GEOLOGICAL SKETCH OF THE REGION OF TUCSON. os 
- The photograph in plate 49, together with the diagrammatic sketch 
section, will serve to give an idea of the general outline of these hills and 
to show their geologic relation to the alluvions of the Santa Cruz River. 
This section also shows the interstratification of a nearly horizontal 
bed of volcanic tufa capped by a basalt on both Tumamoc and Sentinel 
hills. This, which was doubtless a continuous bed, has been largely 
swept away by denudation, as indicated by the broken lines. The remain- 
ing portions of this tufa bed, protected by the overlying hard basaltic 
rock, were evidently spread out in the condition of plastic mud upon a 
foundation layer of basalt. It is now solidified and is sufficiently firm to 
be extensively used for building purposes. Its color is white or gray and 
it is thus in strong contrast with the black of the basalt. 
There are several quarries of this tufa around Tumamoc Hill and at 
other places in this volcanic group. A quarry on the Quijotoa road near 
the Mission of San Xavier yields a light-gray; nearly white, building stone 
of excellent quality for construction. It supplied the stone for the large 
dormitory building at the university. This stone is essentially silicious 
in composition and includes in its substance angular fragments of rock, 
evidently broken by violence from their parent sources and borne along 
in the viscous mass. In the same rock we find interesting orbicular con- 
cretions consisting of concentric layers of different colors and chemical 
composition,* giving evidence of internal chemical changes around frag- 
ments of soluble rocks. 
The tufa bed at Tumamoc is about 25 feet thick, so far as exposed. 
It has a slight easterly dip and passes under and through Sentinel Hill, 
forming the nearly level ridge extending between and connecting the two 
hills. It is underlaid by a bed of sandy material consisting of a mixture 
of grains of clear glass and fragments of feldspar. This vitreous sandy 
bed, some 6 feet or more in thickness, is in turn underlaid by a distinctly 
stratified body of small black, pebble-like but crystalline fragments of 
rhyolitic basalt in an earthy magma. It isa soft, unconsolidated sub- 
stratum, which being easily removed by erosion hastened, by under- 
mining, the downfall of the overlying basalt, which once covered the 
entire area of the tufa. This process of degradation, especially during 
partial submergence, would explain the origin of the great accumulation 
of boulders over the surface of both Tumamoc and Sentinel hills. The 
boulders are often so thickly laid down as to cover the solid basalt from 
view. In many places they are cemented together by caliche which has 
been deposited around them, forming a cemented aggregate of great hard- 
ness. The boulders have for many years been drawn upon for building 
purposes, particularly for foundations in the city. The walls of the Desert 
Laboratory are built of them. 

*For a description of these orbiculites, see Transactions American Institute of 
Mining Engineers, May, 1905. 
