SOUTHERN PACIFIC LINES 191 
Westward from Tucson the railroad crosses the southwestern por- 
tion of Arizona, a region presenting geologic and topographic features 
such as characterize the Basin and Range province of the Southwest. 
While the geology has not been mapped in detail, the principal fea- 
tures have been ascertained by reconnaissances by Bryan, Ross, 
Wilson, Lausen, Darton, and others. Many of the ridges consist of 
the pre-Cambrian granites and schists of a ‘‘basal complex.” In 
places these are overlain by sandstone of Cambrian age, limestones 
of Devonian and Carboniferous age, sandstone and shale of Creta- 
ceous age, conglomerate, lavas, and sands of Tertiary age, and thick 
beds of Quaternary sand and gravel. Igneous rocks of various 
ages cut the schists and sedimentary rocks, and some of the younger 
granitic rocks are not very different from the pre-Cambrian granites. 
The sea covered much of the area for at least a part of Carboniferous 
time, for there are remnants of limestones of this age at many places. 
Outliers of Apache rocks indicate that there was deposition of sedi- 
ments in the region during part of Algonkian time, the products of 
which may have been much more widespread than is indicated by the 
small remnants that are exposed. The features most striking to the 
traveler are mountains or knobs of schist or granite and ridges and 
mesas made up of a thick succession of lavas and other volcanic rocks. 
Many of the knobs rising above the valley floor are the summits of 
ranges which are now nearly buried under the thick valley fill of sand 
and gravel washed from the mountain slopes for a million years or 
more. Before the extrusion of the Tertiary volcanic matter the region 
presented an irregularly eroded surface, doubtless a desert, some areas 
of which were occupied by sands and boulder deposits of earlier 
Tertiary age. These deposits consisted largely of detritus from 
ridges and were mostly laid down by torrential streams under condi- 
tions similar to those of the present time. The lavas came to the 
surface through craters and cracks at various 1 widely, 
probably filling broad valleys and desert flats. Peebles some of 
the earlier ridges were not entirely buried. At intervals a great 
amount of ash, tuff, and other fragmental material was blown out of 
some of the vents. The succession of sheets of lavas and fragmental 
material is 2,000 feet or more thick in some areas, but it varies con- 
siderably from place to place in thickness and in the character and 
order of its rocks. The lavas were later uplifted, tilted, flexed and 
faulted, and widely removed by erosion, so that their original extent 
is not evident. Much of their detritus, together with that of older 
formations, makes up the thick alluvial fill of the present valleys. 
The great deserts of the Southwest at first sight seem nearly desti- 
tute of animal life, but actually they are the habitat of many animals 
in considerable variety, most of them, however, small and not often 
in sight. Most numerous perhaps are the kangaroo rats, which live 
