IN NEW-YORK, MASSACHUSETTS AND VERMONT. 103 



not intruded by transverse rents in llic nuiniur 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 Highlands, 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. 15. 



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 chloritic 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 or 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 inlerstratified, 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 becoffie more exposed to abrading influences, especially 



