134 THE SAN FRANCISCO EAKTHQUAKE AND EIRE. 
In order to get a fair understanding of the effects of the earth- 
quake in San Francisco, a knowledge of the geologic formation and 
the different soils constituting the foundations of structures is neces- 
sary. The city and county of San Francisco comprise the northern 
extremity of a long, narrow peninsula, lying south of the Golden 
Gate, between the Pacific Ocean on the west and the southern half 
of San Francisco Bay on the east. (See map, PL LVII.) The 
boundary line between this county and San Mateo County lies about 
8 miles south of the Golden Gate. The area of San Francisco County 
is 46^ square miles. The population of the city on April 1, 1906, 
was estimated to be 460,000. 
The site of the city has at least four different soil formations. 
Around- the Bay of San Francisco, from Telegraph Hill to Mission 
Creek, which runs from west to east and empties into the bay, and 
on both shores of the creek is a strip which was originally mud 
flats and overflowed lands, having an area of about 354 acres. These 
tracts have been gradually filled in (especially on the bay shore 
and the northern Mission Creek sides), since the days of the Ameri- 
can occupation, by encroachments on the water front, due to business 
and commercial pressure, and wharves and docks, warehouses, fac- 
tories, manufacturing establishments, and large wholesale houses 
have been built on these filled-in lands. At the present time these 
large areas are for the most part included within the sea walls run- 
ning around the officially established water front nearly as far as 
Mission Creek. They are known as " made lands," and consist of 
deep layers of mud, in many places saturated with salt water, and 
overlain by sand, trash, etc., which has been filled in upon them. 
On this soil were built nearly all the commercial and wholesale 
business structures of San Francisco — such as the Union Ferry Build- 
ing — many large hotels, the post-office, the branch mint, and similar 
structures. On the Mission Creek side w T ere originally very large 
areas of marshes that have been filled in with sand from adja- 
cent hills. Adjoining all these made lands is the comparatively 
level ground, composed of a natural mixture of sand and clay, formed 
by the wearing of the hillsides and by the incoming of sands drifted 
from the seacoast. Upon this fringe of soil next to the made lands 
were built many of the largest hotels, tall office buildings, and expen- 
sive structures of brick, stone, and steel. 
A ridge of rocky hills runs from the northeast corner of the city, 
or Telegraph Hill, southwestward along Russian Hill, Clay Street 
Hill, and so on, to Sutro Heights. These hills are composed largely 
of indurated clay, shale, and, on their highest summits, serpentine 
and other rocky formations. A ridge of sand hills runs through the 
western and southwestern portions of the city to the Pacific Ocean. 
The slopes and summits of the hills nearest the business portion of 
