1232 A TREATISE ON METAMORPHISM. 



From the foregoing it is clear that an investigation of the local factors 

 in a district should include both those which are favorable and unfavorable 

 to the concentration of ores, for a study of the latter in many districts may 

 prevent the expenditure of large sums in exploration where the mineraliza- 

 tion is general, but the conditions are not such as to have concentrated the 

 valuable material in sufficient quantity at any one position to warrant 

 exploitation. 



A treatise on ore deposits, including descriptions of individual districts, 

 necessarily deals with the special factors which are important in each 

 district. These special factors may be considered so conspicuous that the 

 entire attention is given to them. But it is to be remembered that each of 

 these is subordinate to the general principles controlling the deposition of 

 ore deposits in all districts. 



SUMMAET A:NT> CONCI/USIOX. 



I shall now make a summary statement of the genetic classification of 

 ore deposits proposed, and consider briefly the relations of these different 

 classes of deposits. It is proposed that ore deposits be divided into three 

 divisions: (A) Sedimentary ores, (B) igneous ores, and (C) metamorphic 

 ores. The sedimentary ores may be divided into two classes: (1) Chem- 

 ical precipitates, and (2) mechanical concentrates. The mechanical con- 

 centrates may be divided into (a) residuary deposits, (b) stream deposits, 

 and (c) beach deposits. As yet criteria have not been formulated by which 

 igneous ores may be subdivided, and therefore there is only one class (1) 

 magmatic segregations. Metamorphic ores are divided into two classes: 

 (1) Gaseous solution ores, and (2) aqueous solution ores. The aqueous 

 solution ores are subdivided into (a) ores deposited by ascending waters, 

 (b) ores deposited by descending waters, and (c) ores deposited by ascend- 

 ing and descending waters combined. The scheme is therefore as follows: 

 Classification of metallic ore deposits. 



1(1) Chemical precipitates. 

 f(a) Residual deposits. 

 (2) Mechanical concentrates. I (b) Stream deposits. 

 I (c) Beach deposits. 



( B ) Igneous ( 1 ) Magmatic segregations. 



(1) Deposits from gaseous 

 solution. 



tn\ Motamnmtiip J I ( a ) By ascending waters. 



(C) Metamorphic. (2) Deposife from aqueous M ^ ^.^ ^^ 



solution. 



(c) By ascending and descending water.. 



