Geology of Arizona. — Blake. 163 
con, and Rillito group may be regarded as the northwestern 
extension of the great mass of mountains in central Arizona 
generally known as the Bradshaws. All these mountain 
ranges consist largely of granitic, gneissic, and schistose rocks 
of pre-Cambrian age with a highly complex folded structure, 
and exhibiting a high degree of meta-morphism. Taken to- 
gether, these mountains may be regarded as the main axis of 
ancient uplift, and of insular land areas in the pre-Cambrian 
and Palaeozoic periods, the beginning of the "Arizona Land." 
Gneiss. The §jpeiss of the southern side of the Santa Cata- 
lina near Tucson is regarded as Archaean. It is remarkable 
for its regularity of stratification and its great thickness, prob- 
ably over 10,000 feet. It occurs in great tabular masses made 
up of thin layers which when seen laterally give the appearance 
of evenly stratified shales and sandstones. The beds are, how- 
ever, essentially granitic with the feldspar spread in nodules 
which make protuberances on the cleavage surfaces of the rock, 
and thus form a porphyritic or augen-gneiss rock. 
Sheets or veins of granite are common in this gneiss. It 
is remarkable that this rock has such an even tabular 
structure without plication, wrinkling or folding and that it 
rests at a low angle generally not exceeding twenty degrees 
upon the massive nucleus of the Catalinas dipping southward 
and passing under the modern detrital accumulations, Qr 
"wash", from the mountains. These beds rise nearly to the 
summit of the Catalinas and then break off precipitously form- 
ing a line of cliffs facing the central part of the range. Some 
obscure traces of an anticlinal fold are visible on the western 
side. 
Huronian or Arizonan. In the same range, but on the 
northeastern side, facing the valley of the San Pedro, another 
formation of thinly bedded and highly crumpled mica schist in 
sharply defined zig-zag folds is referred to the Huronian. This 
is the formation to which I have given the name Arizonan. StiH 
further north occur the heavy conglomerates, red beds, quartz- 
ytes and limestones in which last corals referable to the Devon- 
ian occur. A section carefully measured in detail made here 
may be generalized as earthy limestone in thin layers with sand- 
stone and quartzyte at the base. The strata dip eastwardly at 
a low angle. Their exact stratigraphic relation to the under- 
