18, 3 Smith: Tropical Geology and Engineering 239 
are called tectonic. Those of the last group are most numerous 
and most destructive. The San Francisco earthquake of 1906 
was of this class. 
In an investigation conducted for many years by Father Sa- 
derra Mas6, of the Philippine Weather Bureau, and the writer, 
we found that of the great number of earthquakes that occurred 
in the Philippines from 1599 to 1909 only a comparatively small 
number could be traced to volcanoes; that of the twenty-five 
seismic districts in the Islands only five are near or include 
active or dormant volcanoes; and that two regions of greatest 
seismicity have no volcanoes at all. 
Recently some very interesting and important conclusions. con- 
cerning the connection between seismic phenomena and rainfall 
have been put forward by M. G. Zeil,’° formerly topographer and 
geologist of the Government of French Indo-China and now of 
the Carte Geologique of France. This investigator asserts that 
seismicity in various parts of the world increases with rainfall. 
He mentions only one notable exception, the Valley of the Ama- 
zon, where, although the rainfall is heavy, seismicity is feeble. 
He cites, especially, the example of Agusan Valley in the Philip- 
pines to which Saderra Mas6 and the writer called attention 
some years ago, though not connecting the fact with the rainfall 
of the region. 
It appears that the rapid loading and unloading of a given 
piece of terrain by a heavy rainfall which is discharged quickly 
into the lowlands or the sea, effecting considerable erosion, is 
the prime factor in many of the sudden adjustments in the outer 
shell of the lithosphere. In a heavily forested region like the 
Amazon the run-off of the streams is slow and consequently 
erosion is comparatively slight. 
The rainfall would have still another effect, that of lubrication, 
thus causing slipping of one formation, or of the beds of the 
same formation, over one another. 
M. Zeil in a letter to the writer has pointed out that in Annam, 
in those places where the natives have cut away the forests, the 
number and frequency of earthquakes have increased. Here 
is a splendid example of the need for codperative investigations 
on the part of the engineer, the forester, the seismologist, and 
the geologist. 
This particular subject is one to which we have as yet devoted 
little attention in the Philippines, but we shall in the future 
* Zeil, M. G., Acad. des Sci., Seance de 12 juillet (1920) 117-119. 
