212 GEOLOGY. 



clastic rock and some lava. The beds are for the most part but imper- 

 fectly indurated, and their erosion has locally given rise to the topog- 

 raphy characteristic of " Bad Lands." 



Subaerial formations of Eocene age have not been certainly iden- 

 tified far east of the Cordilleran region. It has recently been sug- 

 gested, though with little probability, that certain preglacial gravels 

 of Indiana may belong to this system. 1 



Igneous activity. — The period of igneous activity which was inau- 

 gurated with the close of the Cretaceous seems to have continued, 

 at least intermittently, throughout the Eocene, for igneous rocks of 

 Eocene age are found in California, 2 Oregon, 3 Washington, 4 Idaho, 5 

 Montana, 6 Wyoming, 7 and Colorado. 8 In some places, the exact age of 

 the igneous rocks associated with Eocene sedimentary beds has not 

 been determined, but volcanic ash and other forms of fragmental 

 volcanic matter form a' part of the Eocene system at so many points 

 in the west, and so many lava sheets are associated with the sedi- 

 mentary beds of the system, that there can be no doubt as to the wide- 

 spread volcanic activity of the time. Igneous rocks of Eocene age 

 are also known south of the United States in the Antillean and Cen- 

 tral American regions. 



General considerations. — Judged by the thickness of the beds in 

 most places, the Eocene period would seem to have been of less dura- 

 tion than most of the periods which preceded. This, however, is 

 not a safe criterion for the estimate of time, since it does not take 

 into account either the discontinuity of sedimentation in any one 

 place throughout the period, or the rate of sedimentation. Even on 

 the basis of thickness, however, the showing of the system is by no 

 means insignificant, as the formations of Puget Sound, Coos Bay, Ore., 

 and southern California show. In the western interior, too, the thick- 

 ness of the beds is often great, especially when it is remembered that 



1 Fuller and Clapp, Patoka, Ind.-Ill. folio, U. S. Geol. Surv. 



2 Hershey, Am. Geol., Vol. 29, p. 349. 



3 Diller, Roseburg, Ore., folio, U. S. Geol. Surv. 



4 Smith, G. O., Mount Stuart folio, TJ. S. Geol. Surv. 



5 Lindgren and Drake, Nampa and Silver City folios, U. S. Geol. Surv. 



6 Weed, Fort Benton and Little Belt Mountain folios, TJ. S. Geol. Surv. 



7 Hague, Absaroka folio, and Iddings, Yellowstone folio, U. S. Geol. Surv. 



8 Telluride, La Plata, Spanish Peaks, Walsenburg, and Anthracite and Crested 

 Butte folios, U. S. Geol. Surv. 



