Geologic Correlation.— Ward. 41. 
once stretched all the way from the one to the other, or that sim- 
ilar lakes or estuaries existed in both parts of the continent at 
the same time. Such is the case when we compare the Triassic 
of the eastern states with that of New Mexico, Arizona, and 
Central America, and although the floras of these widely sep- 
arated parts of the American continent are considerably differ- 
ent, still it has been argued by eminent geologists that the oc- 
currence of a large number of identical species, and a similar 
facies in the type of plants indicate the former existence of a 
great Triassic sea of nearly uniform age, from New Mexico to 
Honduras; and not widely different in age from the correspond- 
ing one which extended on the eastern side of the continent from 
Massachusetts to North Carolina. The same principle could be 
applied to many other epochs in geologic history. 
What then is the specific method adopted in comparing floras? 
It may be briefly defined as the preparation of tables of distribu- 
tion and their discussion. As already remarked, the more com- 
plete the flora of any group to be considered is the more accurate 
will be the comparison. Therefore the first work to be done is to 
make a complete list of all the fossil plants that have been found 
in the given group. This list of species may be primarily re- 
‘garded as wholly unknown geologically. 
If there are several distinct localities, areas, or basins which 
are suspected to be of similar age, the species or forms that occur 
in each of these must first be enumerated separately and compari- 
sons made to ascertain to what extent they are the same or simi- 
lar for the different florules. This is what, in my Triassic work I 
have called the American distribution. The number of forms 
common to any two such areas will indicate the botanical resem- 
blance between such two florules. Thus in the Triassic flora of the 
United States as known at the present time, the following table 
shows the number of species common to twoor more of the basins: 
Areas 
Areas New Jersey New Mexico 
and Virginia. North Carolina and 
Pennsylvania. Arizona. 
Connecticut Valley D 5 6 1 
New Jersey and 7 10 2 
Pennsylvania... 20 2 
Preis hs ee 8s 2 
North Carolina.... 
But as the number of species occurring in the different basins 
, 
