THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. 115 
and a station on Mount Diablo, both on the eastern side of the fault and 53 kilometers 
(33 miles) from it. It also includes the Farallon Light-house on the west side of the fault 
and 36 kilometers (22 miles) from it. There were, in all, 51 old triangulation stations 
which were recovered and their new positions accurately determined by the new triangu- 
lation. ‘The stations had been marked upon the ground by stone monuments, by bolts 
in rock, etc., or by permanent structures such as the Farallon Light-house, Point Reyes 
Light-house, and the small dome of Lick Observatory, or were themselves permanent 
marks; as, for example, Montara Mountain peak (a sharp peak). 
This continuous scheme consists of a chain of primary triangulation comprizing the 
eleven occupied stations, Mount Toro, Gavilan, Santa Cruz Azimuth Station, Loma Pri- 
eta, Sierra Morena, Mocho, Mount Tamalpais, Point Reyes Hill, Tomales Bay, Sonoma 
Mountain, and Ross Mountain; triangulation of the secondary grade of accuracy extend- 
ing from the stations, Mount Tamalpais, Mount Diablo, Rocky Mound, and Red Hill, to 
the Pulgas Base near the southern end of San Francisco Bay, and triangulation of a ter- 
tiary grade of accuracy in three different localities; namely, in the vicinity of Colma, west 
of San Francisco Bay, along Tomales Bay, and in the vicinity of Fort Ross, Sonoma 
County. 
The primary and secondary triangulation are shown on map No. 24, and the tertiary 
triangulation on map No. 25. On these two maps the straight blue lines indicate lines 
over which observations were taken in the new triangulation. The small red circles indi- 
cate stations marked upon the ground, of which the relative positions were fixt by the 
triangulation. Observations were taken in both directions over each blue line which 
is unbroken, thruout its length. Observations were taken in one direction, only, from 
the solid end toward the broken end, over each blue line which is broken at one end. 
A station from which no blue line is drawn unbroken was not occupied. The posi- 
tion of such a station was determined by intersections from the occupied stations. 
In addition to this continuous triangulation, a detached piece of new triangulation of 
the secondary grade of accuracy, connecting old triangulation stations, was done in the 
vicinity of Point Arena. (See map No. 25.) This makes the total number of old tri- 
angulation stations which were recovered and redetermined 61. 
In connection with the new triangulation, astronomic determinations of azimuth or 
true direction were made by observations on Polaris at the stations Mount Tamal- 
pais, Mocho, and Mount Toro. 
Four different observers, each with his own complete outfit and party, were engaged 
in the new work for an aggregate period of 35 months. The observers were all field 
officers of the Coast and Geodetic Survey, with previous experience in triangulation. 
Mr. J. F. Pratt, Assistant, was in the field from August 4, 1906, to July 2, 1907, and 
made the observations at the five primary stations, Ross Mountain, Sonoma Mountain, 
Tomales Bay, Point Reyes Hill, and Mount Tamalpais. 
Mr. W. B. Fairfield, Assistant, was in the field from August 11, 1906, to May 29, 1907. 
He did nearly all of the Tomales Bay triangulation, made the observations at the primary 
stations, Mocho and Sierra Morena, and did a part of the secondary triangulation in the 
vicinity of Pulgas Base. 
Mr. C. H. Sinclair, Assistant, was in the field from July 14, 1906, to April 10, 1907. 
He made the observations at the primary stations, Santa Cruz Azimuth Station, Loma 
Prieta, Gavilan, and Mount Toro, and also did a part of the secondary triangulation in 
the vicinity of Pulgas Base. 
Mr. Edwin Smith, Assistant, was in the field from July 12 to July 24, 1906, engaged in 
making the reconnaissance and other preparations for triangulation along Tomales Bay. 
He was then called away on other duty and Assistant Fairfield completed the Tomales 
Bay triangulation. Between September 26, 1906, and February 26, 1907, Mr. Smith did 
