1885.] 4:dy [Lewis. 



slate ridge, goes through the village, and passing the river, in the bed of 

 "which it may be seen, it follows the summit of Bethel hill into Delaware 

 county, terminating near the road leading from the Lancaster turnpike to 

 the King of Prussia village. This is by far the longest and widest trap 

 dyke of the valley or its borders, its length being a little more than six 

 miles.''* 



In the map published by the Second Geological Survey, under the direc- 

 tion of Prof. J. P. Lesley, Mr. C. E. Hall has connected the two dykes 

 on Prof: Rogers' map to form a single dyke extending from the east of 

 riourtown in a west, south-westerly direction to the borders of Dela- 

 ware county. In his report (C^), Mr. Hall refers to the dyke briefly in 

 several places. He says (p. 19), that "it extends in a nearly straight line 

 from the county line south-west of Mechanicsville, a short distance north 

 of Gulf creek, to Flourtown in Springfield township. East of Flour- 

 town it has not been traced continuously. There are exposures of loose 

 blocks of trap, however, in several localities extending in a north-east 

 direction from the last exposures of the main belt at Flourtown." 



He gives, on p. 23, a map showing some isolated exposures of trap boul- 

 ders in Upper Dublin township, which indicate the probable north-west 

 continuation of the dyke. 



He makes some apparently irreconcilable statements as to whether or 

 not it occupies a line of fault or disturbance. Thus, while saying (p. 20), 

 "The dyke does not seem to mark any line of disturbance. It may occupy 

 a fissure crack, along which no lateral movement has taken place ;" he 

 says on p. 75 : " This dyke, evidently located along the line of some dis- 

 turbance, is undoubtedly the course of a fault or fracture. There is, how- 

 ever, no positive proof of any lateral movement." 



Mr. Hall represents the dyke on the colored map accompanying his re- 

 port as having a length of a fraction over eight miles. 



In the present paper, which is the result of the personal observations of 

 the writer, made during the past two years, it will be shown, that the dyke 

 heretofore described is only a small portion of a long narrow dyke which 

 passes almost entirely across the south-eastern corner of this State, from 

 near Doylestown to Maryland, and which, taken together with some par- 

 allel dykes of similar nature and composition, north-east of Doylestown, 

 forms a series of nearly continuous dykes some ninety miles in length. 



It will be shown that a great fissure was made at the close of the Trias- 

 sic period across the State, which fissure, filled by eruptive diabase, trav- 

 erses strata of Laurentian, Cambrian, Silurian and Triassic age in an 

 almost unbroken line, thus becoming an important feature in the geology 

 of the State. 



We shall follow it continuously from a fine exposure south of Doyles- 

 town along its south-west course through Bucks, Montgomery, Delaware 

 and Chester counties, to the Maryland border, and shall demonstrate by a 

 detailed list of its outcrops, that, although frequently represented only by 



* Geol. of Penna., Rogers, Vol. 1, p. 214, 1858. 



