ADIRONDACK MAGNETIC IRON ORES 33 



to some process of sedimentation as outlined above, but it would 

 appear to the writer more reasonable to regard them as introduc- 

 tions subsequent to the formation of the wall rocks. They 

 apparently antedate the period of deformation during which the sur- 

 rounding rocks were subjected to their final compression and folding. 



As a rule the deposits are more irregular than would be 

 expected in stratified bodies. They have no well defined bounds, 

 but shade off into the country rock. It is seldom that the charac- 

 ter of the hanging and foot shows any marked change that can 

 be taken for original variations in the sedimentation. The thick- 

 ness of some of the deposits is excessive when compared with 

 known examples of bedded iron ores; the Benson body, for 

 example, measures over 200 feet across the strike and the country 

 rock is mineralized over much greater width. 



Though it is believed that the ores are of epigenetic or secondary 

 derivation, there is little basis of facts to support a more precise 

 explanation -of their origin. The view that they were formed 

 before the surrounding rocks had undergone final rearrangement 

 appears reasonable, because they have laminated textures and follow 

 closely the general field structures. Their introduction may thus 

 have taken place before the rocks were metamorphosed, in which 

 case it might have been accomplished by ordinary ground-water 

 circulations, with limonite or carbonate replacing the shales and 

 limestones as the first step. The presence of organic matter in the 

 beds, indicated by their content of graphite, would exercise a 

 reducing action favorable to the formation of magnetite rather than 

 hematite under the ensuing met amorphic conditions. 



Mining and milling in the Adirondacks 



Both underground and open-cut methods are used in the Adi- 

 rondack mines, the latter, however, being restricted to a few large 

 ore bodies or those so situated as to present a considerable surface 

 development. In general the high inclination of the bodies and 

 their narrowness across the strike render a system of underground 

 working the most suitable from the start. Inclined shafts or 

 slopes following the dip of the ore have been generally adopted in 

 preference to vertical shafts which in some instances at least would 

 seem to offer important advantages as regards economy of opera- 

 tion. The deepest shafts are at Lyon Mountain, about 1500 feet 

 measured on the incline. Horizontal drifts are extended on either 

 side of the shaft at more or less regular intervals and the ore stoped 

 out between them, leaving occasional pillars of ore for roof sup- 



