ulrich: major causes of oscillations 



69 



over, in the next recorded age the directions of ensuing move- 

 ments at the two places often were reversed. The phenomenon 

 might be Hkened to a gently convex platform supported in the 

 middle and tilted alternately to the east and west and at other 

 times to the north and south. The condition is recognized by 

 the alternate presence and absence of sediments of particular 

 ages on opposite sides of the tilting platform. (See figure 2.) 



.•S£:a i-£:vst. 



Fig. 2. — Diagram illustrating tilting of interior areas of uplift (for example, 

 the Cincinnati dome), and the consequent variations in amounts of advance and 

 retreat of the sea on their opposite sides. Arrows indicate direction of horizontal 

 stresses. The letters A, A', A", on the one side and B, B' and B", on the other, 

 mark the same points on the flanks of the dome in all of the three stages. In i the 

 sea laps equally on both sides; in 2 the elevation of the dome is accentuated and its 

 summit has migrated to the left, while the sea has advanced much more on the right 

 side than on the left ; in 3 the summit has migrated in the opposite direction so that 

 the deposits of the preceding stage on the right flank are largely emerged whereas 

 on the submerged left flank the new sea widely overlaps the deposits of the two 

 preceding stages (i' and 2'). 



Comparative studies of the Paleozoic deposits in the Appa- 

 lachian Valley region, from eastern Pennsylvania on the north 

 and central Alabama on the south, have brought out over a 

 hundred clearly defined examples of such oscillations. They 

 are manifested by the restricted distribution or local deposition 

 of many overlapping formations having maximum thick- 

 nesses of from 200 to over 2,000 feet. In many cases these forma- 



