-ISOSEISMALS: DISTRIBUTION OF APPARENT INTENSITY. 221 
The most important of these is the relatively large Mission Valley, opening into the Bay 
between Rincon Point and Potrero Point and extending back westward and then south- 
ward, with a minor fork to the northwest, fully a quarter of the way across the penin- 
sula. Mission Creek, with its lagoon and contiguous marsh, before it was filled to provide 
street and building sites, extended from the Bay shore around the northern extremity of 
the hills of the Potrero. Another long narrow marsh occupied a part of the floor of Mission 
Valley, stretching eastward from the present site of the Post-office building for several 
blocks, and then turning southward to the old Bay shore. This marsh also has been filled 
to provide building sites. Another dominant valley is that of Islais Creek, stretching 
back to the southwest between the hills of the Potrero and those of Hunters Point. This 
valley is outside the city proper. . 
The city and county of San Francisco occupy the northern end of the peninsula, bounded 
on the south by an arbitrary east-west line some 7 miles south of the Golden Gate. The 
city, properly speaking, occupies the northeastern third of this area, covering the sum- 
mits and flanks of the sandstone hills known as Telegraph Hill, Nob Hill, and Russian 
Hill, on the north; and other unnamed summits on the west. It covers also the floor 
of Mission Valley and reaches well up on the flanks of the hills which culminate in the 
center of the area. On the outskirts of the city proper, except in the southwestern part, 
are small detached groups of dwellings in the hills or on the sands. 
Market Street is a broad thorofare running southwestward from the Ferry Building and 
the wharves, at the northeast corner of the city, thru Mission Valley to the flanks of the 
high hills in the center of the area. About the lower part of Market Street is the commer- 
cial center of the city. The City Hall, situated about 1 block north of this broad highway, 
and about 12 blocks southwest of the Ferry, was not far from the center of the city proper. 
The zone of faulting where the recent earthquake had its origin past under the sea from 
a point near the head of Bolinas Lagoon, 12 or 15 miles northwest of the Golden Gate, 
to a point half a mile north of the little headland of Mussel Rock, about 8 miles south of 
Point Lobos. The map, No 4, shows its location. 
The entire area of the city and county is east of the fault-zone. The southwest corner 
of the area is less than a mile distant from it. The vicinity of the Ferry Building, at the 
foot of Market Street, was the most remote of any point in the whole area, being between 
9.25 and 9.75 miles away. The site of the City Hall is from 7.5 to 8 miles from the fault. 
The Cliff House, at Point Lobos, the most western point of the area, is about 3 miles east 
of it. Fort Point lies between 5.75 and 6 miles east of it. Potrero Point and Hunters 
Point, as well, are about 8.5 miles from the fault. Hunters Point is the most easterly 
point in the district. 
| GEOLOGY. | 
‘It is desirable to insert here a brief abstract of the geology of the northern part of the 
San Francisco peninsula, for it will appear that the effects produced by the earthquake 
were largely influenced by the character of the underlying formations. Map No. 17 
shows the distribution of the geological formations at the surface. It shows also the areas 
of ‘“‘made” land. These areas were determined by plotting the shore line shown on the 
accurate chart published in 1853 by the U.S. Coast and Geodetic Survey, upon the 
latest accurate chart of the same bureau. In these districts the materials forming the 
surface have been transported to their present position by human agency. The depth 
or thickness of this “filled” stratum is variable and, for the most part, not definitely 
known. 
A little study, comparing the areas of Welk with the topographic contours, shows that 
all the hills are of firm rock, mostly coated with a veneer of soil and vegetation, but fre- 
quently outcropping at the surface. In ‘general, their lower flanks are more and more 
thickly covered with loose sand and alluvium the nearer approach is made to the floor of 
