126 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
Bay, Bodega, and Ross Mountain (see table 2). Some of the internal evidence of com- 
putations of triangulation indicate that Sonoma Mountain moved in 1868. According 
to the general laws of distribution of the earth movement of 1906 as derived from other 
stations Sonoma Mountain did not move much, if any, being far to the eastward of 
the fault, 34 kilometers (21 miles). For these three reasons it is believed to be prob- 
able that the whole displacement of Sonoma Mountain, 1.24 meters (4.0 feet), in azimuth 
183° (8° E. of N.), which certainly took place sometime between 1860 and July, 1906, 
all occurred in 1868. 
Group 2. Southern end of San Francisco Bay. — In this group there are three new 
points not yet considered and Red Hill which has already been considered in group 1. 
The three new stations, Guano Island, Pulgas Hast Base, and Pulgas West Base (see 
map 24), were determined in 1851-1854 and again after the earthquake of 1906. No 
determination was made between 1868 and 1906, hence these points are entered in table 3, 
the combined displacements of 1868 and 1906 being determined, but not the separate 
displacements. 
A study of the errors of the triangulation shows that the apparent displacement of 
Guano Island, 0.21 meter (0.7 foot), is probably due to errors of observation, and that 
there is one chance in three that the apparent displacement of Pulgas East Base, 0.41 
meter (1.3 feet), is also due to errors of observation. 
The determination of the displacement of Pulgas West Base, 0.74 meter (2.4 feet), is 
reasonably certain, there being about one chance in twelve that it is due to errors of 
observation. 
Tho the determinations of the separate apparent displacements of Red Hill in 1868, 
0.65 meter (2.1 feet), and in 1906, 0.30 meter (1.0 foot), are each doubtful, the combined 
displacement as observed, shown in table 3, 0.94 meter (3.1 feet), is certain. 
It is therefore reasonably certain that there was a relative displacement of Pulgas West 
Base and Red Hill as indicated in table 3, Red Hill moving 0.94 meter (3.1 feet), in azi- 
muth 227° (47° E. of N.), and Pulgas West Base 0.74 meter (2.4 feet), in azimuth 344° 
(16° E. of 8.). This lengthened the line Pulgas West Base to Red Hill, 16 kilometers 
(10 miles) long, 0.50 meter (1.6 feet), or one part in 32,000. It also changed the azimuth 
of this line by 11”, from 240° 44’ 35” to 240° 44’ 24”, rotating it in a counterclockwise 
direction. 
The red arrows on map 24, showing apparent displacements, indicate that the apparent 
displacements of Guano Island and Pulgas East Base, which are considered doubtful, 
are not inconsistent with the displacements of Red Hill and Pulgas West Base. Appar- 
ently the area included between these four stations was distorted by stretching and 
rotated in a counterclockwise direction. 
There is no evident method of ascertaining whether the displacement of Pulgas West 
Base took place in 1868 or 1906 or in part at each time. The displacement is nearly in 
the direction corresponding to the laws governing the displacements of 1906, as already 
stated in connection with group 1. Pulgas West Base is to the eastward of the fault of 
1906 and slightly nearer to it than Mount Tamalpais and Ross Mountain and hence, 
according to the laws referred to, should be displaced in the same direction as these two 
points (see table 1), and by a similar amount. ‘This is the fact. 
Group 3. Vicinity of Colma.— There are nine points in group 3 all determined by 
triangulation in 1899 or earlier, and redetermined after the earthquake of 1906 (see table 
1). The earlier determination was made by secondary and tertiary triangulation, ex- 
tending from the vicinity. of Pulgas Base northwest, spanning San Francisco Bay to the 
Golden Gate, and thence southward to Colma. The earlier positions of these nine points 
are subject to the effect of accumulated errors in this chain of triangulation about 60 
