118 PROCEEDINGS OF THE ACADEMY OF [March, 



side, the Paleozoic strata bearing the ore/^ and, since diamond 

 drilling has shown that ore extends well under this cover, the Middle 

 Hill workings are now being extended in that direction. Spencers- 

 discussed the probability of there having been progressive overlap 

 here, and published several cross-sections exhibiting this relation. 



The last exposure of this contact found by the writer is twenty 

 miles west of Cornwall, on the west bank of the Susquehanna River, 

 one and a half miles below the town of New Cumberland. Here, 

 in the Northern Central Railroad cut, the Triassic conglomerate 

 is in solid contact with the limestone, and specimens sho\ving a 

 ''welding" of the two were secured. The plane of contact here 

 slopes more steeply than usual, perhaps 45° to the south, while 

 the conglomerate beds dip about 20° northward. 



It thus appears that all along the line through Pennsylvania 

 the highest beds of the Triassic, usually coarse-grained conglomer- 

 ates, overlap upon the older rocks — limestones, quartzites, and 

 gneisses. The few faults which can be recognized are too limited 

 in extent to have any bearing on the question as to the position 

 of the beds as a whole. 



That the several formations of the Triassic were not deposited 

 regularly and evenly on top of one another can also be inferred from 

 the observed lack of anything like metamorphism — cementation, 

 induration, crystallization — of the lowermost (Stockton) beds. 

 Had these ever been buried beneath the whole 20,000 feet of the 

 two overlying formations, the temperature would necessarily have 

 been raised so high and opportunities for chemical action have 

 become so great, that some changes would surely have been pro- 

 duced. Again, the Stockton is known to thin rapidly northward. 

 Its thickness along the southern edge of the belt, toward the eastern 

 end, is as great as 5,500 feet, but where brought to the surface by 

 the Buckingham Mountain fault (the Flemington fault of New 

 Jersey), ten miles further north, it is only about 2,000 feet. 



It is therefore believed that in the portion of the Triassic basin 

 crossing eastern Pennsylvania the locus of deposition of the beds 

 was gradually shifted northward during the course of the period, 

 so that the successive formations overlapped more and more to the 

 north, the basin being deepened by down-warping rather than by 

 faulting, as brought out by Section B. 



11 Lesley and D'Invilliers, Ann. Rept. Second Penna. Geol. Sun\, 1885, pp. 

 491-570. 



'■-Op. cit., pp. 20, 21, pi. III. 



