18, 3 Smith: Tropical Geology and Engineering 937 
In the course of the coast survey of the Philippine Islands, 
also, considerable discrepancies were frequently noted between 
the trigonometrical and the astronomical location of stations, 
in some places amounting to 30 or 40 seconds of arc; such a 
difference might mean a matter of one-fourth mile or more on 
the surface of the earth. How to adjust this has been a matter 
of some difficulty. The great piles of volcanic rock constituting 
the Zambales Mountains suffice to explain the discrepancies at 
some of the stations on the west coast of Luzon. Near Olongapo, 
it was found-that the plumb bob was deflected to the west instead 
of to the east, though the main mass of the Zambales Mountains 
lies to the east. An examination made by the writer several 
years ago on the Cinco Picos Range, which lies to the west of 
Olongapo, revealed the fact that these mountains are made up 
of one of the densest rocks known, periodotite. This rock has 
a density of over 3, whereas the main mass of the rock in the 
Zambales Range to the east has a density of only about 2.5. 
Therefore, apparently, this discrepancy is explained by our 
knowledge of the geological features of the two regions. 
The work of Hayford has shown so clearly the importance 
of the data of geodesy in the solution of problems of geophysics 
that geologists interested in these broader and fundamental 
problems, such as that of isostasy, cannot afford to ignore them. 
On the other hand, from the example cited, the dependence is 
clearly seen to be mutual. The writer is planning to carry a 
small gravity instrument into the interior of Luzon on some 
of his expeditions within the coming year, as we have no data 
of this kind from the interior. 
Physiographic influence upon economic development.—Under 
this topic it is proposed to cite several examples of how a 
knowledge of the geologic and physiographic conditions would 
be of direct practical value in other matters, aside from its more 
cbvious applications to mining, and so forth, already given. 
The relation to engineering here is only indirect. 
Cebu Island, one of the Visayan group of the islands of the 
Philippine Archipelago, is long, narrow, and mountainous. It 
an interrupted narrow strip of coastal plain on which 
practically the entire population of the island (the most popu- 
lous in the Archipelago) may be found. The people are com- 
paratively poor and live largely on and by the sea. There is 
at present, owing to conditions produced by the war, a partial 
refutation of this statement. There is little productive “hin- 
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