136 The American Geologist. March. i904. 
briefly to describe them so that they may be added to the list 
of well determined cases. 
The territory in question lies about eighty miles northeast 
of Tucson and near the south bank of the Gila river. The 
exact location is in the vicinity of Saddle mountain, six miles 
east of Dudley ville which is situated at the junction of the 
San Pedro and Gila rivers. 
The geologic features of the region consist of an irregular 
open synclinal basin of Paleozoic rocks, extending from the 
Upper Carboniferous limestone down to and including a large 
series of Middle Cambrian quartzytes. In this basin and in 
general lying conformably upon the beds of limestone are 
coal-bearing sandstones and shales from which determinative 
fossils have not yet been obtained, but, which provisionally 
have been assigned to the upper part of the Cretaceous. These 
beds have a thickness varying from ten or fifteen feet on the 
west side of the basin in the vicinity of Saddle mountain to 
400 or 500 feet on the east side where they outcrop along the 
base of Mescal mountain. Overlying the shales and sandstones 
just described is a great mass of andesyte, which doubtless was 
poured out as surface flows in the synclinal basin. There is no 
sharp line of division between the igneous and sedimentary 
rocks, and along the contact there is more or less interbedding 
as though sedimentation was at times interrupted by lava flows 
and then again was resumed under more favorable conditions. 
Over most of the basin the zone of transition from sedimentary 
to igneous is marked bv a thick bed of conglomerate, consisting 
of a matrix of andesitic tuff cementing well-rounded boulders 
of all sizes from a few inches to ten or more feet in diameter. 
Figure i shows the character of this bed as it is exposed on 
Ash creek directly east of Saddle mountain. At this point the 
bed dips to the left at an angle of about 45 degrees and the 
boulders are badly crushed and faulted as a result of move- 
ment in the rocks. The boulders are composed chiefly of gran- 
ite and material derived from the Paleozoic limestones and 
quartzytes. The amount of andesitic material poured out into 
the basin is not known, but in its maximum development it 
probably has a depth of not less than i.ooo feet. 
The Tertiary liistory of the region is complicated, consisting 
of great erosion intervals in which larpc masses of gravel were 
