296 F\ REPORT OF PROGRES8. B. W. CLAYPOLE. 



(1.) The Great Horseshoe dyke, Ironstone ridge, or Cum- 

 berland county dyke. 



(2.) The Little Horseshoe dyke, next, east of, near and 

 parallel to the great dyke. 



(3.) The West Duncannon dyke; and 



(4.) The East Duncannon dyke, next, east of, near and 

 parallel to it. 



The two Horseshoe dykes run south across Rye township, 

 and will be also described in the geology of that township. 

 The West Duncannon dyke will also be noticed in the 

 geology of Wheatfield and Watts towmships ; but the East 

 Duncannon dyke is confined to Penn township. 



(1.) The Great Horseshoe dyke, Ironstone ridge, or Cum- 

 berland Valley dyke. 



The line of this dyke may be detected by loose fragments 

 on the south side of Peter's mountain about f/wo and a 

 west of the river and between the highest terrace and the 

 summit of the mountain. 



Its course is plainly indicated down the slope by the same 

 evidence from terrace to terrace, with a bearing of S. 10° W. 

 into the Cove at the foot of the Horseshoe and almost to 

 the creek. 



Along this part of its course it appears to be the widest 

 of all the dykes in the Cove, but just before it reaches the 

 creek it suddenly and markedly increases and assumes com- 

 paratively gigantic proportions, admirably displaying both 

 the trap and the accompanying rocks altered by contact 

 with it. 



The sandy beds of the Mauch Chunk Red shales (XI,) 

 which are here cut through are changed to a dark brown 

 and chocolate colored material; the red shales themselves 

 are in some places burnt into a mass resembling half made 

 brick, but cot usually much hardened. Some tine shale 

 beds, however, have been SO much changed that they are 

 almost as tough and hard as the trap itself. 



This change in the appearance of the rocks at this point 

 has led to considerable excavation in the belief that the 



