128 
GEOLOGY. 
with the young shoots of the wild oat springing up luxuriantly. It was evident that these 
hills were underlaid by granite at no great depth, as it frequently rose to view in low ridges. 
A trap dyke, traversing the granite, .was noted at a point about half-way between San Pas- 
qual and the Mission, hut I could not ascertain' its direction with accuracy. A short distance 
beyond this dyke, a section of a low hill near the trail exposed a bed of conglomerate, six feet 
thick, dipping gently towards the southeast, at an angle of five degrees. It was full of rounded 
fragments, from one to three inches in diameter, all firmly cemented together, and underlaid by 
loose, sandy beds. The materials of the conglomerate were much water-worn, and it had the 
appearance of beach shingle. 
A second exposure of trappean rock was crossed two or three miles westward of the first. 
The direction of this last intrusion appeared to he nearly the same as the prevailing trend of the 
granite ridges—northwest and southeast—and it appeared to form the last range of hills be¬ 
tween the mountains and the Pacific. Westward from this point the country was entirely open, 
extending off towards the ocean in a broad, gently sloping plain, broken only by slight eleva¬ 
tions around the hay of San Diego. 
This wide slope flanks the foot-hills of the mountains for a long distance, both to the north 
and south of San Diego. Its rate of descent is so gradual that it seems like a plain, and appears 
to be unbroken by a single valley ; but by travelling upon it, it is soon discovered that streams 
have cut their way below the surface, and excavated numerous deep, narrow channels and 
valleys, in one of which the Mission of San Diego is situated. 
When the observer is in one of these excavated valleys, the view of the surrounding country 
is completely cut off by the continuous banks on each side. The surface of the slope is strewn 
with rounded pebbles and cobble-stones ; and in some places these are thrown together in low 
heaps, like the accumulations upon a beach. These water-worn accumulations, together with 
the gentle and uniform slope of the broad plateau, give to it so much of the character and ap¬ 
pearance of a shelving ocean beach, that, when an observer stands near its upper portions, on 
one of the banks of pebbles, and hears the dull sound of the distant surf, it is impossible to resist 
the conviction that it once broke among the pebbles at his feet. 
There is a wide area of cultivable land about the Mission, 1 confined, however, to the valleys 
below the general level of the slope. There were few or no good exposures of the strata forming 
the slope, and no favorable natural section was seen. The strata are nearly horizontal, and 
probably Tertiary. While in camp, a mass of sandstone, filled with Tertiary fossils, was sent to 
me, but its precise locality could not be ascertained. It was probably taken from the adjoining 
hills, or picked up in the bed of one of the streams. On the way from the Mission (six miles 
inland from the bay) to “Old Town,” or San Diego, I passed a slight exposure of the strata at 
the road-side, and found them to be composed chiefly of sand and argillaceous beds, some of 
them quite dark-brown, and nearly black with bitumen, indicating coal or lignite. 
At Punta Loma, there is an exposure of strata made by the undermining action of the sea. 
It is probable that the strata are more recent than those of the slope, having a modern appear¬ 
ance, and not being firmly consolidated. They appear to dip gently towards the east or from 
the sea. 
Before leaving San Diego, I collected many shells along the beach, and found a thick layer 
1 The Mission of San Diego was founded by the Padre Junipero Serra on the 16th of June, 1769, and became, with the 
Presidio, the first establishment of the Spaniards in New California, although the port of San Diego had been discovered 
a long time before. This Mission, which the Franciscans called the Mother Mission, is seventeen leagues north of San Miguel, 
the most northern Mission in Old California, and fourteen from the Mission of San Luis Rey de Francia. 
