PERMANENT DISPLACEMENTS OF THE GROUNDS. 27 
A POSSIBLE ORIGIN OF THE DEFORMING FORCES. 
The reasoning so far has been strictly along dynamic lines and the results may be 
accepted with some confidence; but in attempting to find the origin of the forces which 
produced the deformation we have been studying, we pass into the region of speculation. 
The theory of isostasy, which has been shown to be true on broad lines by geodetic 
observations, requires that there be flows of the material at some distance below the sur- 
face to readjust the equilibrium destroyed by the erosion and transportation of material 
-at the surface. This suggests that flows below the surface may have been the origin of the 
forces we have been considering, for as Dr. Hayford has pointed out,' such flows would 
exert a drag on the material above them. The isostatic flows are the direct result of 
gravity and therefore easily understood, but no explanation has been found for the flows 
suggested as the origin of the forces in the case under consideration; nevertheless, as the 
forces must have been exerted at the lower surface of the moved region, it is worth while to 
determine the character of the flows which could have produced these forces, and leave 
to future observations the decision as to whether they really exist or not. Without 
assuming exact proportionality between the flow and the dragging force it exerts, we 
can say that the flow would be in the same directions as the force and would increase and 
decrease with it. Therefore the flow can be inferred from the diagram of forces in figs. 
13and16. In the first case they consist of a flow to the north between G and O, and a flow 
to the south between O and H; they would not be uniform, but starting with a zero value 
at G and H, they would increase to maxima at D’ and F’, and decrease again to zero at O. 
The force between W and G, H and E, would not be due to flows but would be due to the 
resistance to the displacement of that part of the crust by the undisturbed material below ; 
this displacement being due to the drag of the flows nearer the fault, transmitted elasti- 
cally thru the crust to these regions; this is indicated by the reversed curvature of the 
line of displacements in fig. 12. The principle of continuity would naturally lead us to 
suppose that the flows were connected beyond the northern and southern ends of the 
fault; these portions of the flow would be so far apart and would have so short a length in 
comparison with the portions flowing north or south that their effects would be relatively 
insignificant. It may appear that there is a suggestion here of perpetual motion, but this 
is not so; all steady flows are in closed circuits, and it is only in case we should disregard 
the necessity of a proper supply of energy, that we should fall into the fallacy of perpetual 
motion. 
The line of demarkation between the northerly and southerly flows need not necessarily 
lie exactly in the fault-line, but sufficiently near it for the growing shearing force to reach 
the limiting strength of the rocks at that point before it did at other points; nor is it nec- 
essary to suppose that the flows remain either constant in strength or in position; the con- 
trary seems more probable; for if, as is natural to suppose, the forces which caused the 
earthquakes of 1868 and 1906 were of the same general character, the region of greatest 
shear, that is, the boundary between the flows, must have been in the neighborhood of the 
Haywards fault, about 30 km. (18.5 miles) further east, in 1868. Indeed, the displace- 
ments which occurred between the first two surveys indicate a somewhat different distri- 
bution of the flow from that suggested to explain the later displacements. 
At first thought we might suppose that the movement of Mount Tamalpais in opposite 
directions relative to Mount Diablo in the two intervals between the surveys would indi- 
cate that it was on opposite sides of the boundary during these intervals respectively, 
but this would not necessarily follow. During the whole time that strains were being set 
up all points west of C moved to the north with respect to it; this relative movement in 
the second interval is represented on the eastern side of the fault by the distances between 
the lines C’Q’ and C”Q” in fig. 6; and if we consider the curves in the figure as similar 


1The Geodetic Evidence of Isostasy. John F. Hayford. Proc. Washington Acad. of Sci., 1896, 
vol. v1, pp. 25-40. 
