778 Working v. Fighting . [December, 
neftion of the eastern reefs of Pelelin and Kreiangel with 
the dead elevated coral cliffs all argue against any recent 
subsidence. The author’s own theory is that the growth of 
corals is chiefly influenced by the strength and direction of 
the oceanic currents. Wherever constant and deep currents 
strike a coast tangentially the reefs are compelled to grow 
vertically upwards, and on the other hand many corals have 
the tendency to spread out equally on all sides, as far as pos- 
sible, when shallow currents sweep over them. He considers 
that the peculiarities of the Pelew reefs can all be explained 
by the action of such currents during a period of elevation. 
He observes that certain corals, such as the Astrese and 
Porites, form circular masses of a regularly convex surface. 
The polypes on the summit are as well developed as those 
on the sides, so long as they are never laid dry at time of 
ebb. Larger blocks of the same kinds become flat on the 
summit, which is exposed even at ordinary ebbs, the polypes 
being dead and covered with sand and sea-weed. In still 
larger masses the surface is concave with an elevated 
margin, presenting, in fact, the likeness of an atoll in 
miniature. 
It is obvious that if Professor Semper s theory holds 
good, much of our present conclusions concerning supposed 
areas of subsidence will require to be carefully reconsidered. 
On Mr. Darwin’s hypothesis an atoll in the present day is 
the monument over, or rather around, the grave of a sub- 
merged island. At an earlier date it must have been a 
barrier reef surrounding a small, generally mountainous, plot 
of land. Would it not be important to search whether at or 
near the centre of a lagoon within such atolls there could be 
found traces of rocks other than coral or its transformations? 
It will also be essential to examine the relations of the more 
characteristic atolls, barrier reefs, and fringes, to prevailing 
currents. 
Regretting the necessary shortness of this survey of the 
teachings of two most significant works, I must recommend 
them to the careful study of the reader. 
