36 NEW YORK STATE MUSEUM 
Effect of faulting on the magnetite bodies. The larger ore 
bodies of southeastern New York have not been seriously affected 
by the various periods of deformation through which the Pre- 
cambrian rocks have passed. 
The ore body in the Forest of Dean mine, a pitching chute of 
fairly uniform cross section, has been mined to a depth of more 
than 4500 feet down the pitch without discovering a single fault of 
any magnitude. The main ore body of the Lake mine has not been 
affected, but the ore body of the old Crossway and Mountain mines 
of the Sterling Lake group, nearby, has been slightly offset by fault- 
ing, and the ore body in the Scott mine, of the same group, has 
likewise been faulted to a small degree. 
Wendt * first mentioned the fault which cut the ore body of the 
Tilly Foster mine, and later Ruttman,*® presented an elaborate dis- 
cussion of it. The fault was not, however, of great throw and 
caused no trouble in working the ore. The ore body in the Mahopac 
mine was likewise cut off by a fault, but no details of the magnitude 
of it are available. The old “ Phillips vein,’ along which were 
located the Canada, Sunk, Pratt, Sackett and Denny mines, is slightly 
offset by an oblique fault which lies between the north end of the 
Sackett pits and the south end of the Sunk workings; the apparent 
offset is about 50 feet. Small crush-zones are a common feature in 
the Highlands, and the magnetite deposits have been more or less 
affected by the movements which developed the crush-zones. It is 
difficult to say to what degree faulting has affected some of the 
deposits, since the old workings were in many cases merely shallow 
pits, and these are now caved and water-filled. 
Nature of the Precambrian complex. The gneisses of the 
Highlands comprise a composite belt of crystalline rocks with some 
types certainly of igneous origin and others that represent original 
sediments, now metamorphosed. The igneous members include 
gneissoid rocks of granitic, syenitic and dioritic composition, and 
mixed types which may be regarded as syntectics, all associated with 
and usually accompanied by extensive developments of pegmatite 
which occurs in small and large dikes, bands, streaks, schlieren and 
small bosses. Pegmatite is almost invariably encountered in the 
vicinity of the magnetites, and usually closely associated with them. 
The gneisses of sedimentary origin are exposed in unmixed 
88 Wendt, A. F. The Iron Mines of Putnam County, N. Y. Trans. A. I. 
M. E., 13: 478-88. 1884-85. 
89 Ruttman, F. S. Notes on the Geology of the Tilly Foster Ore Body, 
Putnam County, N. Y. Trans. A. I. M. E., 15: 79-90. 1886-87. 
