MAGNETITE IRON DEPOSITS OF SOUTHEASTERN NEW YORK 2I 
Brewster, Putnam county, seem to close the record of activity in 
the study of the magnetite deposits of southeastern New York. 
Koeberlin first appreciated the significance of the character of the 
minerals associated with the ore in the Tilly Foster mine, and first 
recognized the similarity in origin between the Tilly Foster and the 
Mahopac ore bodies. He correctly described these as deposits 
formed by the replacement of an original body of interbedded lime- 
stone: “It seems probable in the light of these facts that the 
chondrodite-bearing limestone found associated with the ore is what 
remains of an original body of interbedded limestone. This lime- 
stone was subjected to contact metamorphic action by the intrusion 
of an igneous magma, which was probably either granitic or syenitic, 
as both these rocks outcrop in the neighborhood. . . . The 
intimate association of these two minerals (chondrodite and spinel) 
with the magnetite, precludes the possibility of a sedimentary 
origin.” (p. 749-50) 
In the contiguous territory of New Jersey the study of the prob- 
lems involved in the complex geology of the Highlands was actively 
prosecuted. Until 1889 most geologists of the time were convinced 
of the sedimentary origin of both the gneisses and the associated 
iron ores, but Nason ** at this time first threw doubt on the correct- 
ness of the view that all the rocks of the Highlands are meta- 
morphosed sediments. He questioned the value of the subdivision 
of the crystalline rocks into three groups by Britton, and although 
he suggested a division of these rocks into types dependent on 
mineralogical and structural features, he found difficulty in classify- 
ing the Archean rocks in the then existing state of knowledge. Nason 
suggested that the greater part, if not the whole of the Archean 
might prove after all to be eruptive, and conceived that the schistose 
and foliated structures might have been caused by pressure or shear- 
ing. These views did not receive much support, however, until 
nearly 15 years later, and they were rejected subsequently by Nason 
himself. In the meantime the origin of the magnetites received less 
attention, the geological work trending in other directions. Among 
the more important papers published during this interval were 
Correlation Papers, by Van Hise,** in which an excellent summary 
of New Jersey geology provides a fair estimate of the knowledge of 
42 Nason, F. L. Geological Studies of the Archean Rocks. Annual Rep’t 
State Geol. Geol. Surv. N. J., p. 12-72. 1880. 
48 Van Hise, C. R. Correlation Papers. Archean and Algonkian. U. S. 
G. S. Bul. 86, p. 414-15. 1802. 
