2^6 REPORT OF THE AMERICAN COMMITTEE. 
of (a) tlie base, and (b) the summit, and may, as a collateral prob- 
lem, involve the selection of (c) a standard section for the United 
States, (a) The base : Of the region described, this problem 
concerns only the eastern continental area. (In the eastern bor- 
der region the base is above the top of the limestone, and the 
sandstones higher up present no sharp line for division. In the 
central continental area there is a decided gap between the De- 
vonian and the next lower Silurian faunas. In the west there 
appears to be a gap and unconformity separating the base of the 
Nevada limestone (Devonian) from the lower Silurian or upper 
Silurian in places above it.) (b) The top of the Devonian: The 
question involved here concerns the central and eastern conti- 
nental areas of the Devonian. The points to determine are as 
follows : 
(1) Shall we include the Catskill rocks (and when no marine 
faunas occur, up to the base of the Olean Conglomerate and 
equivalents) in the Devonian ? 
(2) Shall the Chemung marine fauna be taken as the upper- 
most marine fauna of the Devonian ? Or, shall a part or the 
whole of the marine faunas between the middle Devonian and the 
Conglomerate which introduces the coal measures be called 
Devonian ? 
I have elsewhere presented the facts which support the view, 
that if an arbitrary line is to be drawn, faunally, it should be 
between the Chemung and the Waverly ; the only place where 
they appear to meet in continuous section is in Western Pennsyl- 
vania. The difficulties are not less serious in England, and the 
Pilton and Baggy Point beds of Devonshire hold faunas which 
it is as difficult to settle on the Devonian or the Carboniferous 
side as it has been with the Waverly, Kinderhook or Marshall 
faunas. The fullest discussion of the problems on this point of 
the termination of the Devonian will be seen in Alex. Winchell's 
paper on the " Geological Age and Equivalents of the Marshall 
Group" (Proc. Amer. Phil. Soc, vols. 11 and 12, 1869 and 1870). 
But in the New York and Appalachian sections the question 
is, — Shall the Oriskany be taken as the base of the Devonian or as 
the top of the Silurian? If we draw lines they must be drawn 
somewhere, arbitrarily or not. The Oriskany, like the Foreland 
grits of Devonshire, may be called passage beds or Devono- 
Silurian, as was proposed by Professor Hull for the Foreland 
