270 PHYSIOGRAPHY. [chap. 



much hastened, at depths between 2,500 and 3,000 fathoms, 

 that an abundant residuum of undissolved shells is left, at 

 the former depth, and none at all, at the latter. Here is a 

 problem the solution of which cannot as yet be given. 



Again, what is the " red clay " which takes the place of 

 the Globerigina-ooze? It has been suggested that it is the 

 residuum left after the Globigerince have been dissolved ; 

 but there is no sufficient evidence that pure and clean 

 Globigeriiia shells contain any appreciable proportion of 

 such mineral matter. 



An alternative supposition is, that the red clay is simply 

 the finest of the washings of the land, which have gradually 

 drifted into the greatest depths of the ocean ; while another 

 explanation which has been offered is, that it is the result of 

 the decomposition of the volcanic ejections which are borne 

 about by the winds, and finally scattered over the surface of 

 the ocean ; and which, as a m.atter of fact, are found floating, 

 far and wide, in the shape of pum.ice. Fragments of 

 volcanic minerals are everywhere found in the Globigerina- 

 ooze ; and, it is highly probable, that a " volcanic shower " 

 is intermixed with the foraminiferal shower a'.l over the 

 ocean. If this be the case, then, in those localities in 

 which the Foramijiifera are dissolved before they reach the 

 bottom, the volcanic minerals would remain as the sole 

 constituent of the ooze ; and, by their decomposition, they 

 might give rise to the red clay. 



From what has been said it follows, that if, in consequence 

 of one of these movements of upheaval to which reference 

 has been made, the present bed of the Atlantic were raised 

 to the surface and became dry land, the many thousand 

 square miles of new dry land thus produced, would be 

 found to be covered, for an unknown thickness (amounting 

 possibly, and indeed probably, to hundreds of feet) with a 



