eotlich.] SWEETWATER DRAINAGE PLIOCENE GROUP. 113 



with the Mobrara Group. We discovered several localities where such 

 fossils were found, hut they were too much weathered to be recognizable. 

 It appears that at some places the bones are impregnated with silica, 

 which renders their preservation almost a matter of course. Wherever 

 this is not the case, however, good specimens can only be obtained by 

 excavating. The identification of the fossils obtained is such as to leave 

 no room for doubt as to the position of these beds. 



An interesting feature in connection with this series was observed 

 near Agate Lakes, directly north of the Sweetwater. Innumerable 

 moss-agates are strewn all over the surface. Some of them are very fine 

 and cut admirably. So far as we could observe, all of them are water- 

 worn, occurring in the form of small pebbles. The area upon which these 

 agates may be found measures about 6 square miles. We endeavored 

 to find the original locality from which they came, but faded to do this 

 to our satisfaction. At some places we observed narrow, irregular 

 seams of quartz or mdky opal running parallel with the strata of white 

 marls. These contain dendritic inclusions of manganese, forming moss- 

 agates. In no instance, however, was the mineralogical character of the 

 quartz perfectly identical with that observed in the pebbles. Northeast 

 of the locality at which we collected the agates we found a number of beds 

 very highly Siliceous, which showed similar forms in the quartz. They 

 extended for a considerable distance. I am of the opinion that the nu- 

 merous agates strewn over the ground were derived from the western 

 continuation of the higher portions of these beds, which are entirely 

 removed. The quartz which appears so plentifully in this region must 

 have been held in solution, judging from the character of its distribution.. 

 Percolating through the loose marls and sands, the siliceous solution, 

 deposited quartz in narrow seams and in layers parallel to the stratifica- 

 tion. Most likely this occurred while the silica was in a hydrous con- 

 dition. It is an accepted fact that the " moss " in agates is but the result; 

 of impeded crystallization. Within this gelatinous quartz, then, this, 

 incomplete process may have gone on until eventually the inclosing 

 material became rigid. During our examinations of this region we saw 

 such enormous quantities of such material that the sum of our observa- 

 tions suggested this method of genesis. 



It is evident that the thickness of a group deposited in such a manner • 

 as this Pliocene series must vary considerably. The best exposures were 

 obtained low down on the Sweetwater and along the northern edge of" 

 the plateau. From what was seen at these localities, I estimate the: 

 maximum thickness of the Pliocene strata at 700 to 900 feet. Toward: 

 the west they grow thinner, until finally they pinch out altogether. 



Wyoming conglomerate. 



This term has been used by Emmons and Hague to designate the wide- 

 spread conglomeratic accumulations of drift which maybe assigned to < 

 the Pliocene period. It is entirely structureless, and composed of the^ 

 most varying material. Essentially it may be regarded as the product 

 of all formations existing within a given area. During the last era of " 

 extensive inundations it was deposited at the most convenient localities. 

 Along the entire northern slope of the Sweetwater and Seminole Hills, 

 we find enormous deposits of this material. No structure whatever can 

 be observed there, and the whole mass forms merely a huge cover of" 

 erratic bowlders. Their size varies somewhat, but does not reach any 

 considerable dimensions. We find the narrow gullies running down from . 

 the hiUsides cut into this conglomerate, and the tops of the ridges are: 

 8 G s 



