294 F\ REPORT OF PROGRESS. E. W. CLAYPOLE. 



of meat depth, for they can be traced along the present sur- 

 face of the earth for a great distance. The trap-dyke de- 

 scribed by Dr. Prazer, in his report on Lancaster f county, 

 runs in a nearly straight line (N. E.) forty miles. Many 

 others exist in Adams, York, Lancaster, Dauphin, Lebanon, 

 Berks, Chester, Delaware, Montgomery, and Bucks coun- 

 ties, and in middle and northern New Jersey, southern New 

 York, and New England. 



The most remarkable of them all starts in the South Mount- 

 ains, and runs in a nearly straight line across Cumberland 

 county (between Mechanicsburg and Carlisle) to the Perry 

 county line, on the crest of the Blue mountain, two miles 

 east of Sterritt's gap.* 



The description of the dyke, on page 366, of Vol. I of 

 this "Geology of Pennsylvania," is erroneous in several 

 important particulars. It reads as follows : — 



"This dyke crosses Cumberland valley and cuts through 

 the Blue mountains 2 miles east of Sterrett's gap. It 

 crosses Fishing Creek valley and the Cove mountain bear- 

 ing N. 10° E., where it forms a low ridge separating the 

 waters of Fishing creek from those of Shermans creek. 



"The dyke next crosses the Cove where its loose frag- 

 ments are strewed over the surface to a great extent. The 

 point where the dyke cuts the northern ridge of Cove 

 mountain is not visible but we find the mass J mile north 

 of Petersburg, (Duncannon,) where, with a bearing of N. 



* Dr. A.. A. Henderson, assistant geologist on the first survey, in 1849, sup- 

 jm)sc(1 that the dyke here entering his district extended to the Juniata and be- 

 yond the Susquehanna into Dauphin county, 30 miles from its southern end 

 at the Boiling Spring, on the Yellow Breeches creek, in Cumberland county. 

 This is evidently not the real southern end of this dyke. It undoubtedly 

 continues southward through the South Mountain rocks into York county, 

 and joins the trap-dykes north of Petersburg, for Mr. A. E. Lehman, assistant 

 geologist on the second survey, has found traces of it in the wooded parts of 

 the mountain. 



It is I >ut justice to Dr. Henderson, who was one of the best geologists of his 

 day, and to whom we owe our first accurate knowledge of the complicated 

 structure of Perry, Juniata and Mifflin counties, to say that he could only 

 with great difficulty at that early day trace the line of the dyke upon his map 5 

 and the line which ho laid down on his map was transferred to the Geological 

 Map of Pennsylvania, made by me in 1842, and published by Professor 

 Rogers in the Atlas to his Final Report, in 1858. [J. P. L.] 



