284 Tertiary. 



Cretaceous, and to bind the Eocene, the Miocene and Pliocene with 

 the present as one connected age. The lower Eocene lake deposits are 

 found superimposed conformably upon the brackish deposits of the 

 Fort Union Group. The Eocene is divided in ascending order, into 

 the Wasatch, Green river and Bridger Groups, though these are found 

 conformable with each other in some places and mark a continuing age. 

 The Wasatch is again divided by having the lower marls called the 

 Puerco Group, and the Green River Group is divided, for convenience, 

 in some places, into an upper and lower Green River Group. It would 

 seem that all other names proposed for the fresh-water Eocene deposits 

 are synonyms, though the equivalency of strata has not, probably, in 

 all cases, been determined. The Miocene is known in the lower part 

 as the Wind River Group, and higher as the White River Group, and 

 sometimes the latter name is used to the exclusion of the former. In 

 some places the upper Miocene is called the Truckee Group. The 

 Brown's Park Group, Sweetwater Group and Monument Creek Group 

 are Miocene, but their exact position is not so fully determined The 

 two latter are supposed to be equivalent to part of the White River 

 Group, and the former may be so too. The Pliocene is very properly 

 called the Loup Fork Group. It has also, in part, received the name 

 of the Salt Lake Group, and a conglomerate of the age of the upper 

 part of the Pliocene is called the Wyoming Conglomerate. The dis- 

 tribution of these Groups and questions of sj^nonymy, have been con- 

 sidered at some length, in preceding pages, and in the near future 

 the nomenclature will no doubt be more definitely established. 



The northern drift does not occur in California, nor on the Pacific 

 coast as far north as British Columbia and Alaska. There are no in- 

 dications throughout the Rocky mountain region of any general ice 

 action. There are no such exhibitions of scratched and grooved rocks 

 succeeded by fossiliferous marine clays and sands with bowlders, as 

 occur in the New England States and St. Lawrence region, nor of 

 scratched rocks and ancient soils succeeded by cla}^ sand and gravel 

 with loowlders, as occur in the central part of the continent; but, on the 

 contrar}^ the whole region may be regarded as an absolutely driftless 

 area, except as to local drift produced upon the shores of the Tertiary 

 lakes, and more or less distributed by the rivers, that in the course of 

 time cut out the canons which drained them. On the borders of the ancient 

 lakes, on the borders of the ancient lake-like expansions of the rivers, 

 and on the borders of the ancient rivers, there are terraces which mark 

 old shore lines at various places from Mexico to Alaska, and especially 



