ENVIRONMENTAL AND HISTORICAL FACTORS. 79 
formed by the disruptive effects of the flow upon the crust, but the 
assistance of slope and gravity withdrawn, the material is immediately 
deposited. I do not think that 50 per cent of the material thus dropped 
has since been removed, except on the slope immediately facing the 
Santa Cruz River. In volume this wash amounts to less than one-fiftieth 
of the material of the hill. It will be seen, therefore, that erosion has 
scarcely touched the hill since it assumed its present form. ‘This recent 
caliche-cemented formation is called later wash to distinguish it from 
the wash formation underlying the tuff. In places it is thoroughly 
cemented with caliche, developed by the leaching of the porous amyg- 
daloidal surface of the lava flows. The pore spaces of the lava were 
originally filled with calcium carbonate and sulphate, which can still be 
found in freshly broken rock. The cement migrates down slope, spread- 
ing out in places and cementing the material of the Tucson Mountain 
slope, which is otherwise free from caliche. 
The Tumamoc wash just described extends out in a thin sheet over- 
lapping the Tucson Mountain wash. ‘The latter is not indicated on the 
map, but covers the andesite everywhere except in spots south and west 
of Tumamoc Hill. It is a subaerial wash deposited under torrential 
conditions. It is excellently stratified. Its material varies from the 
finest sand to bowlders 6 feet in diameter. The fine sand often forms a 
matrix in which the bowlders are embedded. Wind erosion is now rather 
more active than deposition, and in some spots some imperfect examples 
of desert pavement are developing. The wind is also exposing by erosion 
some of the coarse material formerly deposited in the temporary stream- 
channels. I have named these arroyo-trains,’ and the former course of 
the stream can be imperfectly traced by the heaps of this material at 
the surface. 
There is no direct evidence to determine the time in which the events 
just described took place, but the indirect evidence, such as the recency 
of the last faulting, the freshness of the basalt, and the increased length of 
time that has been allotted to the Pleistocene by recent studies suggests, 
as already stated, that the basaltic extrusions took place in the Pleistocene, 
and the andesite and rhyolite were erupted during Tertiary times. 
PETROGRAPHY. 
The rocks of Tumamoc Hill fall naturally into five types or classes, 
which are recognized in the following petrographical descriptions. 
OLIVINE BASALT. 
This is an exceedingly fine-grained, compact rock in which none of 
the mineralogical constituents can be identified without the microscope. 
When fresh it is black or very dark gray, sometimes quite free from cavi- 



1Journal of the proceedings of the Arizona Miners’ Association, 1905-1906, pp. 13-17. 
