SOUECE OF THE METALS. 571 



interruption in the regular flow of the current, as rapidly moving waters 

 deposit much less readily than those whose movement is very slow. 



Admitting the above conditions, it would seem, a priori, impossible to 

 assign any general direction of movement to currents from which ores are 

 deposited, and that each individual deposit must be studied by itself in 

 order to determine, by its geological relations, from which direction the 

 depositing solutions probably came. The admission that ore bodies have 

 been deposited by currents of circulating waters logically involves the 

 admission that they may have been upward, downward, or lateral currents, 

 according as the conditions at time of deposition favored either direction 

 of approach to the locus of deposit. While the determination of this direc- 

 tion in a special district is of the utmost importance from an economical 

 point of view, since by it the explorations for the continuation of ore bodies 

 must be largely guided, its theoretical importance as bearing upon the gen- 

 eral question of the origin of ore deposits seems to have been hitherto much 

 exaggerated. 



Source of metals. In speculations as to the source from which the metallic 

 contents of ore deposits are derived, a distinction should also be made 

 between the immediate and the ultimate source. 



The ultimate source is as much a purely speculative matter as the 

 nebular hypothesis. Since according to this hypothesis the earth in its 

 present condition is the result of gradual cooling from an incandescent mass, 

 and since moreover the specific gravity of the rocky crust which is exposed 

 to observation is very much less than that of the whole mass of the earth, 

 it is a legitimate conclusion that the heavy metals must be in much larger 

 proportions in the interior of the earth than in the rocky crust. Although 

 this view is generally admitted by geologists at the present day, it is evi- 

 dent that its basis is somewhat negative, since, like the nebular hypothesis 

 upon which it is founded, it cannot be proved in the present state of science 

 by actual experiment or observation. Volcanic emanations and thermal 

 springs have been found to contain metallic minerals, as have also the 

 waters of the ocean, but it cannot be stated definitely from what depth they 

 have come in the former case, nor whether, in the latter case, they may 

 not have been ultimately derived from the same indefinite, deep-seated 



