264 GEOLOGY OF THE SECOND DISTRICT. 



The lithological characters of trap are, 



1. Compact greenish masses with a slight granular texture. 



2. Greenish masses, fine grained, but with needlcform crystals. 



3. Trap, greenish grey, with nodules of the same material, which often separate rcadUy from the mass ; it is the first step 



towards a columnar structure. 



4. Greenish trap, divided into horizontal columns, but rarely regular in the number of sides; they are triangular gene- 



rally, and very rarely pentangular, or similar to cohuims of basalt. 



The direction of the dykes is a point worthy of some attention, and there is quite a wide 

 range in this respect. I notice a few, in order to acquaint the reader with their general 

 range, omitting the locahties. They are selected from all parts of the county, and pursue 



the following courses : 



4, N. 60° E. 



2, N. 70 E. 



2, N. 50 E. 



2, N. 40 E. 



1, N. 75 E. 



2, N. 20 E. 



I have never observed a dyke which coincided in its direction with the strata, nor one whose 

 direction was due north and south. The nearest to the latter direction is that of N. 20° E., 

 which, liowever, it is rare to meet with ; and the introduction of the single instance in the 

 preceding column alters the average range, and reduces it to N. 50° E., while I am more dis- 

 posed to receive the average direction as N. 64° E. 



One of the localities at which dykes are quite common, is Tremblcau point, near Port 

 Kent. This point is the termination of the great range of mountains from the southwest, and 

 the hypersthene rock is exposed upon the shore of Lake Champlain. In the distance of half 

 or three-fourths of a mile, sixteen dykes occur, the average direction of which is about N. 

 60° E. Some of them are twenty feet wide, but frequently terminate in a number of subor- 

 dinate branches. 



Another point where dykes are numerous, is Port Henry, where several may be observed 

 in the small stream which comes in from the west, and flows over the ledge of limestone 

 above the furnace. 



The largest and most important of the volcanic rocks belonging to the class under conside- 

 ration, are found in the interior of Essex, near the sources of the Hudson river and Ausable. 

 One of these I have described already, namely, the great trap dyke at Avalanche lake, which, 

 in consequence of being the channel of a small stream, has been broken up to the depth of 

 a hundred feet, and the process is still going on. 



In this region, in almost every direction, may be observed notches in the ridges, which are 

 most of them produced by dykes, the materials of which have been removed, as at the lake 

 just named. 



The general phenomena of dykes are those which accompany mineral veins ; hence it is 

 important to ol)servc them : they are in fact, true veins filled with stony matter, and observe 

 the same laws in regard to direction, shifts, faults, etc. as other veins. 



