Page:The Journal of geology (IA journalofgeology11893univers).pdf/445

The cross section of a great mass of sediments accumulated over a zone parallel to the shore is that of a bi-convex lense. One edge rests against the shore from which the mass at first thickens rapidly and then thins gradually seaward. A broad shallow trough is thus formed by the deeper strata which may be called an original syncline. The authors give data to show that previous to compression such original synclines of deposition existed in the paleozoic strata of the Appalachians in Pennsylvania, east Tennessee, northwest Georgia, Alabama and in other localities.

In the original synclines of the Appalachian province the steeper seaward dip was northwestward and the gentler shoreward dip was southeastward. If strata in such a position be subjected to sufficient compressive force, the original syncline will be exaggerated and the steeper shorter arm will be rotated as between the forces of a couple. If compression is continuous long enough the beds may be overturned.

From this stepfold a thrust-fault may develop in either of three ways. The pressure tending to exaggerate the fold is most efficiently transmitted by the most massive stratum, and any condition which weakens this stratum may lead to a fault. The three conditions under which this massive stratum may be weakened are erosion, fracture, plastic flow; the second being the most common in the Appalachian region, where the massive stratum seems to have fractured, forming thrusting faults under loads of 2,800 to 11,000 pounds per square inch, but to have folded without breaking under loads of 11,000 to 34,000 pounds per square inch.

The authors discuss with the aid of diagrams the mechanics of repeated parallel folds, and show that the parallel folds are later than that located by the original syncline, and are consequent each upon the next preceding it in time and position. In the Appalachians the compressing force was directed both northwestward and southeastward. But when folding began there was a movement from the force towards the resistance. This the authors conceive to have been a superficial flow of a broad zone from northwest to southeast, from the sea towards the land.