160 



hundred feet. Three phenomena connected with this crag are wor- 

 thy of observation. 1. It has the character of a trap vein which has rup- 

 tured the strata in a fluid state. The dip of the sedimentary rocks on 

 its west side is to west-south-west, on the east side to the east, and 

 in both cases at a pretty high angle. We ha^e liere, therefore, evi- 

 dence of the strata having been ruptured, and a fissure formed ; and 

 we have the trap^ the rupturing agent, placed on the fissure, and 

 occupying the anticlinal line, or axis of the upheaving movement. 

 According to the received hypothesis respecting the origin of trap, 

 we might expect this appearance to be common, and yet it is rather 

 rare. It forms the exception, while it ought apparently to be the 

 rule. 2. The trap has mingled tcith the shale, and affected its struc- 

 ture in a very remarkable manner. It has enveloped portions of the 

 shale withiu its mass, and penetrated laterally among the beds to 

 the extent of one hundred feet or more on the east side. On the 

 south flank of the highest part of the crag we have a section exhi- 

 biting the relations of the two rocks. The trap has the form of an 

 arch, of perhaps twenty feet radius, with a long straight tabular 

 mass about three feet farther to the east. A mass of shale lies un- 

 der the arch exhibiting a laminar structure ; in the portions conti- 

 gnous to the trap, its substance is hardened, and the planes of the 

 laminse, instead of being straight and parallel, are arranged concen- 

 trically with the curved surface of the trap which covers them. 

 Portions of shale also lie under and abr)ve the thin tabular project- 

 ing mass of trap, and these likewise conform themselves to its sur- 

 face. The inference is, that, independently of the original slaty 

 form of the shale, a secondary laminar structure was induced upon 

 it, by the heat of the fused trap being propagated through it pro- 

 gressively and unequally, each portion as it was dried and harden- 

 ed, separating from that beyond it. 3. The crag exhibits a striking 

 example of the effects of denudation. The north end of the ridge, 

 which is low, and on a level with the surface of the land westward, 

 has merely had its covering of shale stripped off, and exhibits a few 

 trifling indentations ; but the south end, which rises in bold relief 

 above the adjacent land, has been cut into seven distinct hillocks, 

 separated by transverse ravines, which are steep in the sides, and 

 from ten to sixty feet in depth. The appearance of these ravines 

 shews, that they must have been cut by powerful currents flowing 

 from the west. 



