258 GEOLOGY. 



Drift, then two feet of compact peat, then clay and black soil, and 

 then Drift and blue clay again. The lower Drift here probably 

 represents the period of the first glacial advance. The upper clay, 

 black soil, and peat represent the middle period when the glaciers 

 had retreated and a new forest-bed covered the State. The Drift, 

 immediately on top of this, marks the second advance of the gla- 

 ciers. The Loess on top represents the final retreat of the glaciers, 

 and that era of depression of the surface of the State when the 

 greater part of it constituted a great fresh-water lake into which the 

 Missouri, the Platte and the Republican rivers poured their waters. 

 Old Forest Bed. — This is not observed in sections through the 

 Quaternary in all parts of the State. Sometimes it is apparently 

 absent from whole counties, and probably its removal was caused 

 by a second advance of the glaciers, to be considered presently. In 

 some of the canyons of the Loup region I have found the bed of 

 black soil, but without a vestige of silicified wood. In other sec- 

 tions of the same region it was sparingly present. It is possible 

 that this may have been occasioned by a condition slightly resem- 

 bling the present — that is, a condition of alternating forests and 

 prairies. Though it is evident that the proportion of forest to prai- 

 rie must have been much greater than at present, as east of the 99th 

 meridian in 30 sections that I have examined through the Old For- 

 est Bed, twenty-three contained silicified wood. West of the 99th 

 meridian only three out of fifteen sections contained any. The fol- 

 lowing is a section in a canyon running into the West Loup, where 

 no wood was visible. I examined the exposure carefully for half a 

 mile, and the most dilligent search failed to bring any to light: 



Surface soil 3 feet. 



Loess 21 " 



Calcareous sand and gravel 7 " 



Boulders, flint and gravel 4 " 



Carbonaceous, shaly clay 3 ( ' 



Black soil 4 " 



Silicious clay 2 " 



Gravel and boulders, exposed 3 " 



The black soil of the Old Forest Bed in color and constitution 

 closely resembles the black surface soils of the State at the present 

 time. This is particularly true of the lower half. The upper por- 

 tion has probably been so modified by subsequent glacial and water 

 agencies as no longer to exhibit its original character. As already 

 observed in the eastern part of the State, specially large quantities 



