CLIMATE AND TERRESTRIAL DEPOSITS 275 



dition being a moist soil during the season of growth.^ Although the 

 level of the ground water in flood plains of even subarid climates may 

 lie not many feet below the surface, the alternate stratification of fine 

 sand and clay which is frequently present is very unfavorable for a 

 forest vegetation. The clay is capable of carrying the water upward 

 to a greater height, even as high as ten feet, but transmits it very slowly. 

 The sand, on the other hand, cannot lift the capillary water more than 

 one or two feet, but does this very quickly.^ If the upper portion 

 of a sand stratum is dry, the plants cannot feel the moisture below and 

 will fail to send roots after it. In the presence of such strata a vegeta- 

 tive covering of bunch grass is to be expected, leaving no appreciable 

 organic record. A deep loamy soil favorable for storing water and 

 for its capillary rise is the most favorable condition for the growth of 

 trees and shrubs over semiarid flood plains. The roots in such cases 

 strike downward rather than horizontally and may penetrate to 

 great depths, twenty feet being not uncommon. ^ The angle of pene- 

 tration of fossil roots is therefore a matter of importance from a cli- 

 matic point of view. The strong oxidation acting at the surface nor- 

 mally destroys all vegetable tissues before they become buried in the 

 course of time below the deep zone of oxidation, but there is a chance 

 of finding casts of downward-branching rootlets in massive arenaceous 

 shales and more rarely of vegetable remains buried by superficial 

 accumulations. It is seen, therefore, that in the river deposits of 

 semiarid climates casts of logs are most likely to be preserved in the 

 sands deposited in the neighborhood of stream channels. At a dis- 

 tance from the channels, wetting and oxidation would tend to destroy 

 the logs and larger fragments if such existed, before sufficient time had 

 elapsed for burial. Root impressions of trees in such regions would 

 be of more common occurrence than trunks and confined possibly to 

 what were originally deep loamy sands. The herbaceous types of 

 vegetation, however, are the more common over the well-drained por- 

 tions of truly semiarid flood plains, and the plant impressions recorded 

 in the strata would consequently be of small size compared to those 

 of the large and luxuriant vegetable forms of more rainy climates. 



I Op. cit., pp. 164-75. 



= E. W. Hilgard, Soils, 1906, pp. 202-13. 



3 Loc. cit., pp. 167-83. 



