ISO G. F. Wright — Continuity of the Glacial Period. 



valley the ice extended to Topton, which is the water shed be- 

 tween the two above-mentioned rivers; thus bringing- to light 

 an important and hitherto unsuspected temporary line of Gla- 

 cial drainage into the Schuylkill River.* 



One significant observation of Professor Williams has a spe- 

 cially important bearing upon the date of this earlier glaciation. 

 Just south of Bethlehem, at the site of the new reservoir, on 

 the north flank of South Mountain, the gneiss is very deepty 

 decomposed, showing an extremely long exposure to disinte- 

 grating agencies. But upon the summit of the mountain, 

 where there are evidences of glaciation, the decomposition of 

 the gneiss extends only to a few inches below the surface. 

 Professor Williams' interpretation of this would unquestionably 

 seem to be correct, namely, that the twenty feet of decomposed 

 gneiss upon the north side represents preglacial work, where 

 the material had, in some way, been protected from erosion 

 during the ice-movement, while the few inches of decomposi- 

 tion upon the summit of the mountain represents the work 

 since the deposition of the fringe ; material loosened by pre- 

 glacial decomposition having been removed by the ice from the 

 summit. 



The thinness and irregularity of the deposits over the area 

 which we have denominated "the fringe" in New Jersey and 

 Eastern Pennsylvania are supposed by Professor Salisburyf to 

 be due to the action of the long period of erosion to which it has 

 been subjected. This seems extremely questionable. At any 

 rate, there does not appear to be any definite evidence that this 

 is the true explanation. On the contrary, it is more likely that 

 the original deposition near the margin and for some distance 

 back was slight and irregular, owing to the comparatively brief 

 stay of the ice near the margin of maximum extension and to 

 the small amount of motion which characterized the extreme 

 border.;}: This view is confirmed, also, by the fact that Mr. 

 Leverett in his careful study of the deposits in Southern Ohio 

 is of the opinion that there the attenuation of the border de- 

 posits is due " more largely to original deposition, than to sub- 

 sequent erosion."§ 



With regard to the age of the Philadelphia brick clays and 

 red gravel in the Delaware Yalley, the evidence seems to point 

 more and more clearly to a closer continuity with the Trenton 

 gravels in their deposition than Professor Chamberlin supposes. 



* Williams, Proc. of the Geol. Soc. Am., vol. v, pp 13-15. See further con- 

 vincing facts in the short paper of Professor Williams in this Journal for Jan., 

 1894, pp. 33-36. 



f Annual Report of New Jersey, for 1892, p. 64. 



i See diagram. Fig. 7, and explanation by Mr. Warren Upham on page 185. 



§ See Glacial Succession in Ohio, Journ. of Geol , vol. i, p. 132. 



