THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. . 135 
ranging from more than 52 seconds to zero. The squares to the westward of the fault 
were moved bodily in a northerly direction parallel to the fault, their sides parallel to 
the fault remaining straight and unchanged in length and direction. Their sides perpen- 
dicular to the fault became curved lines concave to the northward and each changed in 
direction by rotation in a counterclockwise direction, the change being more than 31 
seconds for squares near the fault and less for more remote squares. The displacement 
of squares near the fault was twice as great for squares on the western side as for squares 
on the eastern, but the distortion was slightly less for squares on the western side than 
for those on the eastern side. The appreciable displacements extended back much farther 
from the fault on the western side than on the eastern side. 
It is not probable that the actual displacements and distortions were perfectly regular 
as indicated in the word picture of the preceding paragraph, but the apparent departures 
from this perfectly regular ideal, of the displacements and distortions detected by the 
triangulation are nearly all so small as to be possibly due to errors of observation. Atten- 
tion has been called to the few exceptions, of which one can be certain, which have been 
detected. The earth-movements of April 18, 1906, were remarkable for their regularity of 
distribution. 
The triangulation of 1906-1907 has extended eastward clearly beyond the region of 
appreciable permanent displacements by the earthquake of 1906. The disturbed region 
evidently extended to the westward out under the Pacific beyond the possible reach of 
the triangulation. To the northward of Point Arena there is little probability of much 
success if an attempt were made to determine additional displacements by triangulation, 
for the known fault of 1906 touches the coast for but a short distance anywhere north of 
Point Arena, and triangulation to the northward of Point Arena before the earthquake 
consisted simply of a narrow and weak belt of tertiary triangulation. It had been 
intended to extend the triangulation of 1906-1907 far enough to the southward to 
reach outside of the disturbed region. It was supposed until after the observing party 
left the southern end of the triangulation that this had been accomplished, but when 
the additional evidence given by the office computations became available, it was evident 
that the most southern points determined are still within the disturbed region. The 
fact that the visible evidence of the fault of 1906 does not extend farther southward 
than San Juan indicates that there are probabkx few points to the southward of Mount 
Toro and Point Pinos for which the displacements were large enough to be detected by 
triangulation. 
DISCUSSION OF ASSUMPTIONS... . . 
Certain things have apparently been assumed in this investigation; for example, that 
appreciable permanent displacements occurred during the earthquake of 1868 as well as 
during the earthquake of 1906; that the permanent displacemepts in 1906 occurred 
suddenly, and that the stations Mocho and Mount Diablo remained unmoved in both 
earthquakes. ah 
These are called apparent assumptions because in a real sense they are not assumptions 
but are instead facts detected gradually in studying for fifteen months upon a steadily 
increasing mass of evidence. However, treating them as assumptions, their validity has 
been reéxamined in the light of all the eviderice, and to make this report complete, it is 
now necessary to state why they are believed to be true. 
It has been tacitly assumed that the permanent displacements of 1906, detected by the 
triangulation, took place suddenly. It is certain from evidence entirely distinct from the 
triangulation that on April 18, 1906, relative displacements by sliding along the great 
fault of that date took place suddenly, that is within an interval of a few seconds, without 
much crushing or separation of the sides of the fault, and that these relative displacements 
