THE CORRELATION OF GEOLOGICAL FAUNAS. 
By Henry Shaler Williams. 
INTRODUCTION. 
In the year 1881 I began a series of investigations for the purpose 
of discovering the laws which determine the association of fossils in 
faunal aggregates and their modifications in relation to geographical 
distribution and to vertical succession, in order to apply those laws 
as guides to the correlation and classification of geological formations. 
While these investigations have been in progress many other workers 
have joined in the search. Many statistics have been gathered, and 
observations have been extended over a wide field. A few important 
results have been attained, and the nature of the problem is now more 
clearly understood than at the outset. It seems, therefore, that this 
is a fitting time to review the progress already made, and to point out 
the more prominent results achieved and the paths along which future 
investigations may be guided with most promise of success. 
When the investigations were begun it was already known that 
geological formations were marked by species of fossils differing 
greatly for each succeeding formation. In the early days of geology 
this difference was supposed to be due to extinction of old and the 
appearance of new forms for the first time with the income of each 
new formation. With this conception was associated I he idea of 
sharp distinction between formations, each of which had a character- 
istic set of " Leitfossilien." The prevalence of this latter view domi- 
nated all the literature; and the presence, in a newly exploited 
section of rocks, of a species supposed to be characteristic of a given 
formation was assumed to be sufficient evidence of the presence of 
the formation in the new section. On this basis of determination it 
had become a fact that under the name of each formation there was 
catalogued a group of species collected from widely separated regions 
and found in different kinds of rocks, all of them being thus lumped 
together as the characteristic species of the formation considered. 
At the outset of the present inquiry it was evident thai, in order to 
learn how the modification of species lias actually taken place, the 
