72 NEW YORK STATE MUSEUM 
3 The magnetite does not conform strictly to the structure of the 
“ gneisses,” as is commonly stated. On the contrary the ore grades 
into country rock in many cases from “ solid” ore, through dissemi- 
nated magnetite (“lean ore”) into magnetiferous rock and finally 
rock almost free from magnetite. Moreover, the magnetite fre- 
quently cuts the rock structure in stringers and “ veins,” and not 
infrequently occurs in little “ dikes” in the Pochuck granite, which 
is regarded as a manifestation of the very extreme end-stage matter, 
with magnetite very slightly overlapping. 
Evidence for replacement. 1 Petrographic study of both thin 
sections and polished plates proves the magnetite to contain embayed, 
corroded and partially replaced silicate minerals, native to the 
Pochuck-Grenville; these in many cases are affected by sericitization, 
chloritization, epidotization, serpentinization, and by conversion to 
talc; in such a manner and to such a degree as to suggest that 
replacement was connected with the changes mentioned. (See 
plates’ 6;'7, 10, ‘I1.) 
2 Field studies show that the forms of the ore bodies were 
apparently controlled in large part both by preexisting structures in 
the rocks replaced, and by the quality of the replaced material. The 
activity of the deuteric processes was so great, however, that pre- 
existing structures were only partially preserved in some cases. 
Thus, the form of the ore body of the Forest of Dean mine, which 
resembles a synclinal asymmetric fold, is thought to represent an 
inherited (preserved) structure; the forms of the ore bodies in the 
Lake, Scott, Cook, Augusta, Mahopac and Tilly Foster mines are 
likewise suggestive of inherited structures, partially preserved; or 
modified, in some instances, more especially in the Mahopac and 
Tilly Foster mines. 
Evidence for magmatic control of replacement. 1 The inti- 
mate relationship between the magnetite and the pegmatites, the 
latter undoubtedly derived from igneous sources, suggests a similar 
source for the magnetite, and hence magmatic control of replace- 
ment. 
2 In the few cases where interbedded crystalline limestone layers 
or lenses were replaced, the minerals found with the magnetite are 
characteristic contact types usually found where limestones have 
been profoundly affected by the intrusion of igneous bodies (see 
plate 9, figure 1 and plate 14, figures I, 3, 4). 
3 The apatite found in the magnetite is from igneous sources and 
