138 THE WORLD'S LUMBER ROOM. 



everywhere, though less abundantly beyond 1,000 fathoms, 

 and one coral lives at all known depths. 



The. whole of the Atlantic basin, to the depth of about 

 2,200 fathoms, is covered with an almost uniform greyish 

 sediment, which to the naked eye appears like mud, but as 

 seen by the microscope consists of very minute shells and 

 fragments of shells. The upper surface of this "ooze" is 

 of a creamy consistency and is made up chiefly of whole 

 shells, with fragments of sponge spicules, and a con- 

 siderable number of the larger shells of dead mollusks, 

 more or less broken and worn. At all moderate depths, 

 sponges, corals, star-fishes, &c., live in and upon this ooze, 

 The next inch or two below the surface is of firmer consis- 

 tency, the tiny shells being more or less broken up and 

 cemented into a calcareous paste, and beneath this, again, 

 whole shells and even fragments are rare, and the paste is 

 almost uniform. 



What thickness this sediment may have attained is un- 

 known; but in past ages hundreds of square miles have 

 been covered to the depth, in some places, of a thousand feet 

 or more, by a deposit evidently also formed at the bottom of 

 the ocean, and bearing a very strong resemblance to the 

 Atlantic ooze, which may fairly be called chalk-mud chalk 

 which has not yet had time to harden into rock, but which, 

 should it do so hereafter, will greatly resemble the chalk of the 

 English downs and cliffs. For that, too, consists mainly of 

 microscopic shells, which, though almost or altogether in- 

 visible to the naked eye, are of shapes as various and beauti- 

 ful as any of the larger ones, some of which they closely 

 resemble. (Fig. 28.) 



