166 
1. Great types of vegetation are charac- 
teristic of the great epochs in geology, and 
it is impossible for types of one epoch to 
occur in another. 
2. Given a sufficient body of facts—i. e., a 
suitable series of specimens—it is possible 
to determine as conclusively for nearly re- 
lated deposits as for those more widely 
separated. 
3. It is of the highest importance that 
plants should be correctly determined. 
At the same time White, who deplores 
the ‘surprising and painful inadequacy of 
materials relating to stratigraphic paleo- 
botany,’ shows that the present temporary 
obstacles to accuracy in correlation are to 
be found in 
1. The wide vertical range of identical 
species. 
2. The lack of standard paleobotanic sec- 
tions, 1. ¢., a knowledge of plants strictly 
limited to various beds. 
These principles form the basis of meth- 
ods which have been used for the last eight 
years, and are employed with great success 
by members of the United States Geolog- 
ical Survey in determining the ages of de- 
posits. This practise finds one of its latest 
and best expressions in a paper by Mr. 
David White on the ‘ Relative Ages of the 
Kanawha and Allegheny Series.’ On ac- 
count of similarity in the material compos- 
ing it, the similar position of the series as a 
whole in the general lithological sequence, 
and the fact that the Allegheny series has 
been traced stratigraphically with great de- 
tail as far as central West Virginia, the 
Kanawha series has long been regarded as 
in toto the exact, though greatly expanded, 
equivalent of the Allegheny series. Fol- 
lowing out the methods formulated by 
Ward, White has conclusively shown that 
this view is not a tenable one, but that as 
indicated by the testimony of the fossil 
plants, only the upper half of the Kanawha 
series can be correlated with the lower por- 
SCIENCE. 
[N. S. Vou. XIII. No. 318. 
tion of the Allegheny series, in each of 
which identical forms of plants occur; 
whence it appears that the two series over- 
lap in such a manner that the Allegheny is 
in direct chronological succession in the 
lower Kanawha. 
The occurrence of plant remains in the 
coal measures, which were in their general 
features comparable with existing types in 
the tropics, led Schlotheim to regard fossil 
plants as having a direct value in deter- 
mining the nature of the climatic condi- 
tions under which they flourished. This 
idea was later taken up by Brongniart, who 
amplified it, and from the various types of 
vegetation known by their fossil remains, 
determined what he conceived to be the 
climatic conditions of the great geolog- 
ical periods, corresponding to the great 
changes in plant types. The views thus 
adopted have been accepted in principle by 
all succeeding authorities and constitute 
our guide post to day. Writing in 1879, 
the late Asa Gray felt no hesitation in lay- 
ing down the general rule that ‘‘ Plants are 
the thermometers of the ages, by which 
climatic extremes and climates in general 
through long periods are best measured,” 
and in this connection he also pointed out 
that ‘‘ ven very moderate changes either 
one way or the other, in temperature, 
would have excluded either the vine or the 
date palm which for at least five or six 
thousand years, have grown in proximity 
and furnished food to the inhabitants of 
the warmer shores of the Mediterranean.” 
Three years later, in the course of extend- 
ing his important studies relative to the 
Erian and Upper Silurian formations, Sir 
William Dawson laid down the general 
rule that ‘‘ When we can obtain definite in- 
formation as tothe successive floras of any 
region, we thereby learn much as to climate 
and vicissitudes in regard to the extent of 
land and water,” and that with reference 
to such points, ‘‘ The evidence of fossil 
