DERIVATION FROM NEIG0BORIIfG ROCKS. 575 



pure speculation, seems to be in favor of the lateral-secretion theory in its 

 broader acceptation. Geologists whose acknowledged ability and wide 

 experience give weight to their opinion have already made the general- 

 ization that the majority of the ore deposits, whether in crystalline or in 

 sedimentary rocks, are found, if not in aetufil contact with, at least in the 

 immediate vicinity of, eruptive rocks. The experience of the writer would 

 lead him to qualify this generalization by adding that it is with the older 

 and generally intrusive rocks of eruptive origin that valuable ore bodies are 

 most frequently associated, while they are rare in regions where these rocks 

 only form surface flows or are outpourings of actual volcanic vents. As 

 there is no ground for assunu'ng that the latter i-ocks would be freer from 

 heavy metals than the former, the reason for these associations would have 

 to be found in the fact that the older rocks have been exposed longer to the 

 action of percolating waters and the deeper rocks have been more acces- 

 sible to the waters containing materials that would readily dissolve the 

 metals. 



As regards the derivation of ore materials from neighboring rocks, G. 

 Bischof, who has rendered most important services to geology in removino- 

 it from a speculative to an inductive basis, first gave an authoritative and 

 decided opinion in these words: '"As a general consequence of the relations 

 between the matrices of lodes, the rocks adjoining them, and their condition, 

 as well as those between diffei-ent lodes, it may be inferred that all the sub- 

 stances contained in the lodes have been derived from the adjoining rocks." 



Both Breithaupt- and von Gotta ^ admit the probability of this deriva- 

 tion, provided the existence of the vein materials in the country rock can 

 be proved. Bischof had already proved this for the gangue materials, but 

 his investigations had not been carried further, and the existence of the 

 heavy metals in the country rocks still remained to be demonstrated. To 

 this task Dr. F. Sandberger has devoted himself, as he tells us,^ since 1873. 

 Up to that time he had been an advocate of the ascension theory, but after 

 having by careful analysis detected all the vein materials of a certain dis- 



' G. Bischof, Chemical and Physical Geology, III, i). f '8. 

 'Parageuesis der Mineralien, p. 119. lt<49. 

 'Lageratlitteu der Erze. I, p. 177; II, pp. 297 et seq. 1859 

 ■'Unters"filiunsen iibev Erzgange. Wiesbaden, 1882. 



