812 FLORAL ZONES OF THE POTTS VILLE FORMATION. 



evidence offered b}' these species is further strengthened by the pres- 

 ence in the combined flora of the rarer species A^ieimites pottsviUensis 

 and AlethojyUi'is protiuiuUinn^ also typical of that level, as well as 

 l)y Sphenopteris asplenioides and Trigonocarpum HelenKB^ usually found 

 associated with the former in the larger collections. In short, the tes- 

 timony of the (•oml)incd flora is so strongly indicative of the con- 

 temporaneity of the Lower Lvkens Valley coal in this portion of the 

 Western Middle field with Lvkens coal No. 4 of the Southern field as 

 to leave little I'oubt of its approximate synchronism or correlation 

 therewith. The similarity in the stratigraphic position of the coal 

 worked in the Shamokin Gap and at several other points in the west- 

 ern portion of the Western Middle field makes it seem probable that 

 most of the small mines in the Pottsville formation in that region are 

 developed in this coal. No fossils are at hand from the openings in 

 this field on the "upper Lvkens Valley'' coal, which appears to be 

 thin and unstable. 



If the correlations more or less definitely proposed above are accu- 

 rate, the Lvkens eoal No. -i has a relatively wide distribution, not only in 

 the Southern Anthracite field, Init also in the Western ]\Iiddle Anthracite 

 field, and has the greatest extent in workable thickness of all the Lvkens 

 coals, though its thickness is generally less than that attained by Lvkens 

 coal No. 5 in the Lmcoln-Lykens mining district. 



ZONES OF THE POTTSVILLE FLORAS IN OTHER REGIONS OF THE 

 APPALACHIAN PROVINCE. 



Li discussing the distribution of the floras of the several divisions of 

 the Pottsville formation in other basins of the Appalachian province, I 

 shall assume that the dispeTsion and migration of the species along the 

 shore of the interior Carl)oniferous sea were, under the favoring con- 

 ditions of a continuous, broad, Imse-level coastal- plain shore and cur- 

 rcMits l)oth strong and varying, so uniform and so rai)id as compared 

 with the geologic time required for the sedimentation of the terranes 

 that the similar associations of ideiitical species occurring at difl'erent 

 points along the coast are to be regarded as ai)])i'oximately contem- 

 poraneous. Li other words, when regarding the sui-cc^ssion of terranes 

 along the eastern l)order of the great Appalachian l)asin, in which we 

 ha\e in difl'ei'ent districts th(> same regular succession of floras, we are 

 justilied in considering that beds, along a contiiuious and unifoi'in coast, 

 contaiidiigthe same flora are, geologically speaking, synch ront)us. rather 

 than tiiat we ha\ e to do with homotaxy without contemporaneity. 



As aii-eady remarked, in correlating l>eds in regions more distant 

 from the locality of the type section, great weight is attached to the com- 

 position of the iloi-as and tiie \-ertical range of their elements, as Avell as 

 the ))roportion of identical species In other liasins. Referring to the 



