METHODS OF CORRELATION 1131 



province, and most probably not tbe same as a similar series in 

 anotber geologic province. Indeed, from a consideration of tbe 

 pbenomena accompanying marine progressive overlap, it becomes 

 apparent tbat even vvitbin tbe same province tiie two series are dif- 

 ferent, unless tbey are situated along a line parallel to tbe old 

 shore of tbe time vvben tbe strata accumulated. 



2. Stratigraphic continuity. 



When a formation can be traced, with but slight interruptions, 

 over a wide area, the general assumption is, tbat it is synchro- 

 nous in all its outcrops. This is true enough where tbe tracing 

 of tbe formation is parallel to tbe old shore line, or source of sup- 

 ply, but not always true vv'ben at an angle with that line. This is 

 especially the case when the formation in question is of terrigenous 

 origin, formed either as a marine or as a fluviatile deposit. A 

 basal sandstone or conglomerate, formed in a transgressing sea, 

 rises in the scale shoreward ; in a regressing sea it rises seaward 

 (see Chapter XVTII). Tbe Mahoning sandstone of tbe Lower Con- 

 emaugh of northwestern Pennsylvania bad been traced almost con- 

 tinuously around the bituminous coal field and united with the 

 Charleston sandstone of the Kanawha district of West Virginia. 

 It has since been shown, however, tbat these sandstones are part 

 of a series rising and overlapping northwestward, and tbat, where- 

 as the Charleston end of tbe series lies in the Lower Allegheny, tbe 

 Mahoning sandstone proper forms the westernmost part of tbe 

 series, lying at tbe base of tbe Conemaugb. (Campbell.) 



Limestones are, however, much more continuous, and, if traced 

 for moderate distances, are apt to hold their own pretty well. 

 This is especially tbe case with limestone beds of slight thickness 

 interbedded with shales. Thus tbe Encrinal limestone has been 

 recognized in all its outcrops, from Thedford, Ontario, to the 

 Genesee Valley, a distance of over 200 miles, holding its own 

 throughout in litbic character as well as fossil content, though 

 there are other strata which have been mistaken for it farther 

 east. It forms a prominent plane of correlation of the strata, for 

 it seems pretty certain tbat this limestone in all of its occurrences 

 represents simultaneous, or nearly simultaneous, accumulation as 

 the result of widespread uniformity of conditions. (Shinier and 

 Grabau-43.) 



The Agoniatite limestone of the New York Marcellus has been 

 traced from Bufifalo to Schoharie, and southward to Maryland. 



