Williams.] GEOLOGICAL EXPRESSION OF FAUNAL MIGRATION. 37 
local origin, their materials having been brought together and formed 
under conditions which were more or less local in extent. In dealing 
with the classification of such formations the question of their 
sequence, their thickness, and the composition of their materials 
must first be taken into account. In correlating two formations of 
this kind the first question is as to their geographical continuity. If 
we find that a stratum of limestone occupies a similar place in the 
sections of two regions separated by 50 miles of distance, and the 
sequence for both regions is the same, it is safe to assume that we 
are dealing with the same part of the earth's crust. The second 
question, as to whether the two parts of the earth's crust thus corre- 
lated were formed at exactly the same time, does not interfere with 
the conclusion that the formations are the same and maybe classified 
as equivalent. In other words, it is possible (and there are examples 
which show that it is a fact) that the conditions at one particular geo- 
graphical spot have been repeated in the same order at a distance 
removed from that spot, although each episode of the second region 
occurred later in time than its corresponding episode of the first 
region. Such phenomena are generally explained by the supposition 
of the rising of the shores or the sinking of the same in relation to 
sea level, with "transgression of the sea." 
The second set of facts is described by the term faunas. The faunas 
are biological quantities, the term fauna meaning the aggregate of 
organisms living together in a region at a particular period of time. 
Such a fauna lived during the formation of the sediments of a particu- 
lar formation, and on account of this fact is said to characterize that 
formation. 
It does not necessarily follow, however, that another formation, far 
removed geographically from the first, which contains approximately 
the same species, is, on that account, the same formation; but in order 
even to understand what such a proposition means it is necessary to 
differentiate the fauna from the formation and to conceive of the two 
as different entities and as not either intimately or necessarily com- 
bined. The discovery that the limestones of two separate regions were 
not formed during exactly the same interval of time would not be 
sufficient to prove them to be different formations, for the deposition 
of the sediments making up a particular formation may have con- 
tinued at one point after it had ceased and was replaced by the depo- 
sition of sediment constituting another formation in a separate region, 
or deposition may have begun earlier at one spot than at another. 
Such a state of facts follows necessarily from the principle of regard- 
ing a formation as a unit mass of rock instead of a unit division of 
time. 
On the principle of migration of faunas it is quite possible that two 
distinct faunules living contemporaneously in two adjacent districts 
of one basin might be arranged consecutively in a third (also adjacent) 
