284 STUDIES FOR STUDENTS 



progress of evolution has been to progressively specialize life forms 

 for such extremely unfavorable habitats. The present relations are 

 briefly as follows : 



A sparse vegetation occurs over the drier or more alkaline por- 

 tions of the plains. Other portions may be covered with dense scrub 

 as seen in the Australian bush. Along the streams tree growth com- 

 monly occurs, and hence casts of roots and leaves might be left. In 

 the swamps reed growths are abundant and may become fossilized 

 and associated with white or greenish clay strata.' Animal bones 

 embedded in the desert plains stand excellent chances of preserval 

 and observation of the accumulating prairie loess has led Matthew 

 to believe in an aeolian origin for the fine-grained calcareous clays 

 of the White River Tertiary formation covering a considerable area 

 of the Great Plains of the United States,^ though these tracts are 

 presumed not to have been really arid but rather subarid in climate. 

 The fossil fauna of this deposit as Matthew has shown is such as can be 

 explained by the aeolian hypothesis, but not by one of lacustrine origin. 



Ease of recognition in the geological column. — From these character- 

 istic features of the river plains of arid regions it is to be concluded 

 that their fossil representatives should be rather readily recognized. 

 In the marginal region of the delta and for some distance inland beds 

 of salt, gypsum, and marl are interstratified with red shales, in which, 

 however, occasional decolorization may occur. Farther inland the 

 strata will show fewer pure evaporation deposits, but these minerals 

 will still be diffused through the strata. Carbonaceous matter will 

 be practically absent, but well-preserved animal remains may be 

 abundant. Dune sands, characterized by cross-bedding and a high 

 degree of rounding, giving "millet seed " sand beds such as those which 

 Phillips has described. from the English Triassic,^ will be associated 

 with true river deposits, sometimes mud-cracked, and the whole will 

 be characterized by a relative absence of rock decay in the process of 

 sedimentation. 



1 Huntington, "The Basin of Eastern Persia and Sistan," Carnegie Institution, 

 1905, pp. 279-87. 



2 W. D. Matthew, "Is the White River Tertiary an AeoHan Formation?" The 

 American Naturalist, Vol. XXXIII, 1899, pp. 403-8. 



3 J. A. Phillips, "On the Constitution and History of Grits and Sandstones," 

 Quarterly Journal 0} the Geological Society, Vol. XXXVII, 1881, p. 26. 



