XIL] ORIGIN OF METALLIFEROUS DEPOSITS. 225 



laws of the terrestrial circulation, which I have compared to 

 that of the blood distributing throughout the animal frame the 

 elements necessary for its growth. The analogy is not alto- 

 gether new, since a great French geologist, Elie de Beaumont, 

 has already spoken of a terrestrial circulation in regard to cer- 

 tain elements in the earth's crust ; though he has not, so far as 

 I am aware, carried it out to the extent which I now propose 

 to do in my attempt to explain some of the laws which have 

 presided over the distribution of metals in the earth. 



The chemist in his laboratory takes advantage of changes 

 of temperature, and of the action of various solvents and 

 precipitants, to separate, in the humid way, one element from 

 another ; but to these agencies, in the economy of nature, are 

 added others which we have not yet succeeded in imitating, 

 and which are exerted only in growing animals and plants. I 

 repeat it ; I do not wish to say that these latter processes are 

 different in kind from those which we command in our labora- 

 tories, but rather that these organisms control a far finer and 

 more delicate chemical and physical apparatus than we have yet 

 invented. Plants have the power of selecting from the media 

 in which they live the elements necessary for their support. 

 The growing oak and the grass alike assimilate from the air and 

 water the carbon, hydrogen, nitrogen, and oxygen which build 

 up their tissues, and at the same time take from the soil a portion 

 of phosphorus, which, though minute, is essential to the vege- 

 table growth. The acorn of the oak and the grass alike be- 

 come the food of animals, and the gathered phosphates pass 

 into their bones, which are nearly pure phosphate of lime. In 

 like manner the phosphates from organic waste and decay find 

 their way to the sea, and through the agency of marine vege- 

 tation become at last the bony skeletons of fishes. These are, 

 in turn, the prey of carnivorous birds, whose exuviae form on 

 tropical islands beds of phosphatic guano. A history not dis- 

 similar will explain the origin of beds of coprolites and of some 

 other deposits of mineral phosphates. [By whatever means 

 the phosphates have been first concentrated, it appears from 

 the recent studies of Sollas that the so-called coprolites of the 

 10* o 



