116 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
the secondary triangulation in the vicinity of Point Arena and the tertiary triangulation 
in the vicinity of Fort Ross and in the vicinity of Colma. 
These observers remained at their work continuously in spite of many delays and dis- 
comforts due to fog, rain, snow, gales, and roads which were at times nearly or quite 
impassable. To them must be given the credit for overcoming the difficulties and secur- 
ing the observations of the necessary high grade of accuracy. 
THE OLD TRIANGULATION. 
The old triangulation fixing the positions of the points before the earthquake of April 
18, 1906, was done in many years, extending from 1851 to 1899, as a part of the regular 
work of the Coast and Geodetic Survey and without reference to the possible future use of 
this triangulation as a means of determining the movements of permanent character due 
to earthquakes. During the earlier years certain parts of this old triangulation had 
existed as detached triangulation not connected with other parts. Before 1906, however, 
all parts of the old triangulation had been connected with each other by triangulation to 
form one continuous scheme. It was also connected with other triangulation extending 
to many parts of the United States, including many of the interior states, as well as the 
Atlantic and Gulf Coasts. 
In connection with studies of the evidence as to the earth movements set forth in this 
paper, it is important to note briefly the dates of the old triangulation which serves, 
in connection with the new triangulation of 1906-1907, to determine changes in positions 
of marked points on the earth’s surface. 
During the years 1854-1860 primary triangulation was carried from the stations, Rocky 
Mound, Red Hill, and Mount Tamalpais, northward to Ross Mountain, thru a primary 
scheme practically identical with that shown on map No. 24, except that the station 
Bodega was occupied in this earlier triangulation, tho not in 1906-1907, 
Tertiary triangulation, following substantially the scheme shown on map No. 25, was 
also done in 1856 to 1860, along Tomales Bay, starting with the line Tomales Bay-Bodega, 
of the primary triangulation referred to in the preceding paragraph. In connection with 
this work, the station Chaparral of the Fort Ross triangulation, shown on map No. 25, 
was also determined. 
Primary triangulation was done during the years 1851 to 1854, connecting the group of 
stations, Mount Diablo, Rocky Mound, Red Hill, with the Pulgas Base, the scheme being 
somewhat different from that shown on map No. 24, but equally direct and strong. 
During the years 1854-1855, 1864, 1866, primary triangulation was done connecting the 
stations in the vicinity of Rocky Mound, referred to in the preceding paragraph, with 
stations Gavilan, Santa Cruz, and Point Pinos Light-house around Monterey Bay. 
This triangulation, for the greater part of its length, consists of a single chain of tri- 
angles, affording, therefore, comparatively few checks upon the results. 
This practically completes the statement of triangulation done before 1868 which is 
concerned in the present investigation. The extent of the triangulation done between 
1868 and 1906 is stated separately in the following paragraphs. 
Northward of the line Mount Diablo-Mount Tamalpais, but one station of the primary 
scheme, shown on map No. 24, was determined by primary triangulation in the inter- 
val 1868-1906; namely, Ross Mountain. It was determined directly from the stations 
Mount Tamalpais, Mount Diablo, and Mount Helena of the transcontinental triangulation.’ 
During the years 1876-1887, primary triangulation was extended southward (by sub- 
stantially the same scheme as that shown on map No. 24, except that station Gavilan 
was omitted) from the line Mount Diablo-Mount Tamalpais to the line Mount Toro- 

1 See The Transcontinental T riangulation, Special Publication No. 4, pp. 597-608. 
