THE VERIFICATION PERIOD 19 



densities, he succeeded in demonstrating to his own satisfaction that the 

 characteristic minerals of different deposits are contained in the basic 

 silicates of the adjoining rocks, and in 1880 propounded his theory of 

 lateral secretion, according to which the mineral contents of veins are 

 derived, not from some unknown depth, but from the immediate wall 

 rock, being brought in by percolating waters which are not necessarily 

 at a very high temperature. As against the thermal spring theory, he 

 argued that a very small proportion of known thermals contain any of 

 the metallic minerals whatever and none in the state of sulphides ; fur- 

 ther, that the deposits observed in their channels have been precipitated 

 in immediate proximity to the surface and under physical and chem- 

 ical conditions that differ essentially from those that must have pre- 

 vailed at the depths at which veins were formed. 



Sandberger's theory, though for a time it found many adherents, was 

 bitterly contested, especially by Stelzner, Von Cotta's successor at Frei- 

 berg, and by Posepny, professor at the Mining School of Pribram, in 

 Bohemia. They maintained that the facts in their respective districts 

 disproved the lateral secretion theory in the restricted sense in which 

 Sandberger employed it, and they demonstrated by a repetition of his 

 analyses that, owing to imperfect methods, he had not proved the metals 

 he found to be necessarily original constituents of the rocks in which 

 they were supposed to occur. Whatever opinion may be held as to the 

 merits of Sandberger's theory, as such, it undoubtedly contributed to the 

 advance of the study of ore deposits in stimulating what may be called 

 verification — that is, the practical testing of theory in its application to 

 concrete instances in nature. 



In general, it may be said of the period that was now closing that, 

 though facts of observation and experiment had been accumulating, the 

 advances in the study of ore deposits during that time were much less 

 than those that had been made in other branches of geological science 



THE VERIFICA TION PERIOD 



The third period, covering, in a general way, the last quarter of the 

 past century, may be called the period of verification. So fertile had been 

 the imagination of previous thinkers on this subject that at this time it 

 was practically impossible to conceive a theory of origin for a given ore 

 deposit that had not already been proposed or at least suggested. The 

 investigations now to be carried on with more perfect methods, or in the 

 light of recent advances in the science, would seem more properly veri- 

 fications of old theories than the propounding of new ones. 



Method and the microscope have been the two great agents of progress- 

 The greatest improvement in method has resulted from government aid, 

 under which it has been possible for organized bodies of scientific workers 



