IN NEW-YORK, MASSACHUSETTS AND VERMONT. 
103 
not intruded by transverse rents in the manner of injected or plutonic rocks, but are simply 
separated in the direction of their strike, or nearly so, as it would seem by an uplifting 
among them of inferior rocks in the form of parallel ridges. By this means the taconic 
slate is carried more westerly than its general strike at the north ; and it is by this westerly 
thrust that it crosses the river near Poughkeepsie, ranging southward so as to underlie the 
belt of country to the west of Newburgh for six or eight miles ; while the lower or easterly 
members of the same system pass to the east of the primary chain of the Plighlands, and do 
not appear upon the banks of the Hudson till that chain is passed. A good example of an 
arrangement of this kind is furnished at the Rocky Glen Factory in Fishkill, upon the 
creek of the same name. The separation of the adjacent masses is effected at this place 
by a low ridge of granite, which comes up in the form of a slender spur from the High¬ 
lands four or five miles south, where of course it is wider, while at the Glen it has become 
attenuated, and, in the course of a mile or two, disappears beneath the taconic rocks. 
The annexed section explains the arrangement. 
Fig - . 1 5. 
a. Slate, b. Granite, c. Limestone, e. Fishkill mountain. 
In this section, the granite is pushed upward so as to intervene between two rocks (which 
in other localities are in contact), and runs an unknown distance in this relation. The 
width of the granitic ridge is about one hundred yards. It contains a large portion of 
greenish or cliloritic matter: the felspar is flesh-colored, but, as a whole, it has quite a 
resemblance to trap. If now we substitute in imagination a ridge of gneiss or mica slate 
for the granite, we should be very likely to consider it a case of interstratification o. inter¬ 
lamination of rocks of the same age; and were this to occur in the eastern border of the 
system adjacent to the Hoosic mountain, few would doubt that, in truth and reality, slate, 
gneiss, and limestone were interstratified, and therefore belonged to one and the same 
system; and should we substitute quartz for the gneiss, a still stronger case would be pre¬ 
sented, and we could then hardly doubt the truth of the supposition which has been 
advanced. We may believe that these very arrangements do occur, and that mica schist 
actually protrudes upward among the newer slates, appearing like a member of the forma¬ 
tion ; but it is easy to see that, after all, such a conclusion may be false. Not only may 
an inferior rock be forced between two adjacent though different ones, but it may come up 
also between the strata, and thereby separate portions of the same rock widely from each 
other. This being admitted, it leads us still forward to more complicated cases; for by 
changes of this nature, the masses become more exposed to abrading influences, especially 
