792 



SCIENCE. 



[N. S. Vol. XIV. No. 360. 



plete. I trust it will not be found to be 

 false. But the most that I can hope for it 

 is that it is approximately correct as far as 

 it goes. 



It is believed that the principles which 

 have been presented lead to a new and 

 natural classification of the ore deposits 

 produced by underground water. As already 

 noted, ore deposits may be divided into 

 three groups : (1) Ores of igneous origin, 

 (2) ores which are the direct result of the 

 processes of sedimentation, and (3) ores 

 which are deposited by underground water. 



Since the ores produced by igneous 

 agencies and those produced by processes 

 of sedimentation have not been considered 

 in this paper, a subdivision of these groups 

 will not be attempted. 



Ores resulting from the work of ground- 

 water, group (3) above, may be divided 

 into three main classes : 



(a) Ores which at the point of precipi- 

 tation are deposited by ascending waters 

 alone. These ores are usually metallic or 

 some forms of sulphurets ; but they may be 

 tellurides, silicates or carbonates. 



(6) Ores which at the place of precipita- 

 tion are deposited by descending waters 

 alone. These ores are ordinarily oxides, 

 carbonates, chlorides, etc. , but silicates and 

 metals are exceptionally included. 



(c) Ores which receive a first concentra- 

 tion by ascending waters and a reconcen- 

 tration by descending waters. The con- 

 centration by ascending waters may wholly 

 precede the concentration by descending 

 waters, but often the two processes are at 

 least partly contemporaneous. The ma- 

 terials of class e comprise oxides, carbon- 

 ates, chlorides, and rarely metals and 

 silicates, above the level of groundwater, 

 and rich and poor sulphurets, tellurides, 

 metallic ores, etc., below the level of 

 ground-water. At or near the level of 

 ground-water, these two kinds of products 

 are more or less intermingled, and there is 



frequently a transition belt of considerable 

 breadth. 



How extensive are the deposits of class 

 a I shall not attempt to state. Indeed, 

 I have not such familiarity with ore de- 

 posits as to entitle me to an opinion upon 

 this point. However, a considerable num- 

 ber of important ore deposits belong to this 

 class. This class is illustrated by the Lake 

 Superior copper deposits. 



The ore deposits of class b are impor- 

 tant. Of the various ores here belonging 

 probably the iron ores are of the most con- 

 sequence. All of the iron ores of the Lake 

 Superior region now being exploited are of 

 this class. 



It is believed that the ore deposits of 

 class c are by far the most numerous. I 

 suspect that a close study of ore deposits 

 in reference to their origin will result in 

 the conclusion that the great majority of 

 ores formed by underground water are not 

 the deposits of ascending waters alone, but 

 have by this process undergone an early 

 concentration, and that descending waters 

 have produced a later concentration, as a 

 result of which there is placed in the upper 

 100 to 1,500 or possibly even 3,000 feet of 

 an ore deposit a large portion of the metal- 

 liferous material which originally had, as a 

 result of the early concentration, a much 

 wider vertical distribution. 



The depth to which rich deposits of the 

 classes produced or concentrated by de- 

 scending water extend is a very important 

 question ; but time does not permit adequate 

 discussion of it. The factors entering into 

 the problem are very numerous and com- 

 plicated. Only a single one of these will 

 be briefly considered, and this is the to- 

 pography. Where the relief is small, the 

 vertical effect of descending solutions ex- 

 tends upon the average to less depths than 

 where the relief is great. But even where 

 the relief is moderate the descending solu- 

 tions may be effectual to a considerable 



