32 The American Geologist. January, 1892 
one age all rocks from the Hudson River shales to the top of the 
Pocono, for there one finds no interruption, except a streak of 
Lower Helderberg, so thin that only one observer* has seen it in 
place, though others have seen fragments of chert suggesting the 
presence of that group. Nor is the fact that there are localities. 
where the passage is not abrupt, is not marked by destruction of 
the fauna, necessarily a good reason for joining two consecutive 
groups. On such a basis one would have no difficulty in carrying: 
the Carboniferous downward so as to include the Lower Silurian, 
or upward to include the Pliocene. Thus in northwestern Penn- 
sylvania, Chemung fauna lingered into the Lower Carboniferous: 
in south central Pennsylvania and Maryland, Oriskany and Lower 
Helderberg fossils are mingled together in a transition bed.t 
Ordinarily the break between Lower and Upper Silurian is well 
marked, but in southern Pennsylvania,{ the Hudson river forms. 
occur sparingly in the lower Medina, while in southwest Virginiaé: 
Hudson River fossils occur abundantly to within a few feet of the: 
upper Medina; so that even on the easterly side of the Appa- 
lachian basin it would be easy to prove no break between 
Lower and Upper Silurian, Upper Silurian and Devonian, De- 
vonian and Lower Carboniferous, Lower and Upper Carbon- 
iferous. Dr. C. A. Whitel] has told us how the line between 
Paleozoic and Mesozoic disappears in the southwest, while 
to not a few of us the gradual shading away of Mesozoic 
into Cenozoic brought a sufficiency of burdens in the past. 
General, not circumscribed, conditions must be taken as the 
basis of subdivision of the column. The separation between 
Lower Carboniferous and the Upper Devonian is too well marked, 
physically as well as paleontologically, over an immense area to: 
be ignored for any but the most cogent reasons. 
But may not the Catskill as well as some portion of the 
Chemung be contemporaneous with the lower beds of the Lower 
Carboniferous of Ohio? Prof. Herrickf has shown that the base 
of the Lower Carboniferous there cannot come below the Berea 
*Capt. C. R. Boyd, in personal communication. 
+Geology of Bedford and Fulton Counties, p. 86. 
{Loc. cit. p. 92. 
§Stevenson; proc. Amer. Phil. Soc. Vol. xx, p. 138, xxrv p. 89. 
Address as Vice President before Section E of A. A. A. S. 1889. 
*C. L. Herrick; Bulletin Geological Society of America, Vol. 1, p. 34 
et seq. 
