THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. 73 
for example, the locality of the cow incident, the Papermill delta, and Pepper Island — 
the suggestion of lateral compression seems of little avail. 
Another suggestion is that the surface phenomena are essentially representative of 
what occurred at greater depths — that is, that in depth, as well as superficially, the 
faulting left the fault walls farther apart than they were before. Fissure veins show 
that voids have often resulted from subterranean faulting. Unless the surface along 
which the movement occurred is mathematically plane — or conforms to some equally 
difficult geometric condition — the two fault walls should not accurately fit together 
after the movement, but should tend to maintain contact thru only a part of their extent. 
If thru a part of their extent they are separated, the walls are on the average farther 
apart than before. 
There would necessarily be some adjustment thru changes within the rock masses on 
the two sides of the fault. Compressive strains would be locally increased and reduced, 
and there would be subordinate movements among the minor earth blocks of the great 
shear zone whose surface features appear in the Rift. We have evidence of such adjust- 
ments, in fact, in branches of the fault-trace and in a system of bedrock cracks presently 
to be described; as well as in the subsidence of the bottoms of sags in the immediate 
vicinity of the fault. Interpreting other sag phenomena in the light of the long sag of 
the Papermill Creek delta, the fault of 1906 appears to have permitted a very considerable 
volume of material to sink into its fissure. 
The general tendency of this discussion falls in line with a generalization as to the Rift, 
which in the Bolinas-Tomales region appears to show distinctly more local subsidence 
than local elevation. 
Earlier fault-traces. — Because the future is to be judged by the past, there is much 
interest in the question of the frequency and recency of fault movements along the Rift 
previous to 1906. In my later studies of the Rift belt, I have had in mind the possibility 
of discovering fault-traces similar to that of 1906 but less fresh in appearance. In the 
little bluffs at the edges of sags, and in the ponds and marshes, there is abundant evidence 
of early faulting, but it is essentially geologic and does not necessarily pertain to occur- 
rences of the past century or two. The fault-trace, however, is a relatively perishable 
and transient phenomenon, and its preservation might have comparatively definite 
meaning. 
At two localities I thought I discovered old “traces” of the ridge type. In each case 
the features occur on a hill slope where the trace made in 1906 appears in several divisions 
or branches; and what I took to be old traces are distinguished chiefly by the absence of 
cracks. The localities are close together, about 0.5 mile south of the Shafter ranch, and 
may be identified by means of plate 438. The features oceur on the slope at the left, 
but are too indefinite to be recognized in the view. If these old traces have been properly 
identified, they are of very moderate antiquity. I should suppose that the ridges of 
the recent trace would lapse to such a condition in four or five years and that they might 
persist, under pasture conditions, for two or three decades. The history of the recent 
trace shows that a single plowing means effacement, but the general appearance of the 
field in which the old traces occur indicates that it was never plowed. 
Cracks. —In preliminary reports I have classified the earthquake cracks as pri- 
mary and secondary, the primary being occasioned by strains which existed before the 
earthquake, and the secondary being caused by the earthquake. With the multiplica- 
tion of observations this classification has become increasingly difficult, and I now find 
it more convenient to group the cracks as superficial and deep, or superficial and bedrock. 
Many of the superficial cracks are in alluvium. In the field excursions of April and 
May, 1906, they were seen in all alluvial formations within the Rift belt and for some 
distance on each side, The greater number appeared to be merely partings without 
