22 NEW YORK STATE MUSEUM 



Trap rocks 



In addition to the sedimentary bed described above, another 

 and very different type of rock is found within this area. It is 

 usually a dark green or gray color when fresh, but a yellowish, 

 rusty brown on weathered surfaces. When examined with a hand 

 lens, it is often seen to be composed of small, interlocking crystals, 

 some of which have sharply denned and regular boundaries. Jts 

 texture is quite different from that of the sandstone, which is 

 clearly made up of rounded particles held together by some 

 cementing matrix. Moreover, this rock does not occur in thin, 

 regular beds as do the sandstones and shales, but is massive, gen- 

 erally without regular partings, though often broken into irreg- 

 ularly shaped fragments by innumerable cracks. It is. commonly 

 called trap rock, more specifically diabase. By its composition 

 and structure, it is known to have once been in a molten condi- 

 tion. While in this condition, it was forced up from the highly 

 heated interior of the earth into the overlying sedimentary beds, 

 where it cooled and solidified. The trap occurs in several areas, 

 the largest being that of the Palisades. 



The Palisades. The Palisades ridge begins at Bergen Point 

 (N. J.), and extends northward along the Hudson river to Haver- 

 straw, where it curves westward away from the river and disap- 

 pears beneath glacial deposits near the western border of the 

 Newark formation. The same rock is found on Staten Island, 

 but does not form a marked ridge. From Bergen Point to its 

 disappearance a few miles west of Haversftraw its length is over 

 46 miles. There is much reason for believing that this rock 

 extends many miles southwestward from Bergen Point, but abso- 

 lute proof of this is not at hand. 



Width. At the state line its outcrop is a mile in width, but 

 decreases at Sparkill to only f of a mile. Thence northward it 

 increases, attaining a breadth of 1J mile's at Bight, whereas 

 at Nyack its width has again decreased to f of a mile. 

 West of Upper Nyack it attains its maximum width, 2 miles, from 

 which it decreases to less than 40 rods, north of Bockland Lake. 



