Edltorkd Comment. 251 
considerable variety of diverse relations. He pointed out exceptions to 
the normal conclusions deduced from superposition, lithological char- 
acter, and similarity of sequence. We must have a system of criteria so 
varied that if one or more fails others can be employed. All classes of 
evidence are useful, both positive, negative, and circumstantial. 
Major J. W. Powell spoke of the necessity of specialization on the 
part of geologists engaged in the work of correlation. The evidence 
derived from physical and biotic facts might apparently disagree. But 
that a satisfactory result maj' be reached, these two classes of evidence must 
be brought into harmony. He cited an example from his own experi- 
ence of how an identification of synchronous formations might be made 
over a wide area through a union of physical and biotic methods. 
Mr. W. J. McGee remarked that in the costal plain of the United 
States physical correlation alone is employed. The bases accord with 
those outlined by Mr. Gilbert with certain minor modifications and an 
important addition, as follows: 
^ Visible Cdiitinuity: 
For lociil discriininatiou and correlation - Lithologic similarity : 
I Similarity of sequence. 
For correlation throughout the province ) ^'^^''^^^^^:^^^ ^'^ ^^''' "^ g^o^raphy 
Relation to physical events, 
For correlation with contiguous provinces. -\ incliulintr continental movements, 
I transportation of materials, 
land sculpture, etc. 
For general correlation, - - - Homogeny or identity of origin. 
By correlation upon these bases the physical history of a coosiderable 
fraction of the continent may be so definitely ascertained as to permit 
fairly accurate mapping of the geography, and even the topographv of 
each episode in continent growth. After these episodes are clearly de- 
fined, and the fossils found in the formations are studied, it will be pos- 
sible definitely to ascertain the geographic distribution of organisms 
during each episode, then paleontology may be placed on a new and 
higher plane. 
Prof. W. M. Davis showed that it was possible to decipher geological 
history not only through the records of deposition, but also by processes 
of degradation. As an example of this method he explained a topo- 
graphical section from the city of New York westward. In this we 
have evidence of the existence of an ancient '■'■penephiin" or base-level 
lowland of Cretaceous age. This surface was subsequently elevated 
(more toward the west than toward the east) at the end of Cretaceous, or 
at the beginning of Tertiary time. It has since been dissected by the 
excavation of more recent valleys. The Hudson valley lowland was 
cited as an example of this recent dissection 
Prof. E. W. Claypole considered that the different methods of geologic 
correlation differed very greatly in their value. It is improbable that the 
plant or mammalian record will ever equal in its perfection that of the 
marine invertelirate fauna. The marine fauna is to the geologist what a 
primary triangulation is to the geodosist. It marks out the main divi- 
sions, which are subsequently' further subdivided through the aid of 
other fossils, such as plants and vertebrates. 
