226 ORIGIN OF METALLIFEROUS DEPOSITS. [XII. 



green-sand in England result from a petrifaction of sponges "by 

 dissolved phosphates, and similar observations have been made 

 by Edwards with regard to the guano of the Chincha Islands.] 



But again, these plants or these animals may perish in the 

 sea and be buried in its ooze. The phosphates which they 

 have gathered are not lost, but become fixed in an insoluble 

 form in the clayey matter ; and when, in the revolutions of 

 ages, these sea-muds, hardened to rock, become dry land, and 

 crumble again to soil, the phosphates are there found ready for 

 the wants of vegetation. 



Most of what I have said of phosphates applies equally to 

 the salts of potash, which are not less necessary to the growing 

 plant. From the operation of these laws it results that neither 

 of these elements is found in large quantities in the ocean. 

 This great receptacle of the drainage from the land contains 

 still smaller quantities of iodine; in fact, the traces of this 

 element present in sea-water can scarcely be detected by our 

 most delicate tests.* Yet marine plants have the power of 

 separating this iodine, and accumulating it in their tissues, so 

 that the ashes of these plants are not only rich in phosphates 

 and in potash-salts, but contain so much iodine that our sup- 

 plies of this precious element are almost wholly derived from 

 this source, and that the gathering and burning of sea-weed for 

 the extraction of iodine is in some regions an important indus- 

 try. When this marine vegetation decays, the iodine which it 

 contains appears, like the potash and phosphates, to pass into 

 combination with metals, earths, or earthy phosphates, whirli 

 retain it in an insoluble state, and in certain cases yield it to 

 percolating saline solutions, which thus give rise to springs 

 rich in iodine. (Ante, page 143.) 



In all of these processes the action of organic life is direct 

 and assimilative, but there are others in which its agency, 

 although indirect, is not less important. I can hardly con- 

 ceive of an accumulation of iron, copper, lead, silver, or gold, 

 in the production of which animal or vegetable life has not 

 either directly or indirectly been necessary, and I shall be- 

 * See the Appendix to this paper. 



