THE VERIFICATION PERIOD 25 



the heavier minerals to concentrate near the borders. Following out the 

 suggestion offered by this observation, Vogt had proved by field study 

 that certain titaniferous iron ores were actual segregations in the eruptive 

 magma previous to its final consolidation. Based on petrographic studies 

 made by Brogger and himself, and personal observations on ore deposits, 

 principally in Norway, Vogt defined two methods of formation of ore 

 deposits as the direct result of igneous action : 



1. By magmatic segregation. 



2. By eruptive after-action or pneumatolysis (a term first used by 

 Bunsen to describe the combined action of gases and water). 



In the first class (of admittedly infrequent occurrence) are titaniferous 

 iron ores, chromite, and other metallic segregations in basic eruptive 

 rocks. In the second class, commencing with tin and apatite veins, he 

 included, as time went on, increasingly numerous types of deposits. 

 This was in one sense a revival of de Beaumont's theories, but the mod- 

 ern standpoint differed, in that the existence of a liquid molten interior 

 of the earth had been disproved by terrestrial physicists. Vogt held 

 that as no communication could be established between ore deposits 

 and a heavy interior, they must have been derived from a crust, say, 

 ten, twenty-five, or fifty kilometers in thickness, and in great measure 

 the result of eruptive processes within that crust. 



Emmons (in 1893) acknowledged the importance of the magmatic 

 concentrations of metals in eruptive rock, but thought that in most 

 cases such accumulations must have been further concentrated in order 

 to produce economically valuable ore deposits. 



During the second decade the influence of Posepny's paper was felt 

 in an increased adherence among outside geologists and mining engi- 

 neers to the ascension theory. Vogt's views received less attention in 

 this country, because for a long time no ore deposits were studied to 

 which they were found to be applicable. The first case was that of the 

 titaniferous magnetites of the Adirondacks studied by Kemp, who pub- 

 lished his results in 1898. 



The year 1900 was rendered important in the progress of theoretical 

 views on ore deposition by the simultaneous appearance of " Principles 

 controlling deposition of ores," by Van Hise ; " Secondary enrichment," 

 by Emmons and Weed, and "Metasomatic processes," by Lindgren,and 

 by the discussions which they prompted. 



Van Hise's article was a broad, philosophic treatment, based on experi- 

 mental data, of the whole question of underground circulation as bearing 

 on ore deposition. It would be impracticable to give here any complete 

 abstract of his paper, which is probably familiar to most of you, and only 

 a brief statement of such points as bear on the general processes heretofore 



IV— Bull. Geol. Soc. Am., Vol. 15, 1903 



