OSCILLATORY CHARACTER OF CONTINENTAL SEAS 321 



to each other it was exceedingly unstable. The seas were usually small 

 and patchy, and slight tilting of the surface in one direction, then in 

 another, necessitated continual change in their boundaries, and fre- 

 quently complete withdrawals. Numerous comparative studies more or 

 less clearly illustrate this instability of the continental surface. It is 

 indubitably shown on the flanks of the old interior uplifts — ^the Cincin- 

 nati, Nashville, and Ozark islands of the Paleozoic seas. Convincing 

 evidence indicating similarly uneven changes of the strandline is to be 

 seen also around the shores of the Adirondack and Wisconsin peninsulas 

 These are rather fully described in Part II, and others more incidentally 

 in a series of papers in course of publication. That the more contin- 

 uously submerged Appalachian Valley was likewise, perhaps even to a 

 greater extent, subject to such oscillations, is shown by the following 

 discussion of the Chambersburg limestone. 



MIDDLE ORDOVICIAN OSCILLATION INDICATED BY THE CHAMBERSBURG 

 LIMESTONE IN SOUTHERN PENNSYLVANIA^ 



Chamber^sburg limestone in the Chamhershurg-Massanutten hasin — 

 Character and stratigraphic relations. — The very different composition 

 of the Chambersburg limestone in the Chambersburg-Massanutten basin 

 and in the parallel Mercersburg trough affords one of many striking illus- 

 trations of the instability of the floor of the continent during the middle 

 ages of the Ordovician. In the typical sections, between Chambersburg 

 nnd Greencastle, Pennsylvania, the formation has a thickness ranging 

 from 500 to 600 feet at the former town to over 800 feet at the latter. 

 Tlie lower 150 to 200 feet of the last figure varies rapidly in thickness, 

 l)ping very thin or al)sent at Chambersburg and northward from that 

 point. It is absent also to the south of Greencastle, at Pinesburg, Mary- 

 land, and at Martinsburg, West Virginia; nor was it observed at Win- 

 chester, Virginia, but at Middletown and Strasburg, Virginia, its upper 

 part is again represented by 100 feet or more of partly cherty limestone. 

 The variation is due to warping and overlap, the basal part, which con- 

 tains Tetradiam cellulosum and other characteristic Lowville fossils, hav- 

 ing been observed only in the vicinity (2 to 5 miles north) of Greencastle. 

 The up])er part of this lower member of the formation is correlated with 

 the cherty upper member (Leray) of the Lowville of New York. 



Following this variable basal member is the lower Echinospha^rites bed 

 (40 to 50 feet), which carries besides, especially at Strasburg, Virginia. 



" since this was written additional information respecting the Chambersburg lime 

 stone, especially field relations of the formation and lists of fossils, has been published 

 in Folio No. 170, Atlas, U. S. Geological Survey. Whatever discrepancies may be found 

 In the two accounts the present is to be regarded as the newer. 



