Vol. 58.] GEOLOGICAL DEVELOPMENT OF DOMINICA, ETC. 343 
discovered are more interesting, for here the higher part of the 
drowned plateau has a breadth of 35 miles. Its western margin 
is indented by a great amphitheatre or valley-like embayment, 
beginning at the col between the Atlantic and the Caribbean 
basins (which is 3300 feet below the surface of the sea), and 
extending to a depth of over 6000 feet; while the greater portion 
of the plateau is not submerged more than 2500 or 3000 feet. 
(see Pl. X, and also fig. 1, p. 342). It should be further noted that 
on the eastern side of the plateau there are two elevated fragments 
of the drowned tableland (A and B in PI. X), reaching to within 
300 feet of the surface of the sea,—these being repetitions of the 
coastal plains of Antigua, Grand’ Terre of Guadeloupe, and the 
outlying fragment which forms Barbados. About 60 miles east of 
Martinique is another slightly submerged remnant of the drowned 
Antillean plateau, about 12 miles long, and here named Madiana 
Banks. © | 
The channel between Martinique and St. Luciais about 20 miles 
wide, with the higher portions not more than 3300 feet below sea- 
level. From the western side of the col, a valley descends rapidly 
to a depth increasing from 3600 to 6000 feet, and even to more 
than 7200 feet, as it widens out into a broad embayment, all 
within the line between the western shores of Martinique and 
St. Lucia. On the north-western edge of the St. Lucia mass is a 
remarkable cirque, 6624 feet deep, within the 600-foot submarine 
-contour-line. 
St. Vincent is about 25 miles distant from St. Lucia. The 
summit of the connecting ridge is not generally submerged more 
than 1200 feet ; but near St. Vincent is a channel, less than 5 miles 
wide, which dissects the col to a depth of 1800 feet, thus showing 
a rather narrow valley across the submarine Antillean plateau, 
reaching to 3000 feet below sea-level. Towards the west this 
valley descends rapidly, and in a distance of 10 miles the soundings 
show a depth of 6000 feet. This rapid descent appears to be by 
steps, and even within 3 or 4 miles of its head one such step 
amounts to 2500 feet. 
The submarine coastal plains of St. Vincent and the Grenadines 
are generally covered by 100 to 200 feet of water. In reality they 
form one plateau, dissected by a valley with precipitous walls, 
reduced to a breadth of 2 miles. The divide in this valley is 
about 1200 feet below the surface of the sea, or about 1000 feet 
below the submarine plateau. There are no soundings to show 
the development of the valley descending towards the east; but that 
to the westward is seen, reaching in a distance of + miles a depth 
of 2700 feet, and 5 miles farther on to 5500 feet, and still a little 
farther to 6900 feet, below the surface of the sea. This St. Vincent- 
‘Grenadine valley is one of the best-shown amphitheatre- or cirque- 
valleys that indent the submarine Antillean banks.’ (See fig. 2, 
p. 344.) 
: \ 
* See U.S. Hydrographic Chart No. 1279, or the equivalent British Admi- 
ralty Chart. 
23 2 
