1880.] NATURAL SCIENCES OF PHILADELPHIA. 287 



at the edges of the valle}', and has therefore been mistaken for it. 

 The discovery of lignite below it proves its re-stratification in a 

 later age. In manj- places shafts have been sunk over 100 feet 

 without coming to the limestone. The ore, originally derived 

 either from the limestone or from the primal slates, appears to lie 

 below the lignitic strata, 



4. Drift Ore. Resting often unconformabl}^ upon these last, and 

 capping the elevations thi-oughout the valley, is a drift deposit of 

 gravel and boulders containing a workable iron ore. The compo- 

 sition of this drift is most interesting. Its boulders, almost with- 

 out exception, are composed of a loose-grained Potsdam sand- 

 stone, — a formation not now existing either on the North or South 

 Valley Hill at this place, and found only in a limited exposure at 

 the eastern end of the valley. The ScoUthiis linearis is frequently 

 found in these boulders. Moreover, notwithstanding the large 

 extent of Triassic red shale and sandstone immediatel}' to the 

 north of the valle}', and the occurrence of that rock resting often 

 directly upon the limestone, not a trace nor a fragment of Triassic 

 rocks have been found in this drift. 



The evidence is here strong that this drift has not been caused 

 by any flood from the north in a modern age. Additional evidence 

 bearing upon the same point is found in the fact that the Triassic 

 region north of here is absolutely free from drift of any kind. A 

 careful study by the writer of much of that region has shown that 

 not a single drifted pebble is there found. The soil is formed 

 from the rocks below it, and such elaj^s as occur are bog clays of 

 local origin and recent age. That the pebbles of the valley drift 

 have not been derived by weathering from the neighboring lower 

 Triassic conglomerate, which holds often large pebbles, is shown 

 by the fact that such pebbles are here formed entirely of gneiss 

 or gneissic quartzite, and never of Potsdam, and therefore are 

 quite different from those in the valle3^ 



The drift ore and gravel does not lie in hollows, as though 

 locally washed, but is found in patches upon the elevated portions 

 of the valley, as though it were the remnant of a once continuous 

 deposit. 



The facts above enumerated suggest a possible origin at an age 

 when cliffs of Potsdam sandstone, since eroded awaj-, stood as a 

 high barrier between the limestone valley and the Triassic rocks 

 north of it. Such a barrier would effectually prevent Triassic 

 fragments from mixing with the drift of the valle}', and would, 



