24 Mineralizers in Ore Segregations [222 



of its distributaries as a result of their own loads combined 

 with decreased run off. This set of factors combined with 

 the westward tilting that resulted in the Eipley Cretaceous 

 and Midway Eocene seas penetrating up the Mississippi val- 

 ley as far as southern Illinois is sufficient to account for the 

 observed change, of course assuming that there has been such 

 a change. This may be compared to the analogy of the shift- 

 ing of the present Mississippi delta to the eastward by marine 

 currents. 



The remnants of heavy gravels of Tuscaloosa age that have 

 been traced by Wade across Tennessee and into Kentucky 

 appear to represent the gradual migration or shifting north- 

 ward of such a stream. That the western Highland Eim 

 of Tennessee is a middle or late Tertiary planation of pre- 

 vailingly siliceous rocks by the Tennessee River in its lower 

 northward course is probably true but hardly within the 

 scope of the present brief note. 



THE ROLE OF MINERALIZERS IN ORE SEGREGATIONS 

 IN BASIC IGNEOUS ROCKS 



By JOSEPH T. SINGEWALD, JR. 



Though one of the latest groups of ore deposits to be defi- 

 nitely recognized, the magmatic segregations were firmly es- 

 tablished as one of the major types through the classic work 

 of J. H. L. Vogt twenty-five years ago ; and it has been gener- 

 ally felt by economic geologists that the mode of formation 

 of these deposits was so clearly understood that they consti- 

 tuted a group concerning the genesis of which there was no 

 further question. More thorough petrographic studies of 

 many examples of deposits classed with the magmatic segre- 

 gations and metallographic investigation of the ores, espe- 

 cially during the last decade, have accumulated more and more 

 evidence to show that mineralization was not so simple and 

 did not conform strictly to the conception of a magmatic 

 segregation in the sense in which that term is generally 



