100 GEOLOGICAL SURVEY OF THE TEREITOEIES, 



Section of Jurassic heds ex^posed near Bear Creel'. — William S.Holmes. 



No. 



Nature of strata. 



Top. 

 Variegated red and green arenaceous, some argillaceous shales, with 



loose brick-red sandstones and gypsum 



Like No. 5 above, with white or rusty yellow sandstones 



Red arenaceous shales, and purple-red sandstones 



Cherty limestone, bluish 



Red arenaceous shales, like No. 3 



Base. 



Total, (about) 



Thickness 

 in feet. 



200 

 100 

 300 



150 



770 



Near the South Platte the general characters remain much the same 

 as near Bear Creek ; the white sandstones, which in the section (Plate I) 

 are placed near the base of the Jurassic, should more probably be includ- 

 ed in the lower formation as the equivalents of the yellow sandstones 

 at the top of the Trias. The South Platte section was made by Mr. J. T. 

 Gardner. 



The cherty limestone, near the base of this group, seems to be a very 

 persistent bed, and being rather hard, often forms a minor hog-back 

 ridge. It varies slightly in character. At Bear Caiion it is porous, 

 with the silica apparently disseminated through the bed. A rude at- 

 tempt here at making lime from it has failed. At Ealston Creek, how- 

 ever, the silica seems more concentrated in the numerous nodules of 

 pink chert, which are sprinkled thickly through the bed, the matrix 

 being compact and hard. A large kiln here in active operation seems 

 to be making a very excellent quality of lime. 



Between Ealston and near Golden City, and again at several points 

 further south, more or less rude lime-kilns have been built for the pur- 

 pose of making lime from this bed, the product usually seeming to be 

 very white, thougli I know nothing definite of its adhering qualities. 

 Kear Bear Creek a small mill was erected for the purpose of grinding 

 the gypsum which occurs in the Jurassic beds near by, and which Dr. 

 Hay den, in his third annual report, for 1869, p. 136, describes as fol- 

 lows : " The gypsum is amorphous, but very white and pure, and would 

 make the finest of casts and moldings. Some of the layers are sus- 

 ceptible of a high polish, like the California marbles, only they are of a 

 more uniform white color. 



It is possible that in this group also belongs the •' silica in a state 

 of fine division," or an " aggregation of very fine grains of quartz" 

 which has been opened at Golden City. 



THE CRETACEOUS 1^0. 1. 



Next above the last group of strata comes a series of sandstones, 

 which, though only from one to two hundred feet thick, form a more 

 convenient horizon for reference than any group in the sedimentary 

 series. Though this is partially due to its well-defined and pretty con- 

 stant lithological characters, it is chiefly due to the fact that its hardness 

 is so constantly and so considerably greater than the beds either above 

 or below, that it forms a more persistent hog-back ridge than any other 

 group. Except when cut by the streams, this horizon can be traced, 



