PORTAGE AND CHEMUNG FORMATIONS OF MARYLAND 343 

 PORTAGE-CHEMUNG BOUNDARY 



It is very difficult to determine this horizon when the brachiopod 

 fades of the Portage immediately underlies the Chemung. Of the 

 New York section Clarke remarks "It is extraordinarily difficult to 

 fix on a division plane between the Ithaca and the overlying Chemung 

 faunas."^ If this be true in New York, it must be still more true 

 when an attempt is made to correlate that section with those of other 

 states, and the results attained must be open to revision as fuller inves- 

 tigations give increased data. Nevertheless, it seems reasonably 

 probable from the preceding studies, that the horizon in question is to 

 be placed between the strata bearing the Spirijer disjunctus and 

 Spirifer mesacostalis faunas, giving the following succession : 



Chemung, Spirifer disjunctus fauna 

 ( Spirifer mesacostalis fauna 



Portage \ Spirifer pennatus var. posterus fauna 

 ( Naples fauna 



Genesee, Black shales with Buchiola fauna 



Lithologically the horizon is not well defined, the conditions vary- 

 ing at different locahties. In general the Portage is characterized by 

 smooth fissile shales and interbedded sandstones and the Chemung 

 by a larger percentage of sandstones, while its shales are softer and 

 break with a hackly fracture. The transition from Portage to Che- 

 mung is however not sharply defined by any lithological features. 



GENERAL RELATIONS 



Certain general facts seem to harmonize with those of New York. 



1. The general succession of forms seems to be that of New York. 

 At the base occurs the Genesee thinning eastward, followed by the 

 Naples fauna of the Portage. Above the latter is found Spirijer 

 pennatus var. posterus, succeeded by Spirijer mesacostalis, and finally 

 by Spirijer disjunctus. 



2. There is a greater development of the Naples fauna in the west 

 and of the Ithaca fauna in the east. 



3. The shore line was probably eastward as indicated by the fact 

 that there is a marked development of conglomerates eastward, as at 

 Millstone. These diminish toward the west where the lower Jennings 

 is largely composed of argillaceous shales, as at Allegany Grove. 



* Memoir 6, N. Y. State Museum, p. 213, 1904. 



