1l, 
contain a top layer about 15 cm, thick of "red'' érbrown clay. The aver- 
age hanes carbonate content of this layer is 0.5 per cent. 
Below this normal top there are in both cores three graded layers, 
one of which is about 200 cm, thick, The uppermost part of this graded 
layer is composed of clay-size particles, and is distinctly grayer than the 
normal top, but it is in its carbonate content of more than 36 per cent 
that it differs from a "typical" deep-water sediment. Downward in this 
layer there is a very gradual increase in particle size until near the base 
it becomes well sorted calcareous sand largely composed of the tests of 
planktonic and benthic foraminifera, small pteropods, and alcyonarian 
spicules. The carbonate content near the base is 74 per cent. The very 
shallow-water origin of some of the material is shown by occasional par- 
ticles of Halimeda. | 
The shorter one of these two cores ends at 280 cm. still in well- 
sorted material of shallow-water origin. The longer one at 295 cm, re- 
enters normal clay of abyssal facies and a carbonate content of 0.3 per 
cent, thereby disposing of the theory that these layers of shallow-water 
material are evidence of a recent and sudden subsidence of the floor of 
the Puerto Rico Trough. 
Two cores taken on the northern side of the trough contain only 
red clay, although the thicknesses penetrated are 185 cm. and 290 cm. 
Another core from the top of the ridge north of the trough is of "deep- 
water'' facies throughout its length of 260 cm. in spite of the fact that the 
depth here is 5,011 m. (2740 fms.), that is, 2900 m. (1600 fms.) shallow- 
er than the bottom of the trough. 
