. It is here 

 ncarlv 350 feet thick, and at first sight resembles the basal sandstones of 

 the Laramie, 'nit on inspection is found to he calcareous in certain layers, 

 to contain an abundance of the more common Pierre fossils, and in its 

 coarser mineral constituents to differ materially from the purely quartzose 

 beds of the Laramie. 



ironstones. — These are chiefly small, irregularly shaped concretions of 

 hardened, calcareous clay, from 1 to 3 inches in diameter, containing a 

 variable amount of iron. Their color is between an ashes-of-roses and 

 deep rust A larger ironstone, less frequenl than the foregoing, is of much 

 coarser texture and contains a higher percentage of iron. The third occur- 

 rence of iron is in the form of narrow, unstratitied seams of impure linionite. 



Carbonaceous matter. Tills ocelli's ,'IS luillllte fragments Of phlllt tissue dlS- 



tribllted throughout the clays and their contained limestone concretions, 

 ill the latter forming one of the characteristic features. 



Gypsum. — This is distributed in small scales throughout the clays. 



The life of the Pierre is given in the tabular statement of the fossils 

 of the 1 >enver field. 



