ENVIRONMENTAL AND HISTORICAL FACTORS. TT 
grains to bowlders 2 feet in diameter. The stratification is very well 
marked but has a tendency to wedge out in the direction of the strike. 
The individual layers vary in thickness from a small fraction of an inch 
to 5 feet. 
Igneous outbreak again records itself in the rhyolite-tuff mentioned 
above. It overlies the conglomerate south of Tumamoc Hill and the 
breccia in Sentinel Hill. Talus interferes with the determination of the 
extent of the separation of the tuff from the andesite and the basalts by 
the wash conglomerate. It is the most conspicuous formation of Ttumamoc 
and Sentinel Hills, appearing, when viewed from the south, as a white 
band and cap, in marked contrast from the reddish-black lavas (plate 1). 
It is composed entirely of products of explosive action—volcanic dust, 
glass, and pumice, the source of which was some unknown vent to the 
south. Wherever the conditions are favorable for its preservation, the 
Tucson Mountain slope contains similar deposits of tuff, indicating the 
extent of the explosive action. The deposit was largely eolian, and 
only to a slight extent a mud flow; the small size and small amount of 
inclusions, except those of pumiceous character, and the lack of stratifi- 
cation indicate wind as the chief agent of transportation and deposition. 
Very porous pumice occurs in fragments 3 inches in maximum length; 
the other fragments are of plagioclase and olivine basalt less than an 
inch in maximum length. The bottom layers are quite pumiceous, and 
the upper layers south of Tumamoc Hill are largely pure unconsolidated 
pumice. Again, some of the tuff is baked, recemented, and indurated, 
ringing clear under the blow of the hammer. In.color it varies from 
white through gray, drab, pink, and red. The portions of the tuff that 
are moderately indurated make an excellent building stone, light in 
weight and of moderate strength, but unsuitable for foundations, as it 
decomposes rapidly under the attack of moisture and the humic acids, 
doubtless due to the large per cent of alkalies it contains. (See analysis 
in the microscopic descriptions.) South of Tumamoc Hill to feet of 
conglomerate, similar to that underlying the tuff, is exposed and indicates 
a return to the ordinary wash deposition. 
Overlying the upper conglomerate and the tuff is the last of the lava 
flows, B; being similar in composition and appearance to the earlier flow B,. 
The direction of the flow was north and northeast. The platy-structure 
and reversal of dip is especially developed in this last flow, caused by 
a breaking of the first crust and a squeezing up of the viscous lava 
from underneath. The platy-structure is also formed without disturb- 
ance of the crust, developing whenever there has been a second movement 
in a semi-viscous layer. The reversal of dip is shown on the map by the 
sign (/-)and the dip of the formations by the conventional (|). It is 
not easy to determine the source from whence the last flow issued, but 
the considerable development of the platy-structure upon the top of 
