3fesozoic and Coinozoic Geology and Palaeontology. 245 



nearly 150 miles froTn the present coast line, and to an elevation of 

 nearly 400 feet. 



The Miocene occurs in clisconnected patches, in river bluffs and in 

 ravines over the seaboard region, and extending from the shore and 

 the western margins of the sounds 50 to 75 miles inland. It consists 

 of beds of cla}', sand and marl, which are locally filled with shells 

 from 2 to 8 feet, and occasiouall}^ 10 to 20 feet. 



Prof. Theo. B. Comstock* said the Gri'een River Group is used to de- 

 signate that portion of the fresh-water Tertiary strata which lies di- 

 rectly above the coal group, and which is the present surface formation 

 over a large portion of the Green river basin, north of Fort Bridger. 

 The upper limit is not readily definable at present, the transition be- 

 tween the beds of this and the overlj-ing group being rather gradual 

 but the general character of the two formations, both lithologically and 

 palseontologically, diff'ers greatly. The Green river beds are mainly 

 composed of a series of shales, marls, and harder calcareous strata, 

 the latter especially containing quantities of the remains of fresh- 

 water forms of life, with laminated layers, literall}^ filled with the re- 

 mains of land plants of the Phaenogamous series. The outline of the 

 ancient lake basin, in which these strata were deposited, is not fully 

 determined, but there are indications that its eastern boundary wasj 

 outside of the present limits of the Green river basin, and there is no 

 room for doubt that the Uinta mountains, and the Wahsatch chain, 

 then, as now, towered above its surface. Northward it is equally clear 

 that the Wind River Range formed the shore of the great lake, with 

 probably more or less of gently sloping border during a portion of the 

 era of Lower Eocene deposition. The excessive erosion has exposed 

 the beds over the route from Fort Bridger to near South Pass, and 

 generally speaking, the rock contains a considerable portion of calcic 

 carbonate, with an abundance of ferric oxide produced by decomposi- 

 tion and oxidation. G^ypsum and calcite of different varieties are 

 abundant, frequently occurring as thin, papery seams between the 

 rock-layers, at oLher times forming masses of considerable extent. 

 Some of the layers are little more than a pure clay shale, while there 

 are a few quite arenaceous beds and some compact limestones. The 

 texture of the different beds is quite variable, but, in general, the 

 streams which have cut their channels through them are walled bj^ 

 nearlv vertical cliffs, and the buttes and benches for the most part have 



* Rep. of Reconnaissance of Yellowstone river andN. W. Wyoming. 



