depth of 290 m., composed of material, showing the effects of sub- 
aerial weathering, proves that the island once rose above the sea, 
probably before the eocene period. In eocene times or shortly after- 
wards this was again the case but later the volcanic activity ceased 
and the gradual subsidence was then no longer compensated by the 
accumulation of voleanic material; consequently the volcanic cone 
ultimately disappeared below the sea and now lies at 75 m. below 
sea-level, covered by a cap or crown of reef-limestone which nowa- 
days rises fairly high above the sea in some places. In spite of the 
upheaval proved by this latter fact this remarkable boring justifies 
the conclusion that the general trend of the movement of the 
oceanic voleanie group of the Bermudas has for a long period been 
downward. 
What may be the cause of such downward movements? 
It seems to me that from the theory of isostasy we can 
make some deductions, suggesting a possible or even probable 
explanation. 
The doctrine of isostasy developed from a hypothesis (PRart’s 
hypothesis) to a well-founded theory by numerous investigations, 
especially by Hecker and Harrorp, presupposes that on the average 
the mass of the terrestrial crust is heavier under the oceans than 
under the continents. 
It may be assumed that the outer crust of the earth consists 
chiefly of sediments and other rocks of an average density of 
+ 2.6--2.8, of which gneiss and granite are the principal types, 
grouped together by Svrss as sal and sometimes also called litho- 
sphere in a more restricted sense, while under this acid crust a 
shell of more basie rocks follows with an average density of + 3, 
to which Svurss has given the collective name sima, and which is 
also named barysphere in contradistinetion to lithosphere. Basalt is 
a type of these latter rocks and the whole sima has been called by 
Day “basaltic substratum”. 
In order to explain isostasy we might imagihe that the outer acid 
crust is thicker under the continents than under the oceans, but it 
may also be supposed that this lighter granite-gneiss crust also 
comprising all sediments excepting the thin pellicle of oceanic 
(pelagic) sediments, does not form a closed shell round the terrestrial 
globe, but is in general restricted to the continents, so that 
under the oceans the sima or basaltic substratum must immediately 
follow under the certainly not very thick cover of oceanic 
sediments. 
This second explanation is considered the more likely one by 
