222 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
the valleys or the districts of sand-dunes. At the lower levels such loose materials cover 
the whole area very generally. The thickness of these strata must be notably variable, 
considering the uneven configuration of the rock surface where it emerges from this 
mantle, since it is probably no less irregular beneath the covering. Very little informa- 
tion is available concerning the depths to which these uncemented materials extend. A 
well at the United States mint is about 176 feet deep and is believed not to have reached 
bedrock. A boring that was sunk at the corner of 7th and Mission Streets past thru 
sand and clay to a depth of 264 feet, but did not reach bedrock. In general the sands 
and clays fill deeply the major valleys, Mission and Islais. The minor northwest fork of 
Mission Valley, called Hayes Valley along its lower part, is probably less deeply filled. 
This is certainly true of its upper reaches, to which, in this report, the name Upper Hayes 
Valley is applied. Minor valleys and gullies all over the area have thin coverings of sand 
and alluvium which quickly thin out where the slopes of the hills begin to rise steeply. 
From the ocean inland for a considerable distance extends an area covered with sand- 
dunes. This district is limited irregularly at the east by the contour of the hills. The 
sands form a thick mantle near the ocean shore, which becomes thinner and thinner as it 
rises upon the lower flanks of the hills. As in the case of the materials filling the valleys, 
the rock floor upon which the sands rest is probably very irregular. 
Of the hills, the northern ridge is carved out of the firm sandstone of the Franciscan 
series. Along this ridge are the summits of Telegraph Hill, Nob Hill, and Russian Hill, 
with other unnamed hilltops to the west, separated from each other by little saddle-like 
depressions in the surface. The outlying summits of Black Point and Rincon Hill appear 
to belong genetically to this ridge. This body of sandstone abuts on the west against a 
mass of serpentine, which forms a narrow range of hills stretching southeastward across 
the peninsula from Fort Point to Potrero and Hunters Points. 
This serpentine is intrusive in the firm Franciscan rocks, chert, and sandstone. The 
southwestern boundary of the serpentine in the vicinity of Fort Point is determined by a 
fault which has a throw of about 1,000 feet. This fault may possibly extend quite across 
the peninsula along the southwestern limits of the serpentine, but the field evidence does 
not warrant any definite statement. The fault movement occurred so long ago that the 
present land surface gives no unequivocal indication of its position. Mission Valley cuts 
across the body of serpentine, separating the northern hills from the southern group. 
The northern group rises along the western boundary of Hayes Valley. 
The central and southern hills, and the ridge at the northwest of the city, are carved 
intricately from firm Franciscan rocks, sandstone, and chert, commingled with minor 
bodies of irruptive rock of basaltic character. 
The hills of the more remote southwest corner of the city and county are of softer 
rocks of more recent geological origin — sandstones and shales of the Merced formation. 
These are relatively little cemented. Readers interested in a more complete account of 
the geology should consult the detailed report on this peninsula.' 
DESTRUCTIVE EFFECTS AND INTENSITY SCALES. 
To some extent the earthquake caused damage to buildings and other structures in 
all parts of the city and county of San Francisco. The whole area was decidedly within 
the destructive zone. Still, over a large part of this area, far the larger part, the damage 
was slight both in amount and character. Almost everywhere chimneys were thrown 
down or badly broken, but in a few small localities most of the chimneys withstood the 
shock. Some probably were unhurt. Plaster on walls and ceilings was very generally 
damaged. So, probably, were frail partition walls and chandeliers, crockery and fragile 
household furnishings. Such effects were typical of large sections of the city. There 

* The Geology of the San Francisco Peninsula, by Andrew C. Lawson, 15th Ann. Rept., U.S. G. 8. 
