METAMORPHIC ROCKS. 
469 
a chain of low irregular ridges usually contain the limestone in the highest state of crystalli¬ 
zation. These evidently mark the existence of a series of parallel veins of igneous origin, 
the intrusion of which into the limestone has obviously caused its alteration. Between the 
base of the Walkill mountain and the crystalline rock towards the middle of the valley, we 
usually meet with beds of the formation which evince but a partial .amount of change from 
the igneous action ; the limestone retaining more or less of its bluish tint, and presenting only 
a subcrystalline, or even the ordinary earthy texture. These less altered beds, reposing 
upon the gneiss of the Walkill mountain, dip towards the northwest. Towards the base of 
the Pochunk hill, on the other side of the valley, the limestone would seem to have undergone 
a more extensive alteration. Approaching Hamburgh, the crystalline belt contracts conside¬ 
rably in width. About two and a half miles northeast of that village, we find it occupying a 
broken chain of long, narrow, irregular ridges of considerable elevation. These range parallel 
with the base of the Pochunk mountain, separated from it by a tract of low meadow ground, 
about three hundred yards in breadth. In this belt the altered limestone is in great confusion ; 
the calcareous crystalline matter being mingled largely, in many places, with a white friable 
sandstone, referable obviously to Formation I.,* the position of which, w'hen it occurs at all, 
is immediately beneath the limestone. The fused calcareous matter seems in some cases to 
have penetrated the substance of the sandstone. The height of this ridge or chain of ridges 
may be stated to average about two hundred feet; the width at the base being not less than a 
fourth of a mile, and the length rather more than one mile. Besides the mass of altered lime¬ 
stone, these ridges comprise portions of Formation I.,* together with beds of the gneiss, both 
in a highly disordered and shattered condition, with more than one extensive dyke of igneous 
matter, the immediate cause, most probably, of these disturbances. 
“ From the confusion which accompanies all these rocks, none of them can be found, even 
for a short distance, possessing any regularity of dip. The gneiss rocks of the Pochunk 
mountain, on the contrary, are distinctly seen dipping steeply towards the valley to the south¬ 
east. 
“ The most conspicuous display of the crystalline limestone is at the two extremities of 
the ridge which lie nearest Hamburgh, Between these points, towards its central portion, 
the same ridge contains a large well-known deposit of brown or hematitic iron ore, occupying 
its summit and sides, and penetrating deeply into the body of the hill.” 
“ Separated from the Hamburgh belt of altered limestone, by a spur of the Walkill moun¬ 
tain, there exists another band of the crystalline rock on the Sandpond creek, about one mile 
and three quarters southeast of Hamburgh. This commences near the southwestern termi¬ 
nation of a small knob of gneiss, 
“ Though highly crystalline, the limestone still retains considerable regularity of stratifica¬ 
tion, dipping towards the west-northwest at an angle of twenty degrees, being well exposed 
in a quarry (the property of William Edsall), where it is used for making pure lime. It is 
The Potsdam sandstone of the Geological Reports of New-York. 
