54 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
and are present in the form of basaltic lavas spread out originally in 
great sheets in igneous outpourings upon the upturned edges of the rhyo- 
lites and other rocks, as, for example, in the Galiuro Mountains east of the 
Catalinas. Remnants and detached areas of such floods of liquid lava 
are found at the western side of the Babiquivari Valley, at the Mission 
of San Xavier, 9 miles south of Tucson, and at the Sentinel and Tumamoc 
hills opposite the city. 
TUMAMOC. 
The Tuniamoc group as seen from a point a few miles north shows 
two summits, one rising in conical form from the Santa Cruz and known 
as Sentinel Peak; the other has a broad, flattened summit of greater area 
and is connected with the peak by a long, comparatively level ridge. 
The flat-topped Tumamoc Hill is formed largely at the surface of loose 
rough blocks of basalt anciently used by the aborigines for the construc- 
tion of a rude fortification as a place of refuge when driven by roaming 
bands from predatory tribes from their fields and dwellings on the fertile 
bottom-lands of the Santa Cruz. 
A great number of partitions of rude apartments inside of the circum- 
vallation indicate that many people made their homes, for a time at 
least, upon this rocky summit, 700 feet above the valley. 
The buildings of the Desert Laboratory are upon the northern slope 
of this hill, about half-way down and at an elevation of 329 feet above the 
Santa Cruz River, and, like the prehistoric fortress, are built of the loose 
boulders of basalt which abound upon the surface. 
These basaltic hills, of which Tumamoc is one only of a series, present 
to the eye a black, barren rocky surface, with but little visible soil or 
earth for the support of the scanty but peculiar vegetation which finds a 
foothold in the clefts and crevices. 
That these rocks are volcanic in origin there is no doubt, but without 
any crater or indication, by form, of an extinct volcano. There is evi- 
dence in the structure and the interstratification of beds of tufa, and in the 
amygdaloidal interior structure of the basalts, of an igneous outflow from 
some higher ground, but satisfactory evidence of the source has not yet 
been found. 
The basalts of Tumamoc and of Sentinel and southward to the Mission 
present a variety in chemical composition and structure, giving evi- 
dence of flows or outpourings of different ages. These rocks have been 
carefully studied petrologically by Prof. F. N. Guild, of the University 
of Arizona, who finds three varieties: (1) Fine-grained olivine basalt; 
(2) porphyritic basalt with feldspar crystals; (3) quartz basalt. 
In places, notably in the quartz basalt, on the southern base of Sentinel 
Peak, the amygdular cavities are filled with silica in the form of agate 
and also in the form of geodes.* 

* American Journal of Science, vol. xx, October, 1906. 
