MEASUREMENT OF FUTURE MOVEMENTS ON SAN ANDREAS FAULT. 153 
In order to render the monuments thus established available for future measure- 
ments of displacement, it was necessary to have their present relative positions established 
with precision. ‘This work was very kindly undertaken for the Commission by Mr. B. A. 
Baird, Assistant, Coast and Geodetic Survey, a report from whom follows, setting forth 
his methods and results: 
RELATIVE POSITIONS OF THE MONUMENTS. 
By B. A. Barrp. 
OLEMA. 
Description of monuments. — The monuments at Olema are on Mr. Skinner’s ranch, 
just a little north of the dwelling-house. The two piers west of the fault-trace are in an 
orchard on level ground, but the other two, which are just east of the road, are on a hill- 
side, the northeast monument being 
about 15 feet higher than any of 
the others. In order to measure 
and observe between the northwest 
monument and the southeast mon- 
ument, a trench about 3 feet deep 
had to be dug thru the embank- 
ments on both sides of the road and 
somewhat into the traveled portion 
as well. Some clearing of brush 
was necessary in order to make 
the northeast monument and the 
southwest monument intervisible. 
The relative positions of the four 
monuments are shown in the dia- 
gram, fig. 44. The lengths of the 
three heavy lines were determined 
by measurement. The measure- 
ments of the other three lines were 
considered impracticable, on ac- 
count of the great height of the 
northeast monument above the 
others, as compared with the short 
distances between them and it. 
By means of the three measured 
lines, however, a double determi- 
nation is obtained, thru the ob- 
served angles, of each of the three 
lines not measured by the steel tape. 
The lines were cleared sufficiently 
so that all four of the monuments 
could be occupied with a theodolite, 
and then all of the lines were ob- 
served, including the diagonals. In 
order that future movements may Fia. 44. — Monuments at Olema. 
be readily detected by means of ob- 
served angles, the centering of the instrument was considered to be of the greatest impor- 
tance, especially for such very short lines as these. A bronze plate had been constructed and 
set up on each monument, especially designed for supporting in position the Fauth 10-inch 
alt-azimuth instrument of the Civil Engineering Department of the University of California. 
A sketch of the plate is shown in fig. 45. The spindle which screws into the central socket 
of the plate is shown in fig. 47, and the iron cap which protects the plate when the spindle 
is removed is shown in fig. 46. Referring to the sketch of the plate, it will be seen that there 
are three lugs, or foot-plates, standing upon and attached to it. In one is a groove (vertex 
of angle at bottom), and in one a hole (inverted cone), while the third has simply a smooth 
surface. This arrangement prevents any binding of the foot-screws of the instrument, and 
insures that it will always be set in the same position in successive measurements. 


LANO 


GRAZING 



110. 5/2 

