146 THE WORLD'S LUMBER ROOM. 



But some dolomites are evidently of coral-reef origin, and 

 since we know of no plant or animal which could have taken 

 up magnesia in these large quantities, we must conclude that 

 it was deposited directly from the water. 



Wherever the depth of the Atlantic exceeds 2,200 

 fathoms, there the calcareous ooze passes gradually into a 

 very fine reddish or chocolate-coloured clay, derived ap- 

 parently from cosmic and volcanic dust, and the decay 

 of pumice-stone, which floats long distances before it be- 

 comes so water-logged as to sink. The clay contains a 

 few foraminifera, &c., but as these exist everywhere, from 

 the equator to the regions of polar ice, and their shells 

 must be falling in ceaseless showers all over the oceans, 

 one wonders what has become of them, as well as of the 

 shells of those mollusks which float about in multitudes 

 in mid- Atlantic. 



The explanation seems to be that, owing to the greater 

 depth of water they have to sink through, and the larger 

 amount of carbonic acid present at great depths, they are 

 dissolved before they reach the bottom, and the shells 

 of the mollusks dissolve more quickly than those of the 

 tiny foraminifera. 



Quickly, however, they none of them dissolve, for the 

 office of these and other marine animals is just this : to 

 separate the lime from sea-water, and protect it from solution 

 by combining it with organic matter, and they succeed so 

 well that their calcareous structures dissolve with difficulty, 

 even in hydrochloric acid. 



