7O NEW YORK STATE MUSEUM 
ing to W. S. Bayley *’ and A. C. Spencer™*®, who have made the 
latest and most exhaustive studies of those deposits. The magnetites 
of the Brewster district, in Putnam county, New York, and the ore 
in the Forest of Dean mine, in Orange county, have been judged 
to be of magmatic origin also by Koeberlin**? and Ames.**° The 
consensus of opinion, therefore, of the students of these deposits, 
is for an igneous source. The processes operative in the formation 
of the magnetic iron ores of Putnam and Orange counties, New 
York, appear to have been: 
1 Long-continued magmatic differentiation of a basic magma of 
great extent, with the concentration of extremely mobile end-phase 
products rich in those elements which ultimately formed the 
pegmatites; rich in magnetite, rich in quartz and gaseous concentra- 
tion products, all in aqueo-igneous solution. 
2 Subdifferentiation of the concentrate itself, into pegmatite-rich 
and magnetite-rich fractions, and 
3 Transportation, in aqueo-igneous solution, of such fractions 
through the operation of stresses concomitant with batholithic 
intrusion, along directions of least resistance. 
4 The magmatic replacement, by the magnetite-rich fraction, of 
more or less calcareous phases, and in a few cases of interbedded 
limestone lenses, of the Pochuck-Grenville; the whole process sub- 
ject to the structural control of the Pochuck-Grenville. 
5 Forming magmatic- menlonenient deposits, or replacement deposits 
of deuteric origin. 
The feature which the writer wishes to emphasize is the power- 
fully mineralizing capacity of magmatic end-stage emanation prod- 
ucts rich in magnetite, forced ultimately to penetrate a part of their 
own parent body which had already impregnated and injected a 
previously folded and metamorphosed series of sediments; and of 
directions of travel structurally controlled, and loci of deposition 
dependent on the quality of the rock replaced. 
The term “ dewteric”’ was first used by Sederholm’” to indicate 
117 Bayley, W. S. Annual Rep’t State Geol. N. J. 1904. Min. Mag., v. Io, 
1904. 
Iron Mines and Mining in N. J., v. VII, final report series, State Geol. 
ONO) [ROK USA IW Se (EA Siyy iealss 8 hovel) Iskolbioy ion Whi Sy Cay Se) uoOUZ. 
118 Spencer, A. C. Folio 161, U. S. G. S. 1008. 
119 Koeberlin, F. R. The Brewster Iron- bearing District of New York. 
Econ. 'Geol., v. IV, p. 713-54. 1900. 
120 Ames, Edward W. Notes on the Geology of the Forest of Dean Mine. 
Thesis submitted in partial fulfilment of the requirements for the degree of 
Master of Arts, in the Faculty of Applied Science, Columbia University, 1918. 
121 Sederholm, J. J. Synantetic Minerals and Related Phenomena; Bull. 
de la Comm. geologique de Finlande, 48: 141-42. 1916. 
