PRINCIPLES OF STRATIGRAPHIC CORRELATIONS 563 



farther southward before pinching out than do those in the Appalachian 

 Valley proper. These relations suggest the bare possibility that the 

 Allegheny area slipped southward past the more highly folded Appa- 

 lachian part of the continent. In Pennsylvania, for instance, the typical 

 New York Trenton thins southwardly from Bellefonte, where it is still 

 600 feet thick and seems finally to pinch out altogether. The latter 

 point, however, lies some 50 to 75 miles south of where this Trenton 

 might be expected to die out if the Harrisburg axis continued in any- 

 thing like a direct course. Similar conditions are observed in the vicinity 

 of Cumberland Gap, where certain formations of the central Virginia 

 basin extend equal distances southward beyond the line indicated by the 

 Wytheville axis in southwestern Virginia. The Sneedville limestone 

 (Cayugan) and the middle and late Devonian beds in Newman Eidge 

 which are commonly, but erroneously, referred to the Chattanooga shale 

 are the most important of these formations. 



Barrier efficiency of the Harrisburg axis. — The Harrisburg axis caused 

 thinning or complete extinction by overlap of many formations. Some 

 of the latter lapped out in only one or two of the valley troughs and 

 passed on northward to New York in one or more of the other troughs. 

 Others, particularly those which are limited to the valley basins, failed 

 completely to pass over it. 



Of formations in northeastern Pennsylvania, southern New York, and 

 northwestern New Jersey, the Ozarkian Allentown formation, the middle 

 Chazy, which extends southwestward to near Hollidaysburg, Pennsyl- 

 vania, the Normanskill shale, the Lehigh Valley cement rock, and prob- 

 ably the Oswego and Juniata sandstones are confined to the northeastern 

 Pennsylvania basin. Lithologic peculiarities and partial restrictions, as 

 the cutting out of the Cayugan, Helderbergian, and Ozarkian in Dauphin 

 County, are noted in other Paleozoic deposits. 



In the Maryland basin most of the Eopaleozoic formations exposed in 

 the Cumberland Valley fail to pass over the Harrisburg axis. Eeferred 

 to by name, these are the Tomstown limestone, the Waynesboro shale, and 

 the Elbrook limestone of the Cambrian, the Conococheague of the Ozark- 

 ian, and the lower, middle and upper Stones River, the several divisions of 

 the Chambersburg limestone, and a reddish Cincinnatian sandstone that 

 is erroneously referred to the Juniata by Stose. The lower Cambrian 

 quartzite, the Beekmantown, and the Martinsburg shale pass through to 

 New York. West of North and Tuscarora Mountains the Copper Ridge 

 chert, the lower and middle Stones River, the greater part of "the Keyser, 

 and the Greenbrier limestone do not extend north beyond the Maryland 

 basin. 



