52 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
At the base of the highest peak of the Santa Rita and underlying the 
tufas, there is one of the most remarkable beds of large rounded boulders 
of porphyry known. It is exposed to view for a mile or more by the exca- 
vation of an aqueduct for the conveyance of water for the placers at 
Greaterville. The boulders vary in size from a few inches to a yard or 
more in diameter. They are of various colors, are closely compacted, 
and form a thick bed at a low horizon in the series and may be regarded 
as evidence of stupendous cataclysmic action. 
At Poston’s Mountain and Mount Allen and in the Grosvenor Hills, 
southwest of Saléro, there are remarkable strata of snow-white tufaceous 
flagstones so thinly and regularly stratified that slabs yards in area and 
only a few inches thick may be broken out. These tabular masses are 
well fitted for pavements or for building walls. They show ripple-marks 
on their surfaces and little annular projections around what are seemingly 
little pebbles which had fallen upon the surface of the rock when it was in 
a plastic state or in a thick creamy consistency left by a receding tide. 
The whole series gives evidence of deposition in shallow water, and this 
is shown elsewhere by regular bedding and stratification of the Santa Rita 
tufas. 
SANTA RITA TUFAS. 
At the peculiar outcrop of rock west of the arroyo of Josephine Canyon 
and at the head of the great slope down to the Santa Cruz at Tubac the 
tufas from the Santa Rita are in massive and horizontal beds, regularly 
stratified and made up of granular and generally rounded fragments of 
rhyolites and porphyries cemented together with volcanic mud or vol- 
canic ashes. The regularity of the stratification and the average com- 
position is such that pillars are left standing by the weathering away of 
the surrounding rock, and hence the name Los Pilares. The formation 
appears to have a wide extension. It underlies the ancient detrital 
slope which is spread out over it, except where removed by stream erosion. 
The high peak of the Santa Rita with its enormous flanking ridges of 
volcanic ejecta is certainly a monumental relic of a great center or region 
of volcanic activity from which an immense volume of broken rock, 
rhyolites, and ashes was spread far and wide. No distinct crater has 
yet been found, but there is a broad area between the higher peaks which 
has not yet been explored. We are at least certain that in the Santa 
Rita we have an ancient uplift of a thick series of tufaceous and rhyolitic 
deposits at higher levels than in other localities in the region. 
The evidence is conclusive that the greater part of this vast mass of 
volcanic ejecta was spread under water. We not only have the stratifi- 
cation of the beds of the Santa Rita, but of its many spurs and of the 
distant ridges of the Grosvenor Mountains, where the white feldspathic 
flagstones are covered with ripple-marks. 
