Dawson Cañon, which heads in the Temescal Mountains, was explored and found to contain interesting geological features. A fine opportunity is given for the study of the relation of the granite to the extensive porphyry intrusives. For 2 miles east of Temescal Creek no eruptives appear; the rocks being wholly of the Metamorphic Series, with exposures of highly altered sandstones, clay shale, conglomerates, etc., striking northwest and dipping southeast at 45° to 50°. About 3 miles up the cañon the argillaceous rocks are replaced by a coarse granite, rich in mica and quartz. This is the prevailing rock up the cañon for 2 miles, and it apparently extends much farther east. It shows a great variation in appearance; much of it contains large crystals of flesh-colored orthoclase. In this granite, particularly on the north side of the cañon, there are dikes of many kinds of rocks. Large dikes of beautiful diorite porphyrites, both light and dark colored, appear in places. At one spot 4 miles from the mouth of the cañon, there are rectilinear dikes of fine-grained granite, intersecting each other like artificial stone fences. For the distance of a mile east, after the granite begins in the cañon, the hills north show nothing but metamorphic schists. The porphyry in the mountains south does not reach the cañon. The great mass of this rock is dark, but in the vicinity of the granite it is lighter colored and more feldspathic, sometimes assuming a granitic structure. In places it is a gray, hard rock, of almost conchoidal fracture, and faint feldspar crystals. The granite near the contact is usually sharply defined, and has a faintly porphyritic appearance at times. The line of junction of the two formations is sharply defined, not only lithologically, but physically. It is difficult to say which is the older. No granite appears in dikes in the porphyry, but there are many dikes of a porphyry-like appearance, resembling the light-colored porphyry in the granite itself. The line of junction is very irregular, and it is certain that the two formations do not belong to the same eruptive mass.