SUBMARINE DEPOSITS. 283 



Jurassic period have gradually filled the shallow sea formerly 

 existing to the west of the Mississippi basin and of the southern 

 portion of the GuK States. In more recent times, the muds 

 brought down by the Mississippi and other rivers emptying into 

 the Gulf of Mexico have been deposited upon its shores, and 

 have formed the steep continental slope of the Gulf beyond the 

 hundred-fathom line. 



The recent river muds now occupy a narrow strip of the Mis- 

 sissippi basin, between Louisiana and Mississippi, as far north as 

 the mouth of the Ohio. They have also built out the delta of 

 the river to within a short distance of the hundred-fathom line, 

 and extend along the shore of Texas, slowly filling up the Gulf 

 of Mexico. 



I was very much struck, on first seeing the ooze of the deep 

 water of the Straits of Florida between Havana and Florida 

 Keys, with the immense number of dead pteropod and hetero- 

 pod shells which it contained, in addition to the countless tests 

 of globigerinae and orbulinse. These shells belonged mainly to 

 the genera Clio, Hyalea, Triptera, Atlanta, Styliola, etc., all of 

 which swarm on the surface, or a little below it, in all the parts 

 of the Gulf of Mexico which we had thus far passed over. I 

 could at once see how important a part these dead pteropod shells 

 must play in the formation of the sedimentary matter accumulat- 

 ing at the bottom. Globigerinse and orbulinse form, as we know, 

 the bulk of the ooze, but the remaining part of the mud is 

 made up mainly of the dead shells of pteropods in all stages of 

 disintegration, from perfect shells, still filled with the decaying 

 animals, to the most minute grains, in which we can just detect 

 the presence of the pteropod test. This composition of the 

 ooze is the rule in all specimens of the bottom which I have 

 thus far had time to examine. The decay and solution of the 

 test become more rapid with increasing depth, and may be due, 

 as suggested by Mr. Murray, to the excess of carbonic acid 

 present at great depths, or, as Professor Dittmar is inclined to 

 believe, to the solvent action of the sea-water itself. In a vol- 

 canic region hke that of the West Indies, there is no difficulty 

 in accounting for the presence of a large supply of gas. To 

 show how far the dead pteropod shells make up the globigerina 



