Lewis.] 4:Ob [May 15, 18S5. 



cracking on the Other, and letting the water ooze through the narrow 

 crack. The line of fault itself was subject to such pressure as to prevent 

 the entrance of the trap, except perhaps on the New Jersey side of the 

 river, where, as we have shown, the trap seems to have burst through 

 from below. 



The Jurassic age of the dyke and fault is indicated by the fact that while 

 cutting through the uppermost Triassic strata, similar dykes and faults in 

 New Jersey are uncouformably covered by strata of lower Cretaceous age. 

 It follows that the "jogs " in the trap dyke, and also probably in tbe line 

 of serpentine outcrops in Chester county, may be faults of so recent an age 

 as the Jurassic. The relation between the dyke and the faults of the 

 same region is an interesting one, and may hereafter throw some light on 

 the complicated geology of Southeastern Pennsylvania.* 



* Perhaps it is well here to notice briefly an article by Dr. P. Frazer relating to 

 the dyke just described. An abstract of the present paper, omitting details, 

 was given verbally at the meeting of the American Association for llie Advance- 

 ment of .Science, in September last, yiuce then, without waiting for the publi- 

 cation of the paper. Dr. P. Frazer, who was entrusted with the survey of 

 Chester county and who hud omitted to notice the trap dyke, soon afterwards 

 printed a reply entitled, ''Trap dykes in tne Archaean rocks of S. E. Peunsyl 

 vania" (Proc. Amer. Philos. Soc. Oct. 17, 18SJ), in which he endeavors to show 

 firstly, that there is no continuous dyke across the region which he has studied, 

 and secondly, tliat !/ such a dyke exists it marks a fault which he had estab- 

 lished long ago. He concludes by denying that the trap is of uniform compo- 

 sition. 



The title of Dr. Frazer's paper is unfortunate, since but an extremely small 

 proportion of the metamorphic area traversed by the dyke belongs to the 

 Archsean rocks, properly so called. He is also unfortunate in giving the 

 excuse fur the premature publication of his paper that •' error is notoriously 

 fleet of foot, and with a year's start may dely pursuit," before establishing that 

 any error existed. Had Dr. Frazer been aware of the details now first made 

 known, he would hardly have ventured to deny the essential continuity of the 

 dyke in Chester county. Nor is lie accurate in supposing that the dyke marks 

 the fault which he assumed to exist in that county. His supposed fault, not yet 

 demonstrated to be a fact, passed along the north side of the Wouth Valley hiil, 

 while the dyke is south of that area. His final statement that " there were 

 strongly marked difl'erences of texture, structure and constitution between 

 many of the outcrops south of the Chester valley and near Conshohocken " is 

 unsupported by any lacts brought forward by himself, and is directly opposed 

 to the microscopical examinations made by the present writer, which show a 

 remarliabie uniformity in each of these particulars. Dr. Frazer has evidently 

 mistaken other eruptive rocks for the diabase of which the long dyke is com- 

 posed. 



