82 THREE CRUISES OF THE “ BLAKE.” 
more active tidal circulation through them formerly than is now 
taking place. An examination of the cross-sections in the direc- 
tion of the prevailing winds, and in the direction of the tides, 
shows at a glance the mound-shaped mass which forms the base, 
rising bih the general level of twenty-five to thirty fathoms, 
with its abrupt side facing the east. There are also seen the 
deep furrows, more or less broad, which have been scooped out 
of this mass by the action of the currents, such as those passing 
through the Southwest Channel. 
The corals which give to the reefs their peculiar physiognomy 
are the extensive patches of Madrepora (principally M. cervi- 
cornis), the clusters of the two common species of Porites (P. 
Jurcata and P. clavaria) (Fig. 
48) covering more or less the 
shallow tracts of coarse sand, 
and Mæandrina areolata grow- 
ing between the patches of ma- 
rine lawns formed by a species 
of Thalassia, with occasional 
patchesof Anadyomene. In other 
parts of the reefs large holothu- 
rians (Miilleria) lie scattered on 
the bottom, while in somewhat 
deeper regions are pockets filled 
Fig. 48.— Porites clavaria. (Agassiz) with large Diadematidz. Im- 
mense masses of nullipores (Udotea and Halimeda, Figs. 49, 50) 
and corallines grow on the shallowest flats, on the tops of the 
branches of vusdrápodés which have died from exposure to the 
air, either because they have grown up to the surface and so 
have become exposed by extreme low tides, or because strong 
winds have blown the water from the flats. The destructive 
effect which an extremely low tide has on a growing reef is well 
shown on the flats to the southward of Fort ейге! where 
the upper part of the branches of a certain size reaching up to 
a given level are frequently killed off by low tides. Exposure 
to the action of the sun even for a very short time is sufficient 
to kill them. The extreme sensitiveness of all corals to atmos- 
action is well known, so that it becomes plain, as has been 
pheric 
