CHAPTER Ill. 
ENVIRONMENTAL AND HISTORICAL FACTORS. 
THE GEOLOGY OF THE VICINITY OF THE TUMAMOC HILLS.! 
This contribution presents in an informal way the results of the study 
of the Tumamoc Hills, a group of three low basaltic hills about a mile 
west of the business center of the city of Tucson, Pima County, Arizona. 
Tumamoc Hill and the adjacent slopes to the west are the site of the do- 
main of the Desert Laboratory of the Carnegie Institution of Washington. 
I have been requested to preface the geological description of this 
particular locality by a brief discussion of some of those modifications 
of geological processes and deposits induced by moderate aridity that are 
prominently developed in the southern part of Arizona. Attention will be 
directed to subaerial or land deposits; a complex group which it is neces- 
sary to separate from aqueous deposits, both lacustrine and oceanic, 
if sound geographic and climatic deductions are to be drawn. ‘The wash 
conglomerates, volcanic tuffs and lava flows described later represent three 
important varieties of land deposits, viz, torrential wash deposits, eolian 
deposits, and volcanic flows. 
On account of the intimate relation of the geology of the land to botany, 
and because I am convinced that climate is as important a factor in 
determining the detrital covering as the vegetal covering of the earth, 
and because it is within reasonable expectation that the twentieth cen- 
tury will see the determination of the climatic history of the past as a 
result of the critical study of land deposition, I assume that botanists 
will be interested in a short summary of those recent geological studies 
which, using graphic language, have resulted in the discovery of the 
geology of the land. 
In this introduction, space permits only an outline of the results of these 
lines of research and forbids any close examination of the data or meth- 
ods used. Instead of a complete bibliography of this subject, I shall 
note only those important contributions that will give some idea of the 
method, scope, and results of these investigations. I shall omit the 
consideration of the development of the scientific study of topography 
and of glacial deposits, which might well be taken up in this connection, 
the results of which are among the most important obtained by geology 
during the latter part of the nineteenth century, and which drew the 
attention of geologists from the ocean to the land. As Europe is behind 

1This section, pages 67-82, was prepared by request and contributed by C. F. Tolman, 
B. S., Professor of Geology in the University of Arizona. a 
