PACIFIC EXPLORATION: J. F. HAYFORD 
397 
It should be evident that it is extremely desirable to extend to the 
open oceans the proof, which is now conclusive for land areas, that the 
appKcation of corrections for topography and compensation makes the 
corrected results independent of topographic effects and ehminates a 
large part of the systematic error otherwise inherent in the results. 
I am confident that good observations at sea will promptly furnish such 
a proof. They would certainly give a very severe and therefore very 
valuable test of the conclusions as to isostasy which have been drawn 
from the observations on land. 
It is obviously important to determine as well as may be the pre- 
vailing depth at which masses of abnormal density lie. Are they 
ordinarily within 10 miles of the surface, or are they as frequently more 
than 40 miles down? The nearer to the surface such an abnormal mass 
Hes the more rapid will be the space-change, of gravity as an observer 
approaches, passes over, and recedes from the region on the surface 
which lies above the abnormal mass. For this purpose, therefore, for 
indicating the approximate depth of abnormal masses, it should be 
evident that closely spaced continuous lines of gravity observations at 
sea such as could be easily secured would be much more effective than 
are stations on land under ordinary conditions. 
To what is the permanency, or semi-permanency, of the great oceanic 
depressions due? Adequate gravity observations at sea would establish 
conclusively the extent to which the rocks underlying the oceans are 
more dense than those under the land, and thus furnish a conclusive 
partial answer to the question. 
The present indications, from a few gravity stations on such islands, 
is that gravity is in excess on oceanic islands, such as the Hawaiian 
Islands, where vulcanism is active. How far does said excess extend 
out to sea? Observations of gravity at sea would answer that question 
and in doing so might contribute much to our knowledge of the nature 
and cause of vulcanism. 
What is the nature of the deep troughs that occur at various places 
in the oceans and which show a suggestive tendency to be located near 
and parallel to an elongated land area or a mountain chain? Gravity 
observations at sea may throw a Hght upon this question by showing 
the density of the rocks below such troughs. 
I have indicated why I believe it to be especially important to secure 
gravity observations at sea. If such observations are to be made, why 
is the Pacific the ocean upon which they will be most effective? 
Of course it occurs to one at once that the Pacific is the greatest ocean 
and that therefore the largest blanks in which there are now no gravity 
