32 NEW YORK STATE MUSEUM 
cambrian rocks have been thrust. Moreover the isolated hills of 
Precambrian rocks strung out on the western side of the Highlands 
and surrounded by Paleozoic sediments are products of the same 
forces, “stranded fault-blocks”’ * and not “islands in the seas of 
Hudson River time.”” ; 
None of the peaks of the Highlands is especially prominent. Both 
peaks and ridges have a fairly uniform average elevation over wide 
areas, so that if one imagines all the valleys and depressions filled 
to the level of the ridges the result would be an approximately plane 
surface, with a gentle inclination toward the southeast and south- 
west. The tops of the peaks and ridges represent all that remains of 
the Cretaceous peneplane in this area, so that the rugged topography 
of the Highlands is Post-cretaceous in development and has been 
modified by subsequent glaciation. 
STRUCTURAL FEATURES OF THE HIGHLANDS 
Folds. The general structural habit of the Highlands has com- 
monly been described as monoclinal because the banded and foliated 
structure of the gneisses strikes parallel with the trend of the rocks, 
and dips with apparently few exceptions at high angles to the 
southeast, thus giving the impression of a universal monoclinal 
structure. There are exceptions to this general monoclinal habit 
which seem to indicate the probability of a very definite folded 
structure which is, however, somewhat obscure and which may be 
inherited in part from the previously metamorphosed and undoubtedly 
folded Precambrian sediments of Grenville age into which, and 
because of their strong structural control, the later igneous matter 
was introduced, partly by impregnation and partly injected in lit- 
par-lit fashion, thus forming the basal gneisses of the Precambrian 
complex; a process and an origin long ago postulated by Berkey ”° 
and later by Fenner.*® Additional evidence of the existence of folds 
is offered by the structure of the ore bodies at Franklin Furnace 
and Sterling Hill, N. J. The great ore body which lies beneath Mine 
hill, at Franklin Furnace is “ bent upon itself to form a long trough 
with sides of unequal height. The trough lies with its keel pitching 
7 Berkey, C. P., & Holzwasser, Florrie. Geology of the Newburg Quad- 
rangle. N. Y. State Mus. Bul. (Prepared) 
78 Ries, Heinrich. Report on the Geology of Orange County. Rep’t of 
the State Geologist, N. Y., p. 422. 18095. 
79 Berkey, C. P. Structural and Stratigraphic Features of the Basal 
Gneisses of the Highlands. N. Y. State Mus. Bul. 107. 10907. 
80 Fenner, C. N. The Mode. of Formation of Certain Gneisses in the 
Highlands of New Jersey. Jour. Geol., 22: 504-612 and 694-702. 1014. 
