PERMANENCE OF CONTINENTS AND OCEANIC BASINS. 131 
has been buried under coarser material by the advance of the 
continents as they became older. The conclusion seems inevi- 
table, that the sedimentary rocks attained their great thick- 
ness in areas of regular subsidence, the subsidence being due 
to the deposition of material along the shore lines. We 
know of no other cause to account for the deepening of the 
seas, the action of the waves being restricted to a very limited 
depth. 
The effect of the deposition of such immense plateaux of lime- 
stone as the Florida peninsula, the Yucatan Bank, the Pedro 
Bank, and the smaller plateau along the West India Islands, 
should be to cause a gradual depression over the whole of the 
area of the Caribbean and of the Gulf of Mexico. But all the 
indications we have from the deposits of recent formations in 
Florida, in the West India Islands, and at the Isthmus of Pan- 
ama, show, on the contrary, that they are in an area of elevation 
due to volcanic agencies, still more or less active at the present 
day. In like manner, the transfer of such huge masses of silt 
as we have at the mouth of the Mississippi and near Cape Hat- 
teras should theoretically be accompanied by a corresponding 
subsidence. We can only assert, however, that the mud of the 
Mississippi is slowly filling the deep basin off its mouth, and 
that it has been doing this for some time past. 
The distance to which the Mississippi mud is carried from the 
shore shows us how far such deposits, representing immense 
areas of denudation, may share in forming additions to conti- 
nental masses, and determining their relations to the formations 
immediately preceding them. According to the soundings of 
the “Blake,” the presence of Mississippi mud cannot be, de- 
tected more than one hundred miles from its mouth ; beyond 
that we find the usual deep-sea specimens characteristic of the 
bottom of the Gulf of Mexico. Off Cape Hatteras, also, an 
immense slope of fine detritus has been formed by the wearing 
action of the Gulf Stream. As far as agencies now at work are 
concerned, the forces acting to-day are mainly efficient along 
continental masses and along the shores of islands, and must of 
necessity have left the basins separating the continental masses 
practically intact, except along their margins. 
