GREATER PROBLEMS OF PHYSICAL GEOLOGY. 
57 
narrow limits within which those values must be taken. Thus 
the two sciences must work cooperatively and supplement 
each other. 
There is, however, one other branch of physical inquiry 
which bears directly on the foregoing questions. This is the 
investigation of terrestrial gravitation by means of the pen¬ 
dulum. I regret that I have never had time or opportunity 
to acquaint myself thoroughly with the results thus far 
reached by this branch of investigation, and can only speak 
from general knowledge. Pendulum observations are far 
too few for the wants of geographic or geologic science. So 
far as they go they are highly suggestive in the present con¬ 
nection. The pendulum, as a rule, does not show any ap¬ 
preciable variation of gravity, such as would be expected if 
the mean density of all the outer parts of the earth were 
uniform. It indicates rather that the elevated regions and 
continents are composed of lighter matter and the depressed 
regions and ocean basins of denser matter. The exceptions 
are of a character which prove the general rule, and occur 
where we should look for them. The results obtained by the 
India survey upon the Himalayan mass were regarded by 
Archdeacon Pratt as indicating that the plateau was com¬ 
posed of lighter matter than the lowlands to the southward. 
A similar result has been obtained in the great bulge which 
forms the western half of the United States. In other words, 
the pendulum indicates that those elevated regions are nearly 
if not quite isostatic. 
On the other 'hand, the observations of Mendenhall on 
Fujiyama, in Japan, indicated a slight excess of mass, and a 
similar result would seem to follow from Mr. Preston’s work 
in the Hawaiian Islands. From the nature of the process 
by which volcanoes are built these results are to be expected. 
It would also -seem natural to expect that the plumb-line 
would give some indications upon this subject; but ex¬ 
perience has shown that most of the observed deflections of 
the plumb-line are inexplicable. They occur where we would 
least expect to find them—upon broad and level plains, where 
