QUATERNARY AGE. 277 



Here, as in the Republican Valley, the Loess at the bottom of 

 the section (No. 4) has a great many more root marks than the 

 lower part of the Loess above (or No. 2). The same explanation is 

 applicable here as in the former case, namely, that these fossil black 

 soils represent conditions of land surface like the present, when the 

 vegetation of the time penetrated from them into the underlying 

 Loess. Now it is probable that these black soils may have been 

 removed in some districts by erosion at the beginning of the re- 

 newed lake conditions, and left no sign of their former presence, 

 except the fossil root marks below. However abundant, therefore, 

 at some horizons these root marks may be in the Loess of this 

 country or Asia, its origin can easily be explained on the theory of 

 its sub iqueous origin. Again, it is questionable whether these black 

 soils filled with organic matter are ever formed except in the presence 

 of water. The most probable explanation is that the black soils on 

 top of the Loess have been formed when this lake gradually ap- 

 proached dessication or a drained condition. When it approxi- 

 mated the condition of a peat bog the organic matter was retained 

 (as a large portion is always retained when it decays under water), 

 and mingling with the Loess bottom became a black soil when the 

 drainage was completed. This semi-boggy condition endured for 

 ages long enough at least to form a black soil from three inches 

 to ten feet thick. In accordance with this view the highest knolls 

 where the land is rolling have in general the thinnest covering of 

 black soil. This process is still going on in the bogs of the Mis- 

 souri and many of its tributaries in the Loess region. If the Loess 

 was formed, as I maintain, by subaqueous agency, then it is appar- 

 ent that this old lake became dry land gradually. It surrendered 

 its bottom little by little, until modern conditions prevailed. 



Another fact depended on by Richthofen to substantiate his 

 theory, is the difference in level between various points of con- 

 nected Loess regions. This objection is based on the assumption 

 that the Loess districts lie at the same level now as during the 



O 



deposition of this peculiar sediment. No geologist, however, 

 doubts that during Glacial times the continent towards the north 

 laid relatively far above its present level. It is also conceded that 

 during the Champlain Epoch the level of a large part of North 

 America was below w'hat it is now. It is admitted that, partly 

 owing to this depression, and partly to melting ice sheets, temperate 

 latitudes were flooded. The re-elevation of the land drained it. 



