18 SUEVEY OF COLORADO AND NEW MEXICO. 



in an article in Silliinan's Journal, Marcli, 18G8, ^' that all tlie lignite 

 tertiary beds ot the west are but fragments of one great basin interrui)ted 

 here and there by the upheaval of mountain chains, or concealed by the 

 deposition of newer formations." 



As soon as the lignite beds reappear southward, the aspect of the 

 country changes. The distant hills that flank the mountains on the 

 right are still the pebbly conglomerate beds. And in the valleys of the 

 little streams, about four miles south of Spotswood Springs, are several 

 exposures of beds which are undoubtedly older tertiary. There is here 

 shown a deep yellow arenaceous indurated clay layer, passhig down into 

 an ashen-brown grit, with rusty yellow concretions. All over the hills 

 are scattered the greatest number of water-worn boulders. The lignite 

 strata incline in the same direction as those of the more modern deposits. 

 The dip of the former is about five degrees to ten degrees, the latter one 

 degree to three degrees east, from the mountains. There are many 

 other localities where the evidence of non-conformity of the two deposits 

 is perfectly clear. 



A little further eastward on the Dry Creek the ridge is clipped with 

 gray, loosely laminated sandstone; while in the indurated arenaceous 

 bed*^ below are beds of massive rusty sandstone, the same as those that 

 compose the natural fortifications about thirteen miles southwest of 

 Cheyenne. The ridge extends far eastward into the plain, with the beds 

 nearly horizontal. 



Near a high conical butte a little further southward we find the lig- 

 nite beds dipping 85° with a strike nearly north and south. And in the 

 south and southwest we can see the upturned ridges of cretaceous and 

 older sedimentaiy formations composing the flanks of the mountnins. 

 The modern tertiary and the superficial drift deposits have been so re- 

 moved from tlie mountain side — about ten or fifteen miles north of 

 Cache la Poudre — that all the unchanged sedimentary rocks in this 

 region are revealed in the form of inclined ridges, which gradually die 

 out in the plains eastward like sea waves. 



A bed of the laminated chalky marl of JSTo. 3 with Osirea eongesfa and 

 Inoceramus problematicus is particularly noticeable. In the lignite beds 

 harder layers of rusty sandstone, with loosely laminated arenaceous 

 clay, and the softer materials are worn away by erosion, leaving the 

 harder rocky layers projecting above the surface in long lines like walls. 



Near Park station, about twelve miles north of Cache la Pondre, the 

 upheaved ridges begin to spread out, revealing very clearly to the 

 scrutiny of the geologist all the sedimentary rocks, to the tertiary in- 

 clusive. Commencing in the plains about ten miles east of the margin 

 of the mountains we find a series of gently inclined tertiary sandstones, 

 dipping from 5° to 10°. Then come the complete series of cretaceous 

 strata in their order, inclining from 20° to 35°. Underneath the ridge 

 capped with the sandstone No. 1 is a thin belt of ashen-gray marls and 

 arenaceous marls, with one or two layers, two to four feet thick, of hard 

 • blue limestone, which I regard as of Jurassic age. These pass down 

 into light reddish, loose arenaceous sediments. Further toward the 

 mountains, come one to three ridges of brick-red sandstone, and loose 

 red sandy layers, sometimes variegated. Close to the margin of the 

 mountains, sometimes forming the inside ridge, is a bed of whitish lime- 

 stone, underlaid by dull purplish sandstone and pudding-stones, which 

 are probably of carboniferous age. These beds dip at various angles, 

 from 30° to 60°, and, as far as 1 can determine, conform generally to 

 the inclination of the metamorphic rocks which comi^ose the mountain 

 nucleus. 



