52 



AMADEUS W. GRABAU 



conformity, and the highest beds are thus wanting. In Center County, 

 Pennsylvania, the upper beds appear to be completely represented. 

 They are succeeded by 2,335 ^^et of dolomitic limestones, classed by 

 Collie with the Beekmantown, but for reasons given elsewhere' referred 

 by the author to the Chazy; and by 235 feet of limestones of Upper 

 Stones River (Upper Chazy) age. The succession seems to be uninter- 

 rupted, placing this section in the region of non-emergence, while the 

 others cited belong in that of emergence during late Beekmantown 

 time. The section in central Pennsylvania does not, however, show 

 the base of the Beekmantown, which is thus thicker than 2,500 feet 

 (see Fig. 5). There seems no reason for doubting that the higher 



Fig. 5. — Diagram showing relationships between the Mokawk and Central 

 Pennsylvania sections and the character of the overlaps and "off-laps," with the pro- 

 gressively decreasing hiatus. 



beds of the Beekmantown were progressively deposited during the 

 slow retreat of the sea, and that each higher member had, in general, 

 a smaller areal distribution than the preceding one. On this view 

 the successive members have the "off-lapping" arrangement of 

 shingles, except that the earlier and lower formations are continuous 

 beneath the higher ones. This is regressive overlap or "off-lap," 

 and seems to supply the only rational explanation answering to the 

 facts. To assume that the whole of the Beekmantown was deposited 

 before retreat began, not only makes the negative diastrophic movement 

 a cataclysmic one, where the positive movement was a very slow 

 I Types of Sedimentary Overlap, p. 6iq. 



