MAGNETITE IRON DEPOSITS OF SOUTHEASTERN NEW YORK 123 
chondrodite, ripidolite, brucite, serpentine, spinel and allied types, 
as criteria of origin, until Koeberlin’s*"’ paper appeared in 1909. 
He made a petrographic study of thin sections of the ore, finding 
but four minerals, magnetite, chondrodite, spinel and serpentine, and 
concluded from the character of the ore and associated minerals, and 
the presence of chondrodite-bearing limestone found on the dump, 
that the deposit was of contact-metamorphic origin; “a replacement 
of an interbedded limestone in the gneiss series, due to the influence 
of a nearby igneous intrusion.” 
The writer believes this opinion to be essentially correct. Petro- 
graphic study of material selected from the dump shows that the 
magnetite replaces a previously metamorphosed interbedded crystal- 
line limestone; connected with the ore are still unreplaced remnants 
of colorless pyroxene, both monoclinic and orthorhombic (essen- 
tially diopside and enstatite) ; an almost colorless olivine, chondro- 
dite, light-green spinel, phlogopite, clinochlore and serpentine carry- 
ing very little iron, with magnetite encroaching upon and cutting all 
these, in end-stage relations (see plate 14, figure 3). The walls of 
the ore body are Pochuck-Grenville; the interbedded limestone has 
entirely disappeared. It no longer exists in actual contact with the 
ore, but is found in unreplaced remnants within the ore. Thus the 
ore body of the Tilly Foster belongs to the “ contact-replacement ” 
type, to which also belong the ore bodies of the O’ Neill, the Forshee, 
the Red-back, the Standish, the Croft, the Todd and the Mahopac 
mines ; some of these, however, are not such characteristic members of 
this group, as they appear to have been formed by the magmatic re- 
placement of strongly calcareous phases of the Grenville, rather than 
by the “contact-replacement’”’ of actual interbedded limestone; but 
they may be properly placed in the group as distinguished from the 
ore bodies of the Lake, Forest of Dean, and like types, for example, 
in which there is no indication, or but slight indication, of contact- 
metamorphic minerals similar to those enumerated above, and where 
the replaced Grenville was apparently much less strongly calcareous. 
There is no reason to suppose that the ore in this mine was 
wholly exhausted when operations ceased, nor that the ore bodies of 
this mine, and the Mahopac, of like type, were definitely outlined 
and delimited at the time of their exploitation. 
177 Koeberlin, F. R. The Brewster Iron-bearing District of New York. 
Econ. Geol. 4: 713-54. 1900. 
