Philadelphia Meeting of Geological Societg .— TJpham. 97 
ead themselves to belief in one hypothesis or the other. Very few have 
attempted to settle the problem by a posteriori argument, though here, 
as generally in geological questions, this method is probably safer than 
the other. It may be noted that the geographical surroundings of the 
two schools seem to have had something to do with their beliefs. 
European geologists still occupy the ground where a century ago nearly 
all topographic forms were ascribed to marine denudation : the Eng¬ 
lish members of the school in particular are always close to the line of 
breakers that attack their coast, and the production of valleys is about 
as much as most of them will allow to subaerial agencies. American 
geologists have worked in a new field. Members of our western surveys 
have found evidence of vast denudations in regions so remote from the 
sea that hardly any mention of marine erosion is to be found in their 
reports; and under their lead, subaerial erosion has come to be con¬ 
sidered competent not only to produce valleys, but to widen the valleys 
so far as to consume the interstream hills and thus reduce a region to 
almost featureless lowland close to baselevel. I believe it is fair to say 
that the few Europeans who share more or less fully the belief of the 
American school, are precisely those who are most conversant with 
American geology : but this is not a rule that works both ways. 
The particular characteristics of the two classes of plains, by which 
they can be distinguished, may be deduced from a consideration of 
their processes of origin. Whatever valleys may be eroded in a region 
will be extinguished as it is planed across by the sea waves ; and when 
it rises with a cover of sediments on its back, a new system of streams 
will be developed : these streams will be essentially unrelated to the* 
structure of the denuded plain beneath the cover, and when they cut 
down on it, they will with respect to its structure follow the random 
habit of superposed streams. But if baseleveling is accomplished by 
subaerial erosion, the streams will attain a considerable adjustment to 
the structures on which they work so long ; the valleys will mostly fol¬ 
low the weaker strata, and the harder strata will form low divides, 
crossed only by the chief transverse streams. When the plain is intro¬ 
duced by elevation into a new cycle of erosion, the adjustments will be 
extended. Streams of so systematic a pattern can easily be distin¬ 
guished from the random streams that should drain a plain of marine 
denudation. 
If this consideration is valid, it will follow that various plains of 
denudation where adjusted drainage prevails must be referred to 
subaerial agencies. There are, however, many examples of plains of 
denudation to which this test cannot be applied : and it is entirely 
possible that plains whose drainage is not adjusted to their structure 
may have been chiefly denuded under the atmosphere, but afterward 
submerged and buried before being uplifted and dissected. 
This paper was discussed by Messrs. Willis, Reid, Hayes, Vax 
Hise, and Gilbert, who gave instances from different parts of the 
continent illustrating one or the other interpretation cited, or showing 
the influence of the character of the rocks and of isostatic adjustments. 
