226 CORALS AND CORAL ISLANDS. 
was first explained by Sir Wyville Thomson. The limestone 
contains about half of one per cent of iron oxide and earthy 
ingredients ; and these are left behind as “red earth” when 
the rest is dissolved away by the carbonated waters. 
Another source in some regions, if not at the Bermudas, 
is the volcanic dust that is widely distributed by the winds, 
or fragments of pumice and other volcanic rocks that may 
have been brought by the sea and drifting logs. Pieces of 
pumice and augitic lava have been found on the island ; and 
from the sands Mr. Murray obtained magnetite, chrysolte, 
augite. sanidin and other feldspars, mica, and perhaps quartz. 
Twenty miles southwest-by-west from the Bermudas, 
there are two submerged banks or shoals, both reported 
’ 
as having a “corally and rocky bottom ;” one has over it 
a minimum depth of twenty-four fathoms, and the other 
of ten fathoms. Dredging on these banks might make some 
interesting disclosures. 
