Lewis.] '±'±_( [May 15, 



boulders. It appears near the marble quarry, and trends across "Wbite Marsh 

 township in a straight line in a west south-west direction toward Spring 

 Mill, being recognized by its boulders at each road crossed. 



These boulders, where not washed by streams, are decomposed on the 

 surface, and are continually becoming smaller as time goes on. On high 

 ground, where the soil is much decomposed, the dyke is only traceable 

 with difficulty. On the hill east of Spring Mill, half a mile from the 

 Schuykill river, where numerous trap boulders, covered by a crust of 

 soft brown oxidized material, occur on the roadside, large numbers of 

 them were observed to be covered with peculiar markings or striae. The 

 soft crust was marked by grooves each nearly a quarter of an inch ia 

 depth, and running from the angular edges of the boulders in somewhat 

 parallel directions, one set of scratches often crossing another. Two rude 

 sketches of these curious markings are here given, having been taken by 

 the author in the Spring of 1879 : 



These markings, which are common on fine grained ferruginous trap, and 

 may be a foot or more in length, have been mistaken for glacial striae by 

 several writers. Thus one writer,* after describing the scoring and 

 scratching of the boulders in Warrington township, Bucks county, says 

 that they "evidently mark the progress of a glacier, and cannot other- 

 wise be accounted for." Similar mistakes have been made in Chester 

 county. t Even such a well known geologist as Rev. H. W. Crosskey, 

 LL.D., F.G.S.,:}: of Birmingham, has described and figured markings of 

 this character in decomposed trap boulders in Worcestershire, England, as 

 true glacial striae. The excellent plates and the detailed description he 

 has published, show the markings to be identical with those on the trap 

 now under description, which are quite diflerent from true glacial striae. 

 In fact, they are, in all probability, plough marks. They are precisely such 

 as would be formed by the sharp point of a plough passing over the de- 

 composed surface of a trap boulder. As would be expected, they gen- 

 erally occur only on one side of any boulder, the side uppermost, when 

 ploughing was going on. As shown by the author, in his report on the 

 Terminal Moraine^, and more particularly in a recent paper, |1 they are 

 far south of the limits of glaciation. 



* History of Bucks County. Davis, p. 438. 



t History of Chester County. Smith and Futhey, p. 186. 



J The Grooved Blocks and Boulder clays of Rowley Hill. Proc. Birm. Pbilos. 

 Soc, Vol. iii, 1882, p. -159. 



§ Report Second Geolog. Survey of Pennsylvania, Z. 



II On Supposed Glaciation in Pennsylvania, south of the Terminal iloraine. 

 Lewis. Amer. Journ. Science, Arts, xxviil, 1881, p. 2",6. 



