THEORIES OF ORE DISPOSITION HISTORICALLY CONSIDERED. 825 



the mineral |)r()C('ssi's that have ooiic on since Roman times in the 

 masonry of ohl tliernial establishments at IMombieres, Bourbonne- 

 h^s-Hains, and elsewhere, in Avhieh he thonn^ht to trace the actual 

 processes of vein formation. TTis works. Experimental (^Jeolog}-, 

 187!), and Subterranean "Waters, ISST, which contain the first ])hilo- 

 sophic discussion based on experiment of the physics of the rock 

 fractures that constitute canals for the circulation of un(h'r<>:r()und 

 waters, are still classic works of reference for the students of ore 

 deposits. Danbree understood veins to be fractures formed by 

 dislocations of the earth's crust under pressure strains and filled 

 by de])osits from acpieous solutions, generally heated by contact with 

 igneous i-ocks, from which in certain cases they may have directh' 

 emanated. From his observations at Plombieres he inferred that 

 most minerals are soluble if given sufficient w^ater and time, and that 

 great heat and pressure are not absolutely necessary prerequisites. 

 Some of the materials of veins, he admitted, may have been derived 

 from the surrounding rocks. 



The middle of the nineteenth century was characterized by the 

 increasing use of laboratory expei-iments in chemistry and physics 

 as aids in testing the current theories of vein formation, as is sho\vri 

 in the preceding sketch of progress of opinions among French 

 geologists. 



Similar progress was going on in other countries, especially in 

 Germany, which was more particularly a country of mines and 

 mining engineers, though among students of this subject there was 

 less solidarity of opinion than with the French, and their investiga- 

 tions for a long time were rather on chemical than on physical lines. 



A great impulse to increased accuracy in geological investigation 

 was given by the classic work of G. Bischof, Physical and Chemical 

 Geology, 1846^7, which discusses in a masterly way the processes 

 involved in most of the known geological phenomena, largely on 

 the basis of the author's own researches and experiments, and which 

 may be said to have raised chemical geology to the rank of a dis- 

 tinct science. In the course of his investigations, Bischof, having 

 found that the constituents of the gangue minerals of veins are 

 found in the country rocks, thought it probable that the metals of 

 the suli)hide ores might also exist in these rocks in the form of sili- 

 cates. Later, and independently, Forchhammer, the Danish chemist, 

 in the course of his long continued investigations of the waters of 

 the ocean, detected minute amounts of many of the metals of ore 

 deposits both in sea water and in different varieties of rocks. It 

 was long before the suggestions offered by these discoveries had 

 any practical effect on ore-deposit investigations. 



The leading authorities (on ore deposits) of that period were Von 

 Beust, Breithaupt, II. Midler, and Von Cotta. The latter for over 



