STRUCTURE AND STRUCTURAL RELATIONS 341 



the eastern coast of the continent is the generally northeasterly trend of 

 the folds constituting the Appalachian system. This structure holds for 

 most of the territory from Alabama to Maryland and from the Canadian 

 boundary to New York city. In the region of Pennsylvania, however, 

 there is a marked deflection of these parallel folds, on account of which 

 they are found to strike almost due east and west across the state from 

 the Potomac to the Delaware. Beyond the Delaware the formations 

 gradually resume their northerly trend. The change in direction of the 

 folds of the Appalachians is readily seen in the geological map issued by 

 the state of Pennsylvania and in smaller geological maps of the United 

 States issued in the standard text-books. For the present purpose the 

 structure is clearly brought out by a consideration of the areal distribu- 

 tion of the limestone of Ordovician age, which are known by various 

 names — Shenandoah, Lancaster, Chester Valley, Trenton, etcetera — 

 throughout the long course of its development. A study of a map (fig- 

 ure 1) showing the geology of the Virginias, Maryland, and Pennsylvania 

 permits one to trace the extent of this formation from its northeast course 

 along the Shenandoah valley on the west side of the Blue Ridge to the 

 north end of South mountain in the region between Chambersburg and 

 Carlisle, Pennsylvania, where the trend changes from northeast to east. 

 This easterly trend is pursued to Harrisburg and the vicinity of Read- 

 ing, when a series of folds causes the areal distribution to extend some- 

 what farther north, although the structure lines still remain approxi- 

 mately east and west. If one disregards the overlaps of Triassic in this 

 portion of Pennsylvania, it may be seen that the Cambro-Ordovician 

 limestone widens by a series of folds along the Susquehanna and Schuyl- 

 kill rivers and extends back southward along the eastern flank of the 

 South mountain anticline as far as the Potomac river. Within this broad 

 region of limestone may be noticed several anticlines of gneiss bordered 

 by Cambrian quartzite and surrounded by the Cambro-Ordovician lime- 

 stone, which in few instances appears overlain by shales and schists of 

 Ordovician (Hudson River) age. The southernmost portion of the Cam- 

 bro-Ordovician limestone appears in many cases, as at Frederick, near 

 Strassburg, at Downingtown, and on the Schuylkill, to be on the south 

 side of anticlines, and thereby representing the northern limb of syn- 

 clinoria, which possess the general axial features noticed in the Ordo- 

 vician limestone. Overlying the limestone along its southern and eastern 

 borders are the phyllites and less crystalline representatives of the sup- 

 posed very ancient members of the Piedmont plateau. Disregarding the 

 possibility of extensive thrust faults, for which the facts in the area 

 rarely indicate any probability, the most natural inference would be that 

 these shaly phyllites are Ordovician deposits lying in an eastern geo- 



