TRAP. 83 



In general, trap is a dark green when freshly broken ; but when it has been exposed to the 

 weather, it is grey or yellowish brown grey. It is a rock easily affected by atmospheric agents, 

 by which its color is not only changed, but it is broken up into angular fragments. It is 

 tough, and breaks with an uneven splintery fracture, except in a few instances wiicre the mass 

 is vitrified, when it is brittle, and breaks with a conclioidal fracture. 



Such are the general characters of trap, as it occurs in the Second District. I have ob- 

 served it, however, under very different circumstances. Thus, some of the dykes which 

 traverse the Trenton limestone at Montreal are white, with beautiful crystals of white tremo- 

 lite and sulphuret of iron disseminated through them ; which, in hand specimens, without 

 knowing the connection and relations of the mineral, would be mistaken for some other 

 substance. 



Origin and Composition of Trap. 



It is, without question, of igneous origin, and is composed of hornblende, pyroxene and 

 feldspar ; in some instances, it is hornblende with feldspar ; in others, pyroxene takes the 

 place of hornblende. The white dykes of trap spoken of in the preceding paragi'aph, are 

 composed of feldspar, with very little if any iron, except what is combined with sulphur, 

 forming crystals of sulphuret of iron disseminated through the dyke. 



It is scarcely necessary to go into a statement of the arguments for the origin of this kind 

 of rock. The fact that it bears the marks of fusion in various degrees ; the alteration or 

 changes which it produces on other rocks, or the walls of the rock which the dykes traverse, 

 and the difficulty attendant on any other theory, seems to place the question on satisfactory 

 grounds. 



Trap dykes cornjmt'ed with mineral veins. 



The resemblance between dykes and mineral veins, or those filled with metallic substances, 

 is very close. They resemble each other in pursuing ordinarily a direct course, and in having 

 walls of the rock more or less distinct and well defined. There are slight differences worthy 

 of notice : thus, a mineral vein in general is less regular in the direction, width and perfection 

 of its walls. It often bulges out, sometimes to a great width, and then diminishes, or is 

 pinched in ; sometimes so much so, as to bring the walls nearly in contact. The greatest 

 difference, however, consists in the diversity of their contents, which is in part expressed in 

 the terms or names given to masses themselves ; and it is highly probable that the different 

 phenomena exhibited in the two cases, are to be aitributed to this diversity. Those who are 

 conversant with mineral veins of any description, must have observed the great quantity of 

 crystallizable materials, as calcareous spar, fluate of lime, forming what is called the matrix 

 of the vein. Now is it not probable that veins filled with materials of this character may be 

 less compacted together, more liable to have vacant spaces, and points which would support 

 the adjacent wall less securely, than the thick stony and less crystallizable matter which 

 compose trap dykes ? Hence the liability of the walls of the vein to be less securely sup- 



