342 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
outcrops; and the sandstone areas were more disturbed than the serpentine. But these 
differences are minute. 
In the case of the made land and old marsh land of San Francisco, where the apparent 
intensity reached X, there can be no question as to possible local shocks, since the exces- 
sive disturbance was so strictly limited to the area lying outside of the original shore line 
and marsh border. 
In the low ground about San Francisco Bay to the south of the city, we have another 
instance, on a rather large scale, of high apparent intensity determined by the incoherent 
water-saturated condition of the underlying formations. San Francisco Bay in general, 
and the southern portion of it in particular, lies in an alluviated valley which has been 
deprest so that its central portion is now below sea-level. This submerged valley-floor 
passes insensibly into the Santa Clara Valley which encloses it on the south and extends 
southward between the Santa Cruz and Mount Hamilton ranges. Treating San Fran- 
cisco Bay and Santa Clara Valley as one physiographic feature, it may be stated, without 
going into the evidence in detail, that depression and alluviation have both been greater 
in the southern end than in the northern. This southern portion of the valley con- 
stitutes a great artesian basin, and many wells have been sunk in it. The deprest trough 
is not, however, wholly filled by alluvium, since several wells have past through late 
Quaternary strata containing marine fossil remains. It would appear, from the sections 
revealed by these wells, that with the progress of subsidence, marine deposition alternated 
with alluviation. The deposits, whether marine or alluvial, appear to be incoherent 
or unconsolidated, consisting of clays, sands, and gravels, in layers of irregular thick- 
ness and extent. Many wells have past through several hundred feet of such materials 
without reaching bedrock. One well, on the edge of the marsh near Alvarado, reached 
rock at a depth of 730 feet. At the sugar-mill at Alvarado, and at the Contra Costa 
pumping plant, in the same vicinity, there are several wells from 300 to 400 feet deep, 
passing thru clay, sand, and gravel without reaching bedrock. At Roberts’ Land- 
ing-there are 2 wells, one 574 feet deep and the other 540 feet deep, which past thru 
alternations of clay, sand, and gravel, but did not reach bedrock. A well 1.5 miles 
south of Milpitas past thru 11 layers of gravel aggregating 166 feet and 12 layers of 
clay aggregating 218 feet — total depth 384 feet — but did not reach bedrock. The 
wells in the vicinity of San Jose range in depth from 35 to 500 feet as a rule. One well 
on the bank of Guadaloupe Creek, however, was sunk to a depth of 1,100 feet, but did 
not penetrate bedrock. A well at Stanford University is in gravel at 412 feet. On the 
west side of the Bay there are several hundred wells, most of them less than 100 feet in 
depth, while the deep ones are usually a little more than 300 feet. Wells are even bored 
in the bottom of the Bay and an abundant supply of fresh water is obtained from them. 
These brief statements will be sufficient to afford a general idea of the extent to which 
the valley has been deprest and filled in with deposits as yet unconsolidated. To the 
south, the rocky floor of the valley appears at the surface in the vicinity of Coyote, 12 
miles south of San Jose. Beyond this, however, the valley again opens out and is 
deeply alluviated. 
On the floor of this valley, from San Bruno Mountain southward, on both sides of the 
Bay, and southward a few miles beyond San Jose, the intensity was abnormally high. 
On the rocky slopes between the western edge of the valley-floor and the fault, the 
intensity had dropt from X at the fault to VIII at the base of the hills. On the valley- 
bottom it again sharply rose to IX. On this ground were Stanford University, Red- 
wood City, San Mateo, the 44-inch pipe of the Spring Valley Water Company, San Jose, 
Agnews, Milpitas, and Alvarado. On the eastern side of the Bay the intensity of LX 
did not persist to the base of the hills, but extended only about halfway from the shore 
line to the edge of the valley. There was therefore a distinctly diminishing intensity in 
