48 } NEW YORK STATE MUSEUM 
the north end of Middle Branch reservoir. Nothing is visible of 
this famous old mine but a small pond and the surrounding dump, 
which is now more or less overgrown with vegetation. 
The Mahopac mine, equally noncommittal as to surface appear- 
ances, is situated near the highway leading to Kent Cliffs, about 
three-eighths of a mile to the north, and the same distance to the 
west, of Kirk lake. 
It is about 134 miles north of Mahopac falls, and 7 or 8 miles 
west of Brewster. 
The underground workings were of considerable extent, but the 
mine caved near the entrance, and water has filled the depression, 
forming a small pond, which is all that is visible of the mine today. 
(Seeiig.) 12.) 
Geologic relations of the ores. The associated rocks. The 
magnetite bodies of the Highlands are so intimately related to the 
rocks associated with them that a thorough understanding of their 
origin, habit, history and character is possible only provided the 
complex geology of the region is itself comprehended. 
This has been somewhat briefly mentioned in the discussion of 
the general geologic setting (pages 31-40), but it seems best to 
enlarge upon the discussion a little, even at the risk of repetition, 
not only because of the extreme importance of the relationships 
between the ore and its associated rocks, but also in order to estab- 
lish rock names which will serve to identify the various units and 
mixed types most intimately connected with the magnetite, so that 
the descriptions of the various mines may be more intelligently 
followed. 
The Grenville. The oldest formation of the region is a series of 
intensely metamorphosed, strongly folded rocks largely of fragmental 
origin; these consisted originally of a succession of lime- 
stones, sandstones, argillaceous, silicious and calcareous shales and 
smaller interbedded lenses of limestone of varying degrees of purity. 
The extent and thickness of these sediments is not definitely known. 
It is important to emphasize, however, that according to such field 
evidence as the writer has been able to gather, these rocks suffered 
regional deformation, resulting in profound metamorphism and 
folding, before the intrusion of the various magmatic units which 
laier invaded them; this series may be correlated tentatively with 
the Grenville. As previously mentioned, the rocks of this series are 
seldom found in unmixed development in the Highlands of south- 
eastern New York because they have been so thoroughly impreg- 
