68 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
America in the study of these newer phases of geology, attention will 
be directed to the development of these ideas in the latter country. 
In 1897 Davis' presented an excellent summary of some of the cri- 
teria to be applied to distinguish lacustrine from fluviatile sediments, 
although of course the ideas developed were not wholly original,? and 
Gilbert, Hatcher, Johnson, Grabau, Barrell, and others? have, by an exten- 
sion of these and other principles, rescued a number of the clastic deposits 
of the eastern United States from the domain of the ocean, and have so 
changed the ideas in regard to the Tertiary and the Quaternary* that these 
two periods may be defined as an age of subaerial deposits instead of an 
age of lakes. It is thought that throughout this time the conditions in 
western North America were much as now. ‘There were periods of local 
humidity when deposition in lakes was important, times of humidity and 
cold when lakes and glaciers were formed, and periods of warmth and _ 
aridity when the subaerial deposits, always important, took complete 
ascendancy over all other forms of deposition. Volcanic activity was 
pronounced, and marine sedimentation is recorded largely at the margins 
of the continent. 
This new concept of the importance of sedimentary deposition upon 
land surfaces made possible the next step, viz, the recognition of climate 
as one of the factors controlling subaerial deposition, and, as a corollary, 
the reading of the climates of the past times by the evidence presented 
in their respective deposits.> It is scarcely necessary to refer to the 
Asiatic expeditions of the Carnegie Institution of Washington as the best 
example of a critical study of glacial, fluviatile, and lacustrine deposits 
which has developed climatic deductions of great importance. 
A moment’s consideration will show that a relation exists between 
climate and erosion and deposition, but that this relation is not simple. 
For example, deposits piled at the foot of a mountain of considerable 
height will resemble in some particulars those formed at the foot of hills 
under the intense atmospheric and torrential action of a more arid cli- 
mate. Still further, climate itself is a complex, not a simple factor. 
Assume, for instance, a certain moderate number of inches of rainfall 
and then increase the temperature, and one discovers that the gentle 
erosion retarded by a covering of vegetation gives place to the intensive 
erosional and depositional action of the semi-arid climate. Again, by 

*Davis, W. M.: Is the Denver Formation lacustrine or fluviatile? Science, n. S., 
VI, 619. 
“Walther, J.: Einleitung in die Geologie, 1904. 
*Grabau, A. W.: Types of sedimentary overlap, Bull. Geol. Soc. Am., xv: 567-636, 
1906. Barrell, Joseph: Origin and significance of the Mauch Chunk shale, Bull. Geol. 
soc! Ani, XVIN, 449-476. Barrell, Joseph: Relative importance of continental, lit- 
toral and marine sedimentation, Jour. Geol., xrv, 316-356, 430-459, 524-568, 1906. 
*Chamberlin and Salisbury: Geology, 3, 193-194, 196-215, 243-248, 258-276, 296-318, 
472-483. 
Baers especially, Ellsworth Huntington: Some characteristics of the glacial period 
in non-glaciated regions, Buli. Geol. Soc. Am., XVIII, 351-388. 
