SOUTHERN PACIFIC LINES 183 
approach the railroad between Pantano and Vail.* Three miles east 
of Vail, a cavern known as Colossal Cave extends far underground 
with many interesting galleries, some of them with stalactites and 
stalagmites. Formerly it contained guano which was excavated for 
shipment to Los Angeles for use as fertilizer. The interesting relations 
of the rocks in this vicinity are shown in Figure 
Near Vail and westward the wide, level desert plain extends east 
and south to the Empire and Santa Rita Mountains, north to the 
Rincon and Tanque Verde Mountains, northwest to the Santa Cata- 
lina Mountains, and west to the ragged peaks of the Tucson Moun- 
tains. It slopes west into the wide arroyo of the Santa Cruz River. 
e prominent Santa Catalina Mountains, just north of Tucson, 
present a formidable array of rugged cliffs, bem by jach-walled 
canyons heading deep in the range. These mountains consist of a 
large central mass of very coarse nesiialibe and granite, which 
weathers to picturesque pinnacles and balanced rocks; they are the 
latest of a series of granitic intrusions, one of which, known as the 
Oracle granite, is exposed in a large area around Oracle. On the 
southwestern slopes of the lesa the rocks are dominantly 
gneisses, which are eo layered and which form the rugged crests 
and slopes of the numerous canyons. On the northeastern slope there 
are extensive outcrops of the Apache group, comprising the Scanlan 
conglomerate, the Pioneer shale, the Barnes conglomerate, the Drip- 
ping Spring quartzite, and the Mescal limestone, which is here a shale. 
These have been greatly metamorphosed by the later granites of post- 
Paleozoic age, which have also changed the overlying Cambrian, 
Devonian, and Carboniferous strata. Moore, personal 
communication. 
36 The larger ridges consist of massive 
ie apa limestone, Martin (Devon- 
and Escabrosa (Mississippian), 
group, lying on granite. The diabase 
closely resembles in character and rela- 
tions the diabase in the Apache group 
bed 
8s silica in large bodi 
asily be mistaken for quartz 
se the Martin limestone is the 
Abrigo limestone, mostly very impure 
agments of 
which has been found by Stoyanow to | shown in section 
sonteti Middle Cambrian fossils. Next | the plane of displacement slopes to the 
below is hard reddish bedded quartzite | southwest at a low angle. In this por- 
with intrusive sills of diabase, which | tion of the area a 38-foot bed of quartz- 
resembles the Dripping Spring quartz- 
ite of the Apache group. Under this 
is red shale like the Pioneer shale and 
at the base a conglomerate resembling 
the Scanlan conglomerate of the Apache 
ite of unknown age overlies the Abri 
and apparently separates it from the 
Martin li ne, as in Bisbee and 
Johnson areas, 
