OSCILLATORY CHARACTER OF CONTINENTAL SEAS 329 



basin, near Greencastle, Pennsylvania, only 0-90 feet were deposited in 

 the corresponding time within the Mercersburg area, and (4) the fossil, 

 Nidulites favus, is always present in its proper horizon within the Massa- 

 nutten basin — likewise the overlying Christiania fauna — but neither has 

 been seen in the Mercersburg belts.^* 



DISPLACEMENTS OP THE 8TRANDLINE 



Stratigraphic overlaps — General discussion. — The numerous osci na- 

 tions mentioned, and the many more described in succeeding parts of this 

 work, naturally suggest corresponding local or general stratigraphic over- 

 laps. Obviously, too, an overlap suggests preceding sea withdrawal and 

 cessation of marine deposition in the areas thereby emerged. Though a 

 period of sea transgression without such immediately preceding retreat 

 is conceivable, I have met with no instance that may be unqualifiedly 

 described as meeting the requirements. The break that marks the base 

 of the overlapping sediments also defines the top of the next underlying 

 deposits, and the break between them would not be recognized if sedi- 

 mentation had not been interrupted; and discontinuance of deposition 

 ordinarily means sea withdrawal. It was only in the relatively deeper 

 parts of the continental seas, in which, on account of the persistence of 

 the downwarps of the surface, the older deposits are usually buried be- 

 neath the newer, deposition may often have continued during the minor 

 sea withdrawals. In these covered areas, then, the breaks in sedimenta- 

 tion observed in regions where the older formations are exposed to view 

 doubtless are less, and in many cases the record there may be much more 

 continuous and possibly unbroken. The general truth of this statement 

 is established by innumerable deep wells driven in the synclinal spaces 

 between the Paleozoic anticlinal arches and domes. That most of the 

 Ordovician and Silurian formations thicken under cover of later deposits 

 to the west and south of the Adirondack uplift has long been recog- 

 nized, and a similar increase in volume of deposits is shown on the flanks 

 of all of the interjor areas of frequent uplift. That this phenomenon is 

 due to frequent emergences alternating with more gradual submergences, 

 hence to overlap of deposits in an alternately retreating and advancing 

 sea, seems incontrovertible. 



" Considering these striking provincial differences, tlie propriety of using one and the 

 same name, Chambersburg limestone, in the two areas may well be questioned. I do not 

 think that two distinct names are needed, but there should be some method of qualifying 

 the term in use, so that either phase might be referred to as desired. Perhaps the bes; 

 and easiest way would be to eliminate the upper Chazy beds from the formation, in 

 which case the difference between the two areas would not be greater than usual In 

 overlapping formations. (See pages 386-300.) 



XXIII — Bull. Geol. Soc, Am., Vol, 22, 1910 



