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GEOLOGY OF THE SECOND DISTRICT. 
CHAPTER IV. 
SUBORDINATE ROCKS. 
1. Trap. Greenstone Trap. 
General characters of these rocks; composition, structure, origin, etc. — Trap dykes and 
mineral veins compared. 
The term trap has long been used in geology, and has at different times been applied to 
rocks apparently of different characters and origin. All those rocks in which hornblende 
forms a constituent part, have been called by the general name of trap. The term, as now 
employed, is more restricted, being confined to rocks which are without doubt igneous, as the 
stony veins called dykes, and the columnar and amorphous masses resembling basalt; for an 
example of which, I will refer the reader to the rocks called the Pallisades, on the west bank 
of the Hudson river, a few miles above the city of New-York. Other rocks of a similar 
origin are sometimes called tray, in a general mode of speaking, as basalt and amygdaloid : 
the former being a black, fine-grained, or compact homogeneous rock ; the latter, a mass 
originally full of cavities, like those of lava, most of which have been filled with various 
crystalline minerals, as lime, prehnite, stilbite, analcime, etc. 
The trap of the northern counties of New-York exists only in the form of stony veins, 
which traverses all the other rocks in a direction varying but a few degrees from an east and 
west course ; the walls of which are generally distinct and parallel, and may be traced for 
great distances as readily as a road or a highway. 
Trap is a compound mass, in which we may usually discern various minerals by the 
unassisted eye, as hornblende and feldspar in close and intimate admixture : these form the 
base or ground, in which crystals of hornblende and pyroxene are frequently disseminated. 
Sometimes the dykes are columnar ; that is, there is a division of the mass into short irregu¬ 
lar columns, with from four to six imperfect sides disposed at right angles to the strike of the 
vein, and never in the direction of its length ; and also in masses resembling amygdaloid, the 
cavities of which are sometimes empty, and in others filled with calcareous spar. It is again 
very compact, like basalt, slightly crystalline, with short needle-form crystals disseminated 
through the mass. 
