MAGNETITE IRON DEPOSITS OF SOUTHEASTERN NEW YORK 57 
mine (figure 3). Other ore bodies are merely mixtures of ore alter- 
nating with country rock, of such a character that the unmixed, 
solid ore may not be more than a few feet in thickness, although the 
ore zone may be one hundred or more feet wide. In some cases 
there is more than one “vein” or “ bed,” which may be worked 
through a single shaft, as in the Mahopac mine (fig. 13); others 
are irregular, shapeless masses, only approaching a lenticular or pod- 
like form, as was the Tilly Foster ore body.*” 
Moreover, the ore does not always accord strictly with the struc- 
ture of the inclosing country rock, as has so frequently been stated ; 
in many cases the ore cuts the rock structure in tongues and in nar- 
row dikes, so that an abrupt, clean separation between ore and ore- 
free wall rock does not always exist (figure 4). Where there is 
more than one “shoot” of ore, the separate bodies at times merge 
into one another at different levels, as was the case in the Mahopac 
mine) lin short, the! forms) ‘of jthe /ore bodies are purely 
adventitious and are dependent, according to the writer’s belief, on 
(a) preexisting structures in the Grenville, which were inherited in 
part by the Pochuck, and which determined the course of replace- 
ment to a large degree, and (0) the composition of the Grenville; 
where highly calcareous, or where actual interbedded limestone 
lenses happened to lie in channels of activity, there the character of 
the deposits closely approached contact-metamorphic types with the 
production of garnet, chondrodite, spinel, tourmaline, and other 
typical contact minerals, mixed with the ore, as at the Tilly Foster, 
Mahopac, Croft, Todd, O'Neill, Forshee, and to a less degree the 
Red-back, and other mines. The shapes in these cases were deter- 
mined by the structures in the calcareous lenses replaced, the 
intensity of the mineralizing processes and the quality of the Gren- 
ville or Pochuck-Grenville which was invaded. 
Folds. No field evidence has been found to substantiate the 
belief that the iron ores and their containing rocks have ever been 
intensely folded since the formation ore bodies. 
There is sufficient evidence, however, to show that the Grenville 
rocks were intensely metamorphosed and strongly folded before the 
invasion of the igneous masses which so profoundly changed the 
Grenville strata and which were responsible for the deposition of 
the magnetite. 
103 Wendt, A. F. The Iron Mines of Putnam County, N. Y. Trans. A. I. 
M. E. v. XIII, p. 478-88. 1884-85. 
Ruttman, F. S. Notes on the Geology of the Tilly Foster Ore-body, Put- 
nam County, N. Y. Trans. A. I. M. E. v. XV, p. 79-90. 1886-87. 
