1078 A TREATISE ON METAMORPHISM. 



During the lateral journey of the water, before trunk channels are 

 reached, the transported metals may be largely precipitated. Frequently 

 such precipitates are very widely dispersed and are not sufficiently rich to 

 constitute ore deposits. Exceptionally the amounts of precipitated material 

 during the lateral movements in the dispersed channels may result in the 

 formation of a product of sufficient richness to constitute an ore. Pre- 

 cipitation may result from any of the causes or combinations of causes 

 which are spoken of (pp. 1081-1088; see also pp. 113-123) as producing 

 precipitation from ascending waters in trunk channels. For the purpose 

 of illustration, one class of compounds may here be mentioned. Salts 

 traveling as sulphates may be reduced to sulphides and precipitated by the 

 direct reactions of carbonaceous materials, by the reaction of previously 

 precipitated sulphides, or, in the case of copper, silver, etc , by the reaction 

 of abundant ferrous compounds. The Crystal Falls volcanics " furnish an 

 illustrative case -where rocks have been so profoundly altered by metaso- 

 matic changes as to leave scarcely an original mineral present. In them 

 there are sparse, widely disseminated secondary sulphides. In this forma- 

 tion there is no organic material, and the natural cause to assign for the 

 precipitation is the reducing action of the ferrous compounds which are 

 abundantly present. The possible reactions are given on pages 1111-1112. 



From the foregoing it appears that ores are carried to trunk channels by 

 laterally moving waters. Lateral secretion is, therefore, an essential step in the 

 first concentration of ore deposits, although I use the term lateral secretion in 

 a broader sense than did Sandberger. 



The places where the ore deposits themselves are found will now be con- 

 sidered. As already noted, these occur mainly in or adjacent to the more 

 continuous larger openings. These openings are occupied by the trunk 

 streams of circulating waters, and therefore the water is in the latter part of 

 its course. Hence these trunk streams, as has already been shown (p. 583), 

 have in general an upward rather than a downward vertical movement. 

 The waters reaching the trunk channel at any point immediately begin their 

 ascent. At any given cross section of a channel there must pass all of the 

 water contributed below. At great depth this amount has already been 

 seen to be small. From a small amount the waters steadily increase in 



« Clements, J. Morgan, and Smyth, H. L., with W. S. Bayley, The Crystal Falls iron-bearing 

 district of Michigan: Mon. U. S. Geol. Survey, vol. 36, 1899, pp. 73-154. 



