that the mesas have been formed partly from the bowlders which they brought down. This mesa, as well as the gravels at the head of the grade, has an elevation of 1,500 feet. The source of the porphyry bowlders and the garnetiferous schists of this old river is a matter of great perplexity. The gravel deposit has every characteristic of an old river channel, and not that of an elevated arm of the sea; besides, the presence of gold in the gravels indicates their derivation from the country farther east. The gold may have been derived from Julian or Mesa Grande, or some more remote point. The river must have flowed across the gold belt, but then the question arises, how could a river of such magnitude have existed so near the summit? The only way out of the difficulty is to suppose that a great uplift has taken place along the crest and western slope, coupled with an enormous amount of erosion; and that this stream once, before this great change took place in the configuration of the country, headed many miles to the northeast, far beyond the drainage of the western slope. The bowlders consist largely of hard rocks, and are very smoothly rounded and polished, indicating that they have been transported a long distance, and subjected to attrition through a protracted interval. It is quite possible that this river emptied into or near San Diego Bay, and that the immense beds of bowlder conglomerates about the bay owe their formation largely to this river action. The first outcrop of crystalline rocks in Mission Valley is about 3 miles above the old Mission, where the San Diego River enters a cañon. It is a volcanic tufa, consisting of grayish to greenish fragments of a fine-grained rock imbedded in a brown matrix. This has a width of about half a mile. Along the cañon, dikes of a greenish amygdaloid have been intruded in the rock, and are particularly numerous north of the river. One of these dikes in the cañon was observed to be amygdaloidal in the center. Farther up the cañon there is a great variety of tufas. The first contains feldspathic and hornblendic fragments nearly blended in a base consisting of crystallized feldspar and dark chloritic particles. Above this is a dike of brownish crystalline rock, much altered; the only distinguishable mineral being feldspar, in small crystals. Then follows another tufa, with nearly blended micaceous fragments. The next rock is a fine crystalline one with very regular bedding planes, a foot or more thick; dip 30° to 40° southeast, strike north 35° east. Then follows a dark, aphanitic, structureless rock for some distance. At one point a branching dike of almost pure feldspathic material spreads out into this aphanitic rock in radiating arms. Apparent bedding planes run through them, as well as the country rock, showing that these planes are not those of sedimentation, but are due to some secondary cause. These rocks occupy the cañon for 1½ miles, and are all undoubtedly of volcanic or intrusive origin.