<22 CORRELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
and muds to cover the shells and other hard parts of the fossils, these 
were ground up by the action of waves and currents, and their sub- 
stance, though not their forms, was preserved. 
A zone may be traced from place to place, as may the formation 
itself, and whenever a zone runs out or thickens, or breaks up into 
alternate barren and fossiliferous zones, the facts relate to the con- 
tinuity or discontinuity of the zone. Contin u ity and discontin u ity are, 
therefore, terms describing physical conditions, and are applicable in 
describing the persistence or reappearance of the same or parts of 
the same zone in different localities. But a zone is a part of a phys- 
ical formation and is not a fauna or a flora; the term connotes the 
geological position occupied by the fauna, as the term province con- 
notes the geographical area of distribution of a fauna. 
Just as it is presumable that the separate observed localities of a 
living fauna are continuous, and that all of them together make up the 
geographical area or province of the distribution of the fauna, so it is 
presumable that all the outcrops of the same fossiliferous zone were 
originally connected, and thus that there has been a continuous zone 
representing the geological range of each particular fauna whose 
remains characterize the zone. 
If no changes in geological conditions were to take place the geo- 
graphical distribution of any fauna at any particular time, recent or 
geological, would constitute its geographical province, and thus define 
the geographical limits of the fauna. It is, however, evident that 
geological changes have been and are constantly going on, resulting 
in the migration of faunas from place to place. It is quite conceiv- 
able, therefore, that the lapse of time represented by the presence in 
the strata of the species of the same continuous fauna may be nonsyn- 
chronous for two seel ions not many miles apart and belonging to the 
same geological province. 
This fact would be explained as a case of migration of the fauna as 
a whole over the bottom of the ocean. Such a case may be stated in 
the following way: The fauna was a littoral fauna, living along a 
shore facing an ocean to the west ; the land in relation to ocean level 
was gradually sinking during the life period of the fauna, causing the 
littoral conditions of the water to transgress toward the east. As the 
sinking progressed we may suppose the fauna as a whole to creep 
along eastward, retaining its relationship to the littoral conditions of 
environment without modification of its species or loss of its faunal 
integrity. After a long time of such movement in the same direction 
it is quite conceivable that the whole area of bottom originally occu- 
pied by the special fauna might be deserted, and that too within the 
life period of the fauna, which, in the case of the Hamilton formation 
in central New York, was a time long enough for the accumulation of 
over a thousand feet of argillaceous shale strata. 
The record of such a migration would be left in the strata of the 
