LOWER PALEOZOIC. 205 
dovician system. Everywhere tins is unconformable with the 
Cambrian below, but conformable with the Calciferous, Chazy, 
Trenton, etc., above. 
Another change of sediments and unconformity occurs at the 
Medina, and this inaugurates the Silurian age. Still another is 
marked by the Oriskany, the natural base of the Devonian, and 
by the Chemung, which I claim is equally the natural base of 
the Carboniferous, according to physical history. 
The changes of fauna are nearly parallel; the facies of the 
Cambrian fauna continues into the Potsdam, but the species are 
all different; so the aspects of the Trenton fauna reach into the 
Silurian, but out of 10,000 species only about ten pass over. 
The subsidences which resulted in the formation of the Silu- 
rian (Upper Silurian) and Devonian strata were much more lim- 
ited in time and area than those in which the Ordovician and 
Carboniferous systems were produced, but locally the breaks at 
the Oriskany and Medina are complete. Certainly the classifica- 
tion which makes the Oriskany the upper member of the Silurian 
and divides the Calciferous to form the top of the Cambrian and 
the base of the Ordovician is " forcing the balance." 
Another argument in favor of adopting the Potsdam, Medina 
and Oriskany as bases respectively of the Ordovician, Silurian 
and Devonian is that these were chosen by the New York geolo- 
gists in framing their standard section, and that choice was con- 
iirmed by Lyell and Verneuil. Hence that system of classifica- 
tion has become a part of our geological literature, is most familiar 
to teachers and students, and could not be exchanged for a new 
and less tangible and natural system, without confusion, and, I 
think, more serious evils. Hence, I would say, " better bear the 
ills we have than fly to others that we know not of." 
The commingling of Oriskany and Corniferous fossils at De 
Cewville, etc., Canada, first described by Billings (Canada Geol. 
Rep., 1863, p. 359, etc.), and recently investigated by me, renders 
it impossible to assign these strata to different systems. 
As regards the Chemung, I will only say that between the Mis- 
sissippi and Atlantic this marks not only a great physical break 
(unconformity), but a great change of fauna as well ; the inau- 
guration of the reign of the Productidse and other Carboniferous 
forms. Above the Chemung we find perfect conformity, and a 
gradual change of fauna that makes it impossible to establish any 
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