BETWEEN FLORIDA AND CUBA. 911 
shape of a rough, rocky floor, without great imequalities ; 
this formation obtains its greatest breadth, of about eighteen 
miles, a little to the east of Sombrero Light, and tapers off 
to the west, where it ends in about the same longitude as the 
end of the reef; toward the east and north it approaches 
nearer the reef, and ends gradually between Carysfort reef 
and Cape Florida. This bottom, which is called the ‘ Pourtalés 
Plateau’ in Professor Agassiz’s report, is very rich in deep-sea 
corals, and most of the species described in the memoir [of 
Pourtalés] were dredged on this ground. Outside of the 
rocky bottom the Globigerina mud prevails and fills the 
trough of the channel. 
“On the Cuba shore the bottom is rocky and the slope 
very abrupt, particularly for the first four or five hundred 
fathoms. Along the Salt Key and Bahama Banks the slope 
is also exceedingly abrupt, but the underlying rock is often 
covered with mud.” 
The rock of the bottom of the Pourtalés Plateau is a lime- 
stone made from the calcareous relics of the species (corals, 
shells, echinoderms, etc.) living at those depths; and it is 
still increasing, well exemplifying what limestone-making 
may go on below the limit of reef-making corals. Mr. A. 
Agassiz has a cut on page 287 of his “Cruises of the 
Blake” representing a specimen of the material. 
The Salt Key Bank is mostly a submerged atoll-like area, 
setenty miles long, having emerged islets and cays along its 
northeastern and northern sides, or those to windward, and 
falling off southward to a depth of four and a half to six 
and a half fathoms and then steeply to depths of two hun- 
dred and fifty fathoms and beyond to those of the channel 
along the north side of Cuba. 
Prof. L. Agassiz describes the banks and keys embraced 
