the intrusion of the granite with the tilting of the rocks of the range. The occurrence of the granite adjacent to the faults in the Franklin Mountains, the association of the faulting with the general deformation of the range, and the fact that tilting of the strata would probably accompany the intrusion of so large a mass of granite suggest the general contemporaneity of these events. The inclined Cretaceous strata which apparently dip conformable with the Paleozoic rocks on the southwestern flanks of the range 7 miles north of El Paso imply a relatively late date for the tilting of the range. It seems plausible, therefore, that some of the granite may be as young as post-Cretaceous, but proof is not available. All that is definitely known is that, in part at least, the granite is post-Carboniferous.

SYENITE PORPHYRY.

Distribution.--There are several outcrops of syenite porphyry at the base of the Hueco Mountains, near Hueco Tanks. The two largest masses form hills of elliptical outline, trending north and south, each occupying less than 2 square miles and rising several hundred feet above the surrounding wash-covered slopes. The other bodies, of igneous rock in this vicinity are considerably smaller, and they also are flanked by wash, except a small mass east of Hueco Tanks, which is in direct contact with the Hueco limestone. (See fig. 9.)

Character and composition.-The rocks are almost bare of vegetation and are much broken by joints. The principal trend of these joints is north and south; another transverse set is less conspicuous. Parting planes rudely parallel with the surface are well developed and weathering has rounded the outcrops into spheroidal masses. The syenite porphyry is readily acted upon by subaerial influences and a number of erosion hollows have been formed, across some of which dams have been constructed to impound rain water. These are known as the Hueco Tanks. (See fig. 9.) The weathered surfaces are brownish, but fresh specimens are light colored. The fresh syenite porphyry is a light-gray, holocrystalline, fine-textured, slightly porphyritic rock composed of pre-