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ENVIRONMENTAL AND HISTORICAL FACTORS. 5 
unnecessary to describe in detail the distribution of the formation, or to 
divide the descriptions under the numerous subheads usually employed 
in the analysis of more extensive areas. I shall, therefore, combine in an 
informal way the development of the geological history of the region, with 
descriptions of the rocks, their outcrops, and their structural relations. 
The andesite flow.—The supposedly oldest rock of the Tucson volcanic 
series is an andesite, described microscopically as a “‘mica-andesite.”’ It 
is covered on the north, west, and south by rhyolite flows, is intruded by 
basaltic plugs, is covered by small separate basaltic flows in places along 
its eastern edge, is largely hidden under a covering of desert wash, and 
is bounded on the southeast by the Santa Cruz alluvium. The structure 
of the flow has not been deciphered. Its crystalline character suggests 
a very thick flow, or perhaps it is intrusive in part. It appears on three 
sides and underlies the basalts of the Tumamoc Hills. 
The rhyolite extrusions—The next event of the district was the outflow 
of many sheets of rhyolite. These are only recorded indirectly in the 
-region mapped by the pebbles in the later conglomerates. The back- 
bone of the Tucson Mountains consists of this rock, which includes not 
only typical rhyolite, and rhyolite breccia (the fragments of which are 
products of explosive action and foreign material picked up during the 
extrusive processes), but also all the grades between a vitreous rhyolite 
and a mud flow, the variations being formed by different mixtures of 
lava, ash, cinders, and superheated steam. Accurate estimates of the 
thickness of these flows can only be made after careful mapping on 
account of the extensive faulting, but it probably amounts to several 
thousand feet. 
The first period of faulting.—This period also is not registered in the 
portion mapped, the block-faulting being especially developed in the 
rhyolite flows. The faulting was regular, the blocks tilting in a north- 
easterly direction at an angle of about 20°, the fault scarps representing 
the broken edges facing southwest. The topographic effect of this frac- 
turing is noticeable from any point a few miles south of Tucson, the 
larger mountains (each a fault-block) showing the outline of an asym- 
metrical triangle. 
The intrusions and extrusions of the basalis—Next occurred a number 
of basic intrusions and flows, the history of which is somewhat varied 
in different localities, one phase of which is represented in the Tumamoc 
Hills. Here the first of the basic flows is a basalt, marked B, on the 
map (plate 27), and described under “Petrology” as plagioclase-basalt. 
It appears on the southeastern corner of the map, is well exposed along 
the Nogales road, and is noticeable on account of its large, glistening 
feldspars, some of which have measured up to 2 inches in diameter. The 
broken fragments of these feldspars, set in a dense cryptocrystalline 
groundmass, indicate plainly two outward movements of the mass, in 
