12. PENN TOWNSHIP. F a . 299 



lowed by the color of the soil and the loose blocks lying 

 about across the fields to the main road up the cove, which 

 road it crosses just east of a farm-house. 



Running on thence with the same bearing, it may be seen 

 alongside of the road (which here turns to the south-south- 

 west) for about 200 yards, where crosses a lane running off 

 at the next angle in the road. Here its presence is marked 

 by the usual red clay and bowlders. Beyond this point I 

 have not traced it. 



This set of parallel dykes in the cove constitutes a dis- 

 tinct and interesting feature in its geology. Ranging across 

 it from mountain to mountain, like four walls, they divide 

 it into five separate portions, and were they all as heavy as 

 the last mentioned, would form serious barriers to commu- 

 nication, and difficulties in the way of the farmer. As they 

 are it is often a task of great labor to dig out and carry 

 away the fragments from the fields and pile them up at the 

 roadsides where their subangular form and rusty color 

 make them conspicuous objects to the passer by. 



They all consist of the same tough, hard dolerite, show- 

 ing some but very little variation in composition and fine- 

 ness at different places. 



The most remarkable thing about these dykes is this : — 

 Not one of them has ever been detected at the top of either 

 of the two mountains ; the East Duncannon and Great 

 Horseshoe dykes alone rising above the highest terrace, so 

 far as known. The West Duncannon dyke does not appear 

 to rise into the mountain at all, its exposure ceasing sharply 

 at the foot. It is not however impossible that further ex- 

 amination may modify this assertion which is based on neg- 

 ative evidence only. 



The trap-dykes north of the Cove. 



The West Duncannon dyke is the only one of the Cove 

 dykes which exists to my knowledge north of Peters' 

 mountain and west of the Juniata river. 1 can most easily 

 describe it by beginning at its northern end. 



Its first appearance is on the turnpike road about one 



