HAYDEN ON ARTESIAN BORINGS IN WYOMING. 185 



Fourth series. — Arenaceous clays, Upper Miocene, turtle-shells; 

 no other fossils observed. 



The third series of beds contains the plants and shells that were found 

 in such profusion near Barrel Springs on the Muddy. 



The first series is the Laramie or Lignitic group; the second, the 

 Wahsatch or Vermillion Creek group, the former name having the 

 priority, and having been attached to the great group of reddish 

 sands, clays, and conglomerates west of Fort Bridger, in 1870. This 

 group has since been found to extend southward through Western 

 Colorado into New Mexico. The third series embraces the Green Eiver 

 group. The fourth series is an extension eastward of the Bridger group. 

 The Wahsatch group includes the lowest of the purely fresh-water beds 

 in this region, and the evidence seems to be quite clear that it is the 

 equivalent of the purely fresh-water Lignitic strata in the northwest 

 along the Missouri Eiver. There is every reason to believe that in this 

 region, as is so well shown at Bitter Creek, the group is an exten- 

 sion upward of the brackish-water Lignitic strata without any break in 

 the continuity. So in the Northwest, at the base there is a mingling of 

 brackish- water forms with the fresh -water species, though on a far less 

 extensive scale than in Wyoming, Utah, or Colorado. In the brackish 

 beds in Wyoming, several species of Mollusca were determined by Mr. 

 Meek to be identical with well-known forms of the Upper Missouri, as 

 Viviparus trocMformis, V. conradi, and Unio priscus. As it is not the 

 purpose of this brief article to discuss the age of these groups, we may 

 conclude with the remark that the results of each year's explorations 

 show more clearly the remarkable unity of the geological structure of 

 the interior of our continent. All the older formations, from the Silu- 

 rian to the Cretaceous inclusive, may even now be correlated with a 

 good degree of certainty, and the vast number of Tertiary basins are 

 brought into close relations with each other. 



13 BULL 



