12. pexn township. F 2 . 295 



30° E., we trace it for many miles. It reaches the Juniata 

 1^ miles above the head of Duncan's island where it can be 

 seen well exposed on the turnpike. From the Juniata it is 

 traceable to the Susquehanna where it appears immediately 

 below Montgomery's Ferry. It cuts the Half Falls mount- 

 ain a mile from its northeast extremity. 



"Where the trap-rock comes in contact with red shale 

 the shale is indurated and altered to a dark brown and pur- 

 plish color. 



k 'The surface of the dyke is from 60 to 120 feet wide, 

 though the actual thickness of the intrusive wall of rock 

 beneath the surface is probably much less.'' 



"From the Blue mountain to the point where it reaches 

 the Susquehanna its length is 15 miles, but it crosses the 

 river and passes into Lykens valley." 



In Cumberland county there is in fact but one dyke 

 crossing the Great valley ; and this dyke is so conspicuous 

 a landmark that it has been adopted as the boundary line 

 between four townships : Silver Spring and Monroe on the 

 eastern, and Middlesex and South Middleton on the western 

 side of it. 



But in Perry county the crust of the globe was cracked 

 not merely along one line, as the old map represents it, but 

 along several lines, each of which produced a separate trap- 

 dyke. 



On the township maps of Penn (Plate XXX) and of Rye 



(Plate XXXIII; I have drawn the lines of four trap-dykes 



-ing the Cove, and its bounding mountains, one of which 



is much longer than the other three, and extends northward, 



through Wheatfield, bevond the head of Duncan's island. 



Plate XLV. 



There is another dyke in Watts (see Plate XLIY » which 

 is almost in exact line with the course of the long dyke in 

 Penn, before it makes its bend, a mile north of Duncannon, 

 and this is what deceived Dr. Henderson into supposing 

 that there was but one long dyke, the whole length of 30 

 miles. 



I shall describe the four dykes in Perry county under 

 four special names : 



