168 The American Geologist. September, 1893 
Terrestrial Subsidence southeast of the Worth American 
Continent. By Prof . J. W. Spencer, Atlanta, Ga. The con- 
tours of the submarine continental slopes east, south, and wesl 
of Florida, in the region of the Gulf of Mexico, the West In- 
dies and the Caribbean sea, as made known liy U. S. Coasl 
Survey and other soundings, reveal submerged valleys of riv- 
ers, once land surfaces but now covered by 2,000 to 12,000 feet 
of the ocean. The steep submarine escarpments of the conti- 
nental plateau and of the Antillean island plateaus are found 
to be cut to these profound depths by stream courses, often 
at right angles with the adjacent or intersected mountain 
belts. The author believes, therefore, that this part of the 
earth's crust has experienced, at least for large areas, twice 
as great uplift and ensuing subsidence as the 5,000 to 6,000 
feet lately estimated by Jukes-Browne and Harrison from their 
studies of the geology of Barbados. The time of these great 
epeirogenic movements is regarded as late Tertiary and Qua- 
ternary. But the extreme depths of the West Indian seas, as 
the Bartlett Deep, southeast of the Grand Cayman island, 
sinking more than 20,000 feet beneath the sea level, are to be 
attributed to synclinal folds or troughs of deformation of the 
ocean bed. The author supposes that North and South Amer- 
ica were united by w T a}^ of the West Indies during the Pliocene 
period, and that probably up to the Pleistocene the Carribbean 
sea and Gulf of Mexico opened into the Pacific ocean. 
Mr. W J McGee, in discussion, granted the probability of 
such former elevation and subsidence, but would refer them 
to an earlier time, for accordance with his studies of the 
much smaller Tertiary and Cretaceous oscillations of the At- 
lantic coast extending thence north. 
Mr. UfHAM noticed the similarity of the West Indian sub- 
merged valleys and those of the Delaware and Hudson rivers, 
respectively about 2,400 and 2.S00 feet beneath the sea. 
These valleys both at the south and north are cut in the 
gently descending plain of Tertiary and underlying Creta- 
ceous strata, showing their age to be late Tertiary or Quatern- 
ary. In high latitudes such epeirogenic uplifts, approximately 
contemporaneous with these in tropical areas, or alternating 
with them, are thought to have been the causes of the accu- 
mulation of the Pleistocene ice-sheets. 
