ENVIRONMENTAL AND HISTORICAL FACTORS. 69 
lowering the temperature, the disruptive action of frost and the repres- 
sion of vegetation may under certain conditions develop intensive action. 
It does not seem probable, therefore, that theoretical analysis of the 
complex relations that obtain between climate and deposition will accoim- 
plish what it has in the case of deposition by running water or by glacial 
action. The problem will be solved by detailed studies in each region. 
In each case the disturbing factors must be evaluated and the intensity 
of each process gaged.! 



IDEAL SECTION THROUGH A PLAYA 
A= Angle of rock surface 
” 4 erosloTe 
B= 
C= 4 4 slope deposition % 







Fic. 3. 
One of the most inviting fields for such a study is the “bolson”’ or 
undrained area developed under conditions of aridity. The typical 
bolson? is an expression of aggradational processes and is excellently 
portrayed in the smaller undrained flats of southwestern Arizona. An 
idealized section through a bolson is attempted in figure 3. The angle A 
represents the slope of the top of the mountain chain or peak. This 
upper surface is often parallel to the rock structure and is therefore flat 
or sloping as the strata are horizontal or inclined. The higher moun- 
tains reach up into a region of forests and abundant grasses which pro- 
tect the surface and preserve the flat top. The angle B is the slope 
developed by the attack of torrential precipitation upon the rock struc- 

1Since writing the above I have received the Journal of Geology, xvi, No. 2, which 
contains the first installment of an article under the title ‘‘Relation between climate 
and terrestrial deposits,’ which is to be continued in Nos. 3 and 4. It appears that 
Professor Barrell has not been deterred by the difficulties inherent in any general anal- 
ysis of the relations between climate and deposition. The portion published indicates 
clearly the great value of his contribution, and the completion of his article is awaited 
with impatience. 
2Hill, R. T.: Topog. Atlas of the U. S., Folio 3, U. Se Gra tooo.) Kmever: Cas. 
Bolson plains and the condition of their existence, Am. Geol., xxxIv, 160-164. Tight, 
W.G.: Bolson plains of the southwest, Am. Geol., XxxXv, 271-284. 
