26 CONTRIBUTIONS TO THE GEOLOGY OF MAINE. [bull. 165. 
Taxonoraically, we are able to indicate with precision the point in 
America at which this change from the Silurian to the Devonian sys- 
tems occurred. However many species may have survived the change, 
the time of income of the peculiar Oriskany species in Gaspe, New 
York, and the Appalachians is to be correlated with the Gedennien 
sandstones and shales of the Rhenish provinces, which hold a sparse 
but strictly marine fauna, with the corresponding grits of South Devon, 
with the plant-bearing Foreland sandstones of North Devon, and with 
the base of the Old Red sandstones of Wales, northern England, and 
Scotland. This point can be seen in any section in which the fossils 
are preserved, and furnishes for America one of the most definite 
boundaries wherever the Oriskany fauna appears. 
This transition in the faunas is evident in all the sections in which 
the Oriskany appears. In this eastern province the termination of 
the marine faunas occurs at the same place in the faunal succession 
that was adopted by Murchison as the boundary line between the Silu- 
rian and the Old Red sandstone. 
Thus in America we have the means of determining the original 
upper boundary of the Silurian where it passes into the equivalent of 
the Old Red sandstone, and we are able to locate the Lower Helder- 
berg positively in the Silurian system, in spite of the fact that the 
typical Lower Helderberg fauna of New York presents close affinities 
with the earlier marine Devonian faunas of Europe. 
Having thus determined for America the taxonomic equivalent for 
the original boundary between Silurian and Old Red sandstone in 
Wales, the fixing of the boundary in sections in which the marine 
faunas continue upward to form a Devonian instead of an Old Red sys- 
tem is a matter of local correlation. The Gaspe and Square Lake 
limestones of Maine both contain unmistakable evidence of the Lower 
Helderberg fauna of New York, and both of these limestones occur 
below the boundary line. The Lower Helderberg is, therefore, proved 
to belong to the typical Silurian system of the American continent. 
The question of the relation of the species of the Lower Helderberg 
fauna to lower or higher faunas in America does not enter into the 
problem. Wherever in the sequence the local conditions continued 
uniform, it is reasonable to suppose that the successive faunas suffered 
very slight and slow modification with the passage of time. 
On the other hand, any grand geologic event which disturbed the 
relations of land to sea over wide territory would naturally be expressed 
in more or less complete changes in the composition of the successive 
faunas. 
Nor is it safe to give too much weight to the evidence of apparent 
similarity of faunas in regions so distant as central Europe and the 
North American continent. It may be true that the Lower Helder- 
berg fauna resembles the Hercynian fauna in its genera and types of 
