GEOGRAPHICAL DISTRIBUTION. 353 
much as it is contracted on the western coasts. / Moreover, the 
trend of the coast and its capes produce other modifications in 
the direction of the currents, the most of which are apparent 
in the actual distribution of coral reefs. On the shores of /~ 
the continents it is observed that there are few extensive 
reefs, and the coasts on which they occur are those which, , 
owing to the dryness of the climate, have no great rivers to ° 
pour freshwater and detritus into the sea. Thus the influence 
of continental waters and detritus on the distribution of reefs, 
is shown to be very marked. But about the Pacific Islands, 
where streams are small, the same cause has had little effect, 
seldom doing more than modifying somewhat the shores and _ 
bottom of a harbor. It has been further demonstrated that 
in different groups, as the Ladrones, Sandwich Islands, Navi- 
gators, New Hebrides, there is an inverse relation between | 
° ° | 
the extent of reefs and the evidences of recent volcanic ac- | 
tion in the island; and that the largest reefs exist where | 
there is no proof of former igneous action, or where it has 
long ceased. ‘The existence of large reef-islands in open seas 
where the neighboring lands are mostly destitute of coral 
reefs, harmonizes with the conclusions announced, since such 
islands are in general removed from the deleterious influences 
| 
| 
ana} -- 
just mentioned ; yet it is very probable that in many cases 
of this kind the region of the open sea may have undergone 
a subsidence not experienced equally by the lands either side ; 
for the cases in which such seas contain coral islands are 
many. 
The modifications of form and interruptions of reefs, 
arising from abrupt or sloping shores, and tidal or local 
currents, have also been exemplified. 
23 
