128 NEW YORK STATE MUSEUM 



Rather than contact developments along an intrusive mass of gab- 

 bro, they have been spoken of as basic segregations in syenites. 

 Both these interpretations are opposed to the still older belief that 

 the ores are of sedimentary origin and the question may be perhaps 

 stated with the arguments pro and con at this point. It is the more 

 appropriate because among those actively engaged in mining and 

 widely also among geologists having occasion to deal from time to 

 time with other magnetite bodies in the Adirondacks, the rocks and 

 associated ores are regarded as sedimentary in origin.^ The writer 

 while favoring the igneous conception disclaims any personal bias 

 toward it, other than that it has seemed to be the simplest and 

 least objectionable interpretation of rocks, confessedly puzzling. 



The ores do certainly imitate to a marked degree the folds and 

 similar structures of the stratified rocks, with perhaps this qualifica- 

 tion that the folds are of an extreme type, being overturned, 

 stretched and doubled up together in a very violent way. If sedi- 

 mentary they must have been folded under such extreme pressure 

 that the rocks flowed after the manner of viscous materials. In no 

 other way could the Tefft shaft ore body have been pinched away 

 from the main mass of the "21 " pit. These folds are undoubtedly 

 not essentially different from others well recognized in regions of 

 metamorphosed, sedimentary rocks. The cross sections of the 

 Alps for instance show many cases of the same sort. 



On the other hand if one imagines a molten magma, difterentiated 

 into layers of contrasted composition, layers which range from 

 acidic extremes to basic, then squeezed into folds either while yet 

 viscous or after consolidation, the result would be the same. That 

 this differentiation takes place in magmas is one of the growing 

 convictions of students of eruptive rocks. It is certainly well 

 enough established to justify giving it serious consideration. It 

 may perhaps not unjustly prevent us from taking the sedimentary 

 nature of the rocks as positively established because of the folded 

 structure. 



Another feature which has been esteemed proof of the sedi- 

 mentary origin is the persistence and faithfulness of the ores in 

 stratigraphic position. In the case of the Barton Hill group, 

 they certainly do extend as much as a mile on the strike and are 

 persistent for this distance. If we unwrap the Old Bed group 

 from its folds and reconnect the faulted blocks they will extend. 



^ Sec for example A\'. L. Cuming:?. ''On Sedimentary ^Magnetites." En- 

 gineering and ^Mining Journal. July 7, 1906. p. 25. 



