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 IncUa 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. 



