622 
have been recombined with it. Sueh a movement involves that asa 
last trace, bearing witness to the former existence of such an oceanic 
voleanie island, the spot where it onee stood can be indicated by 
an atoll, rising from the bottom of the ocean to the surface. Ob- 
viously such an atoll will only be formed if there is a certain ratio 
between the rate of subsidence of the island and the upward growth 
of the reefs, whereas moreover the reef must have grown uninter- 
ruptedly during the whole period of downward movement, 
This harmonic ratio will of course exist in rare cases only and 
it is to be expected that in many of the sinking oceanic islands the 
contact with the sea-surface will have been broken, which the 
upward growing corals are always trying to keep up. It is clear, 
moreover, that the majority of the bodies built up by submarine 
voleanie activity will have sunk back again without ever having 
reached the surface. We may therefore expect that the vestiges of 
submarine voleanic activity nowadays mainly will be found as a 
ridge or elevation’) of inferior depth to the surrounding sea 
extending longitudinally in the direction of that line of least resist- 
ance of the terrestrial crust, along which the voleanic material was 
ejected. On this gentle ridge submarine hills must be found, formed by ° 
voleaniec masses, in all stages of slow subsiding. Here and there 
from the same ridge atolls will rise up to the surface as colossal 
reef-built structures, while in other spots, where the volcanic activity 
lasted longer or still continues, voleanie masses will be found 
to protude above the level of the ocean to the present day as moun- 
tainous islands, surrounded by barrier reefs. 
The above hypothesis does not give a satisfactory explanation of 
the upheavals which occasionally for a time interrupt the process 
of subsidence and cause some true oceanic coral islands to 
project fairly high above the sea. In comparison with the over- 
whelming large number of coral islands for which no rise can be 
demonstrated, the cases of rising observed are so few that Darwin, 
in my opinion rightly, drew the conclusion that the positive move- 
ments observed represent oscillations only in a direetion opposite 
to the general downward trend. 
From the hypothesis outlined above, some stringent deductions 
1) Such submarine ridges of very feeble relief are indeed indicated on the 
already cited excellent “Tiefenkarte der Oceane” by Max Gron. From the most 
north eastern of -these ridges in the Pacifie the Sandwich Islands rise above the 
sea-level. 
