SOUTHERN PACIFIC LINES 195 
Rillito and Marana are small settlements sustained by irrigation 
with water pumped from the underflow of the Santa Cruz River. 
West from Naviska siding the region is a wide desert. 
cuss Occasional sahuaros are in sight from Avra siding and 
aactiasistie ata westward nearly to Picacho. The village of Red 
New Orleans 1,520 Rock is on this wide desert plain, which — north 
me: to and beyond Phoenix and far to the west. Thi 
Wad Rock plain 4 is floored with sand and gravel, in <i places 
Blevation 1,88 fet very deep, and the subsurface geology is not known. 
Population 40. The embankments at intervals along the railroad in 
New es $31 this vicinity were built for protection from flood 
waters. Many steep-sided mountains rise out of this 
plain, mostly of granite, schist, or volcanic rocks, their rugged out- 
lines indicating rapid disintegration.“ The valley floor bears a sparse 
vegetation of small mesquites and other plants, widely spaced on 
account of the arid climate. 
A railroad branches to the southwest from Red Rock to Silver Bell, 
18 miles distant, a small town with a large copper mine. The werk. 
ings are in a group of high ridges, consisting in part of rhyolite and 
tuff of voleanic origin, and a succession of 3,700 feet of quartzite and 
limestone, the latter containing Carboniferous fossils (C. F. Tolman). 
An extensive intrusion of alaskite porphyry carries blocks of the 
limestone, one of which, according to Stewart, is nearly 2 miles long 
and 2,000 feet wide, and: there are later dikes of andesite and trachyte 
porphyry. The ore reduction works at Sasco are visible from Red 
Rock. The Waterman Mountains, a small range 6 miles southeast 
of Silver Bell, consist of porphyry, quartzite, and a limestone that 
contains fossils of Permian age (Naco limestone). 
Northwest of Red Rock, on the left side of the railroad, is the 
prominent peak known as Picacho or Saddlerock Picacho (see 
*' Rock disintegration proceeds rap- | and along the larger streams. Running 
idly in the desert regions of the South- | water containing sediment in suspen- 
west. The great difference of tempera- | sion is a powerful erosive agent, and 
ture between hot afternoons and chilly | wind-blown sand is especially effective 
dawns is an it agent, causing | in removing decomposed or soft roe 
great expansion and con » and Joints in rocks are cracks, erally 
he fr not of great length, due to shrinkage in 
ing disintegrati of lime- | cooling if the rocks are of igneous origin, 
stones and decomposition of minerals | or ins of various kinds, 
in crystalline rocks are factors which | earth movements. They may run in 
produce large results in a few centuries. | various directions or may be 
ost rocks are traversed by joints or | in sets of nearly parallel cracks which 
wR and along these disintegration | intersect other sets at approximately 
es. It finally isolates spalls | constant angles. Joints differ from 
or * blocks of the rock, and these fall and faults i in being much smaller fractures 
eventually crumble into detritus, which vertical 
is worked down the slopes and becomes displacement of the rock along the 
valley fill or is carried by freshets into 
