OCEANOGRAPHY. 



21 



as to be able to form enough shells and skeletons 

 to cover every available part of its surface. 



Far away from continental lands, and at great 

 depths, the character of the sea-bottom com- 

 pletely changes. At a distance of 100 miles 

 from the coasts of America or Europe, for ex- 

 ample, the land deposits have already found 

 their resting place, and the animal life in the 

 depths of the Atlantic is poor in skeleton- 

 forming genera. However, the surface waters 

 of the ocean teem with creatures of all sorts 

 which, as they die, drop down 

 their skeletons and shells in a 

 gentle shower to form a fine 

 deposit on the bottom. When 

 we get beyond the mud-line, 

 then, and use the dredging or 

 sounding apparatus in depths 

 of 1500 to 2500 fathoms, we 

 find that the bottom is largely FlQ l 



composed of the shells of Such Globlgerina Shell from 



surface animals as the Ptero- a deep - sea ooze ' 

 pods and Globigerinas, and according to the 

 relative abundance of these forms it is called 

 Pteropod Ooze or Globigerina Ooze. In still 

 greater depths than these the character of the 

 bottom again changes, and we find a deposit 

 which is commonly known as the Red clay. The 

 explanation of this change of character depends 

 upon the fact that sea-water exercises a slightly 

 solvent action upon carbonate of lime, and the 

 shells of the Globigerinas and other forms are, in 

 seas of a depth of over 2500 fathoms, dissolved 

 before they can reach the bottom. The only 



