82 GEOLOGY OF THE SECOND DISTRICT. 



CHAPTER IV. 



SUBORDINATE ROCKS. 



1. Trap. Greenstone Trap. 



General characteis 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 arc sometimes called trap, 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 wliich have been filled with various 

 crystalline minerals, as lime, prehnite, stilbite, analcime, etc. 



The trap of tiie 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 pyr-oxene 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 am.ygdaloid, 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. 



