MARINE PHENOMENA. seal 
after the first interruption and blurring of the record by the shock itself, and this 
coincidence in time suggests that the fall in the water near Fort Point was due to a 
negative oscillation generated at the line of the fault. The effect produced would have 
been brought about had there been a slight drop of the sea-bottom on the outer side of 
the fault. But there is independent evidence, to the north and south of this particular 
segment of the fault, that there was no drop on the west side, so that this explanation 
can not very well be entertained.’ It is also possible that the effect observed might 
have been brought about by a slight expansion of the confines of the Gulf of the Faral- 
lones, due to the differential movement along the fault, but this would not explain the 
coincidence in time. The period of the east-west oscillation of the waters in the Bay 
of San Francisco, between West Berkeley and Fort Point, has also been computed by 
the Tidal Division of the Coast and Geodetic Survey to be about 40 minutes. This 
agrees fairly well with the two or three oscillations recorded by the gage after 6 o’clock, 
and indicates that the drop of the water-surface outside of the Golden Gate generated 
an east and west oscillation in the Bay of San Francisco. 
Tidal observations conducted at Fort Point for a period of 1 year from the date of 
the earthquake indicate that there was no change of the relative altitude of sea and 
land at that point, as compared with the conditions prevailing during the 3 years pre- 
ceding. A review of the observations for the past 9 years, by the Coast and Geodetic 
Survey, reveals, however, the interesting fact that in that period of time there has been 
an apparent subsidence of the coast at that point of 4.8 inches, practically all of this 
having been accomplished in the first 6 years of this period. There has been no move- 
ment in the last 3 years. (April 18, 1907.) The only other tidal gage maintained on 
the coast of California is that at San Diego, and the marigram obtained there shows 
no abnormal movement of the surface of the sea referable to the earthquake. 
The only other report indicating that the level of the ocean was affected along the 
coast is by W. W. Fairbanks, of Point Arena, who says: ‘I have endeavored to learn 
of any unusual action of water along the sea-coast, and can relate but one instance of 
anything approaching the character of a tidal wave. On the day of the shock I traveled 
by wheel and on foot from Albion to Point Arena, 25 miles. At the mouth of Navarro 
River, at 8 o’clock on the morning of the 18th, I learned from reliable sources that a 
section of about 10 acres of low, flat land about the mouth of this river was entirely 
submerged for some minutes immediately after the shock.” 
The shock felt by ships. — Information regarding the perception of the shock on ships 
at sea or in harbors has been collected by Prof. George Davidson, and the following 
notes are chiefly the result of his inquiries: 
The U.S. T. S. Pensacola, moored to the pier at the U. 8. Naval Training Station, 
Yerba Buena Island, San Francisco Bay, felt the shock on the morning of April 18, 1906. 
Surgeon L. W. Curtis reports that while in bed on the Pensacola he felt a vibratory shock 
lasting about 30 seconds, with one heavy jar about the middle period of the shock. A 
gentle rumbling sound coincided with the shock. The phenomenon closely resembled 
vibrations which are at times set up in the ship’s hull on starting the dynamo, and it was 
mistaken for that, tho much more active and exaggerated than ever before observed. 
The vibration shook down some loosely piled books and papers from a table. 

1 This explanation is, however, advocated by Prof. H. F. Reid. In a note received while these pages 
are in proof he says: “ If a depression occurred on the western side of the fault-line, extending for some 
distance to the westward, it would start a wave of depression towards the Golden Gate which would take 
9 minutes to reach Fort Point and this is just about the time recorded by the gage. The time necessary 
for the recovery to normal level would depend upon the extent of the area depressed. If this were a 
narrow block, a wave of elevation would follow quickly upon the wave of depression and we should have 
a rapid elevation of the tide-gage above its normal position. As no such wave appeared and recov- 
ery was very gradual we must suppose that the deprest area extends for some distance to the west- 
ward, so that the recovery was slow. This is the only explanation so far offered, that would produce 
the effects observed.” 
