MAGNETITE IRON DEPOSITS OF SOUTHEASTERN NEW YORK IOI 
a The form. The peculiar heart-shaped or “fold-shaped”’ form 
of the ore in the Forest of Dean mine is familiar to all who have 
had occasion to study the magnetite deposits of the State of New 
York. By most of those who have offered any explanation, it was 
thought to be due to folding. Thus, Stolz** remarks “The ore 
body and high micaceous inclosing gneisses . . . probably lay 
originally in a horizontal plane, the syncline being developed during 
a period of folding and pressure.’ And likewise Ames **’ remarks 
“ Dynamic stress caused flexing of the original rock, which already 
had a strong gneissic or gneissotd structure, and igneous intrusion 
took place at the same time, along the lines of weakness developed 
by this deformation.” 
It is not possible that this ore body has been affected by either 
Taconic or Appalachian folding, since the late Precambrian basic 
dikes, thin, tabular and standing almost erect, cut the ore and are 
not themselves disturbed at all. 
The ore is conceded to be of magmatic origin and related to the 
igneous intrusives. These show no signs of ever having been 
folded since their emplacement; moreover, had severe folding 
occurred during the formation of the magnetite such dynamic condi- 
tions would have disturbed the thin tabular mass of rock running 
from the horse into the hanging wall, cutting the ore, but intimately 
related to it in time of formation. (See figure 3.) 
There is no structural evidence in the ore body and its intimately 
associated pegmatite to indicate that folding occurred during or after 
the deposition of the ore. 
On the other hand, the inclosing rocks are indubitably folded; 
hence the contention of the writer that the forms of the ore bodies 
are due to the replacement of previously folded rock, and that the 
structures seen are inherited, seems justified. 
b The horse. The mass of rock which splits the ore body, and 
which remains constant in its relations with depth, and which in 
part gives the ore its peculiar heart-shaped form (see figure 3), is 
typical Pochuck-Grenville (plate 4, figure 1), cut by dioritic 
pegmatite which likewise forms the thin tabular dikelike mass 
cutting the ore and extending from the horse into the hanging wall 
(plate 2, figure 3) The horse is simply country rock, similar to the 
14 Stoltz, Guy C. The Forest of Dean Iron Mine, N. Y. Eng. and Min. 
Jour. 85: 1091-93. 1908. 
145 Ames, Edward W. Notes on the Geology of the Forest of Dean Mine. 
Unpublished thesis submitted in partial fulfilment of the requirements for 
the degree of master of arts, in the Faculty of Applied Science, Columbia 
University (1918). 
