CHAPTER III. 
FAITXAIj dissection of middle and upper devonian 
OF THE NEW YORK PROVINCE. 
The collecting of statistics to illustrate the laws of faunal history 
has been carried to a higher degree of perfection for the Devonian 
faunas than for any other fossil faunas of North America. 
This is partly because a great amount of information regarding the 
individual species of the faunas had been acquired before these par- 
ticular investigations were begun, and partly because for a number 
of years definite attention lias been given to gathering and recording 
the exact statistics needed for the purpose of solving practical diffi- 
culties in this particular field of correlation. 
The method of investigation which has brought out these facts is 
formally stated in Bulletin 41 of the U. S. Geological Survey, under 
the head of " Geographic and chronologic relations of the faunas," 
as follows: 
It is necessary to recognize the effect of geographical conditions upon faunas as 
well as the changes incident to chronological sequence if we would interpret the 
confusion existing in the Devono-Carboniferous deposits of the eastern portion of 
our continent. But the assigning of the Marshall fauna to the period of the Cats- 
kill group does not settle it. Neither does the expansion of the Chemung forma- 
tion to receive the Waverly fauna nor the pulling down of the Carboniferous 
system to cover the Portage formation relieve us from the main perplexities. 
It is only by disentangling these faunas and ascertaining the true geographical 
and chronological relations which they hear to one another that the difficulty is 
to be met. This is to be attained, not by clinging to any sharp limits of a strati- 
graphical or a lithological nature, or to any absolute division between one forma- 
tion and the following, but each fauna must be traced upward and downward and 
its modifications noted until it is replaced by another, and whatever on the way is 
interpolated or is added to it must be traced to its origin or to its center of occur- 
rence. By this method a scale marking the chronological sequence in the life his- 
tory of the organisms and faunas may be prepared which may serve as a definite 
standard for determining the relative age of formations quite independent of the 
lithological characters of the sediments which were being continuously thrown 
down, these being in main part determined by local conditions of the disintegrat- 
ing shores and distance away from them. By themselves the rocks, as rocks, 
present no features which may serve as indications of the particular stage in 
geological time at which they were deposited. « 
Previous work in correlation had been conducted on the funda- 
mental assumption that identity of fossils is sufficient evidence of 
a On the fossil faunas of the Upper Devonian— the Genesee section, New York, by Henry S. 
Williams: Bull. U. S. Geol. Survey No. 41, 1887, p. 21. 
42 
