-7' ' 
The American Geologist. 
1003. 
creek, of Salt river below the Salt 
springs, of the middle Cherry Creek 
valley whete not covered with lava, and 
of the region between the Tertiary aud 
the Plateau south of Sombrero butte. 
They are also occasionally exposed on 
upper Canyon creek. These rocks re- 
present two distinct systems. The up- 
per one is composed of crystalline 
schists embracing hornblendic to mica 
schists. Syenytes are also found in this 
upper formation. The schists always 
underlie the later groups. The lower 
system is granite. This granite except 
in the Apache mountains is a coarse, 
granular aggregate of quartz grains, 
and orthoclase, the crystals of the latter 
being mostly white, and mottling the 
weathering rock. The most extended 
out-crop of this granite that was noticed 
by the writer is near the confluence of 
the Canyon creek and Salt river. At 
this place as in all localities where ex- 
amined it was found to be characterist- 
ically affacted with joints. Mr. Marvine 
also noticed this peculiar jointing of the 
granite of the Pinal mountains where 
he crossed them in 1871 and in writing 
about it he says the granite "is at first 
strongly affected with joints having a 
southern trend, and inclined 6o° to 8o° 
eastward with a subordinate system of 
east and west joints, dipping north, the 
two together tending to stud the surface 
with large tombstone like slabs. 4 
The crystalline schists, micaeous, 
talcose, chloritic, and hornblendic, are 
extended most in outcrops along 
the northeast flanks of the Apache 
mountains from whose crests they 
continue to and in places beyond Salt 
