1877.1 ^^^ [Krazer. 



to about lialf the breadth which it exhibits on tlie Delaware. It is wortliy 

 of note that this contraction of tlie area of the New Red has been brought 

 about by a sloping inwards towards the median line of both its north and 

 south boundaries, so that if the l)readth of the Mesozoic rocks on the Dela- 

 ware were continued to tlie Maryland line, this ore region would fall but 

 little below the axis of the belt. Furthermore, this position would accord 

 well with that of the copper ore deposit in Bonnaughtown, about eight 

 kilometers (or five miles) east of Gettysburg. 



The same argument may be applied to the iron ores of the Altlaud 

 mines, whicli are cut in specular, somewhat magnetic and very cuprifer- 

 ous ore, for this localitj^ lies 11 kilometers (or 6f miles) S. E. of the N. W. 

 limit of the New Red. 



If the same view be held to explain the Cornwall deposit and the Alt- 

 land deposit the latter must be of ante-Potsdam age. This, however, is 

 not at all yet sustained b}- observation. 



Again, if the source of all this magnetic-micaceous ore be assumed to be 

 the older slates, then it seems to indicate that the greater part of the sand 

 obtained by wearing the shores of the Mesozoic sea was obtained from tlie 

 Huronian rocks, for over every kilometer (| mile) of the breadth of the de- 

 posit in York and Adams will be found some flaky "iron-glimmer, " besides 

 many belts of shale and sandstone, colored green with the debris of the Hu- 

 ronian chlorites and sparkling with the hydro-micas of the same age. 



Now supposing that this were true, i. e. that either the greater part of 

 the Mesozoic sandstone rocks, or at least a large proportion of them, taken 

 in any part of the belt, consisted of the debris of the Huronian schists 

 worked over, it might prove a connection between these ore deposits, but 

 a few miles from the South Mountain, and those of the latter: independently 

 of whether the shallow but monoclinal structure of these beds (as explained 

 by the wave strewing hypothesis of H. D. Rogers ; or the deepening trough- 

 bottom hypothesis of J. D. Dana),* or the normal deposition, folding and 

 subsequent erosion shall be assumed, for even in this latter case a margin 

 of a certain Avidtli along the coast line, Avhere the water was shallow, 

 would show in the deposits the characters of the original rock forming 

 its shelving bottom. 



So that in any case we should look for the Huronian source under the 

 present position of the ore, because the waves whicli broke up the sliallow 

 bottom would strew the debris in the immediate vicinity of the parent rock. 



[In Rogers' hypothesis the direction of the apparent motion of the 

 wave is the really important factor, and it is difficult to understand why 

 we should not have S. E. dips on the S. E. margin of the Mesozoic es- 

 tuary, since the waves are supposed to produce layej^ dipping in shore. 

 (See diagram. Vol. II, Part II, p. 812, Final Report on tlie Geology 

 of Pennsylvania.)! Or vice versa if the suspension hypothesis (ibid, dia- 

 gram p. 813) be preferred.] 



But, abandoning this wave-strewing hypothesis as altogether inadequate 

 * Manu\] of Geology, 187.5, p. 421. t.l8o8. 

 PROC. AMER. PHILOS. SOC. XVI. 99. 4d 



