48 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
of structure. This preservation is due to the permeation of the rock by 
silicious waters which have changed the organized structures from car- 
bonate of lime to silica, while the investing, or surrounding, limestone 
remained unchanged. The fossils thus became silicified and then, being 
less soluble than the limestone, are left by the weathering standing out 
above the general surface in bold relief. 
Silicified fossils in limestone arecommon at many localities in Arizona. 
A fine section of the stratified rock formations of the northern end of 
the range is found along the Southern Belle Creek, which extends from 
the summit of the range eastward. The strata are uplifted and dip 
eastwardly at an angle of about 15 degrees. The series consists of regular 
strata of red sandstone and shale, quartzites, sandstones, and limestone, 
resting upon diorite at the west end, where also we find a strong vein of 
auriferous quartz traversing the red rock approximately parallel with the 
contact. At the eastern end of the section strata of sandstone and lime- 
stone abut upon granite and are much altered in composition. ‘These 
limestone beds are the upper members of the series of the section and are 
probably Devonian, as indicated by beds of corals. The underlying 
strata of sandy shales are distinctly fucoidal. The most prominent strata 
of the series are the massive white quartzites, with repeated outcrops due 
probably to faulting. The red shales of the lower series pass into beds 
of red sandstone of compact and even grain, regular freestone, suitable 
for buildings. The underlying diorite penetrates this series of shales and 
sandstones and is itself underlaid by the coarsely crystalline porphyritic 
gray granite which is largely developed around the northern portion of 
the Catalinas, extending to Oracle and beyond. It has a wide extension 
toward the east and north, reaching nearly to the San Pedro River, and 
on the west it extends to the Canada de Oro, where strata of quartzite 
and limestone rest against it. At and near Oracle the granite weathers 
into huge boulders of decomposition, giving a very picturesque and 
diversified surface overgrown by groves of evergreen oaks. 
There are several localities of remarkable conglomerate in the north- 
ern and central portions of the Santa Catalinas. The component pebbles 
are chiefly from quartzite; they are all much rounded and show the violent 
action of currents and waves indicative of shallow seas and insular condi- 
tions in remote geologic time. 
THE RINCON RANGE. 
In the southern prolongation of the Catalinas known as the Rincons 
the central and higher portion consists of gneissic rocks like those of the 
Catalinas, but toward the railroad pass to Benson these crystalline rocks 
are flanked by an extensive development of Paleozoic strata, chiefly 
quartzites, shales, and limestones uplifted and contorted, and under- 
laid by a coarse granite. Silicified corals abound in places and indicate a 
