178 H. S. WILLIAMS SHIFTING OF FAUNAS 



various rock formations. For this task he has small need of help from 

 the paleontologist. His second task, however, is more difficult — that is, 

 the classification and correlation of his formations after they have been 

 defined. For this task the work of the paleontologist is in most cases 

 indispensable. The order of sequence may be a means of classifying 

 formations in a continuous section, but so soon as the continuity is 

 broken, either by faulting or by concealment of the rocks under surficial 

 cover, fossils are required for identification of "the particular division of 

 one section which may be repeated in the next. Formations differ, 

 from place to place, in the various characters by which they are defined, 

 and because likeness of definition alone does not furnish a reliable means 

 of classification, geologists have adopted the principle of classifying them 

 on the basis of supposed contemporaneity of deposition. Fossils are 

 the means of determining this contemporaneity. Huxley called our 

 attention to the fact that formations supposed to be contemporaneous 

 on the basis of fossil evidence may not in fact be contemporaneous, but 

 that only homotaxis — that is, like order of succession, can be proven by 

 likeness of the fossils. When rocks from two distinct geologic provinces 

 are compared, this is undoubtedly true in general ; but whether equiv- 

 alent formations be regarded as contemporaneous or only homotaxial, 

 the determination of their equivalency rests on the evidence of their 

 fossils. If the range of all fossil species were restricted within the limits 

 of the formations in which they are commonly found, it would be a 

 simple matter to classify formations by their fossils, and to assign each 

 formation its exact place in a standard time scale. The facts are in 

 evidence, however, to show that species which in one section are so re- 

 stricted in two successive formations, in another section may occur 

 together, thus proving that in part of their life-history they were con- 

 temporaneous. 



Fossil Faunas and their Movements 



In the following remarks illustrations will be given of the way in 

 which, recognizing the lack of perfect conformity between the forma- 

 tional limits and the range of their common species, the combination 

 of fossils we call faunas becomes a more delicate test of contemporaneity 

 than individual species, and how the movements of the faunas them- 

 selves may be used directly in classifjdng the formations carrying them. 

 The examples are taken from a series of Devonian sections in the states 

 of New York, Pennsylvania, and Ohio. The accompanying chart 

 represents the range and distribution of the fossil faunas through a 

 series of formations, extending from the top of the Onondaga to the 



