THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. 83 
ing from some adverse condition, but elsewhere the normal green color was continued 
to the lowest limit. At the head of the lagoon and just to the east of the fault-trace is 
a considerable tract of Salicornia, of which the low-lying parts showed a brown color, 
but the distribution of vigorous and sickly plants was less simple than on the island and 
its causes were not fully understood. I afterwards visited the north slope of the spit 
to see if the condition of its vegetation corresponded to that on the islands, but found 
the evidence complicated by another factor. The overflow of the spit by waves during 
the past winter had washt a considerable amount of sand down the north slope, and this 
sand suffocated large tracts of Salicornia and other plants. 
In the discussion of these data, the first point to be noted is that the killing of Salicornia 
thru the lower part of its zone definitely indicates a lowering of the ground on which it 
stands. The plant normally travels down the slope as far as it can tolerate the tidal 
submergence and there stops; and its inability to sustain itself in a well-defined belt 
constituting the lower part of its former range shows that the submergence in the belt 
has become intolerable. The amount of submergence is shown by observation 
on McKennan Island to be at least 10 inches, and if allowance is made for a certain 
amount of lag in the response of the plant to change of condition, the lowering of 
the land may have been several inches more than this. If McKennan Island and the 
eastern part of Pepper Island subsided the same amount, it is probable that the only 
change on Pepper Island was a subsidence of its eastern part, the western part remain- 
ing at its former level. In that case the amount appears sufficient to account fcr the 
overwashing of the spit, altho no measurement is there practicable. 
The tract of land, whose subsidence appears to be demonstrated by the botanic evidence 
and the overtopping of the spit by waves, is bounded on the southwest by the fault, but 
its other limits are not known. In the immediate vicinity of the fault it may reach the 
head of the lagoon; that it does not extend beyond is rendered probable by the fact that 
there is no vertical dislocation in the fault-trace at a point about one mile northwest of 
the lagoon where the trace is favorably exposed on flat ground. It may be possible that 
the area of subsidence is limited on thé northeast along an old line of dislocation which 
coincides approximately with the northeast side of the lagoon. ‘This dislocation has not 
been determined by a study of the geologic structure, but is indicated by the physi- 
ography, and was presumably concerned in the making of the basin occupied by the 
lagoon. 
The evidence of elevation west of the fault is less coherent. Dr. Southworth’s observa- 
tions on the clam patch give a presumption in favor of elevation, but they are not well 
supported by the evidence from barnacles and plants. The botanic evidence indicates 
that the entire dislocation shown by the measurement on Pepper Island is a subsidence 
on the east and does not include elevation on the west. The evidence from the barnacles 
suggests, without proving, a slight elevation at the Bolinas wharfs, but by no means indi- 
cates so great an elevation as would be necessary to account for the increased facility in 
reaching the clam patch. Dr. Gleason’s report of the shoaling of water in a channel near 
Pepper Island undoubtedly shows a local change, but such a change may have been pro- 
duced by a horizontal shifting of unconsolidated material such as occurred in Tomales 
Bay. On the other hand, it is not possible to explain the phenomena of the clam patch 
by a hypothesis of shifting bottom, for the sand in which the clams live is contained in 
shallow basins of visible bedrock, and any change in the relation of surface to tide at that 
point is a bedrock change. As the Rift belt with its numerous earlier dislocations ex- 
tends nearly to the clam patch, it is not impossible that there were differential movements 
west of the fault-line and that the ground occupied by the clam patch and the abalone 
patch rose independently of the western division of Pepper Island. 
