56 KEMPS ORE DEPOSITS. 



1.06.15. It is believed that under the above heads are included 

 all the forms of ore bodies which constitute well-recognized and 

 fairly well understood geological phenomena. To these catego- 

 ries year by year we are enabled, by the results of extended and 

 careful investigation, to refer many that have been obscure. A 

 number of familiar terms for ore bodies in mining literature fail 

 to appear, but are mentioned in the classifications quoted from 

 others. Many of these refer only to form, and geologically con- 

 sidered are only convenient admissions of ignorance as to origin. 

 Some other ore bodies whose methods of origin are involved in the 

 processes of regional metamorphism are placed by themselves 

 farther on. The explanations of them are as yet hypothetical. A 

 few comments on the scheme may now be added, although in the 

 main it explains itself. 



1.06.16. I. Much attention has been given of late years* to 

 processes of rock formation from igneous magmas. Of these the 

 excessively basic are the only ones primarily concerned with ores. 

 It is well known that in the series of igneous rocks we have suc- 

 cessively those with less and less silica. It is quite conceivable 

 that local developments might bring about such a decrease of the 

 silica and such an increase of one of the commonest of the bases, 

 iron oxide, that the limits of an ore might be reached. Such bod- 

 ies are almost always highly titaniferous, so much so that in this 

 country they are not available. Not a little attention was direct- 

 ed in earlier years to the Cumberland Hill outcrop in Rhode 

 Island, but it was found to be too high in this element to be suited 

 to furnace practice. No analyses are at hand of the Brazilian ex- 

 ample, but a considerable percentage of titanium might be ex- 

 pected. The presence of these ores in Canada, the Adirondacks, 

 and Minnesota is very familiar. It is possible, as indicated by 

 Yogt, that when magnetite crystals had formed in the still fused 

 magma, they became aggregated by magnetic currents in the 

 earth. And it is also conceivable that these early and heavy 

 crystallizations may have settled to the lower portions of the mag- 

 ma aiid have become concentrated. Much that is more or less 

 speculative is involved in these explanations. 



1.06.17. Under II. 1, the precipitating agencies are men- 

 tioned, which are the chief causes in the chemical reactions of de- 

 position, and these run through all the subterranean cavities as 

 well. The general application is esteemed self-evident. The 

 large part played by organic matter, both when living and when 



