34 The American Geolog ist. January, 1892 
PRINCIPLES AND METHODS OF GEOLOGIC COR- 
RELATION BY MEANS OF FOSSIL PLANTS * 
By Les'rer F. Warp, Washington, D. C. 
In all work on geologic correlation, whether by means of fossils. 
or of stratigraphy, the modern doctrine of homotaxis should, I 
think, be carefully kept in mind, as it is now well recognized that 
identical forms do not necessarily indicate indentity of age. 
In the eighth chapter of his well known work on Paleontology, 
M. Pictet lays down the following general principle: ‘‘Contem- 
poraneous deposits, or those formed at the same epoch, contain 
identical fossils. Conversely: deposits which contain identical 
fossils are contemporaneous, ’'t 
Schimper, in his Vegetable Paleontology accepts this state- 
ment and adapts it to plants in the following form: ‘‘Contempo- 
raneous deposits, or those formed at the same epoch, contain 
floras, if not completely identical, at least homologous, and con- 
sequently deposits that contain identical or homologous floras are 
contemporaneous. ’’+ 
Nine years after the appearance of the second edition of Pie- 
tet’s work, above quoted, and seven years before that of Sehim- 
per’s first volume, viz., on the 21st of February, 1862, professor 
Huxley, in his annual address as president of the Geological 
Society of London, gave utterance to sentiments widely at vari- 
ance with these, but the soundness of which has been more and 
more clearly felt with each addition to geologicaf knowledge. 
Although in this address Prof. Huxley did not cite the above 
propositions of Pictet, and contented himself with making a much 
milder statement of the position of paleontologists, he took up 
the question of the assumed contemporaneity of the deposits con- 
taining identical fossils and apropos thereof expressed himself in 
the following language: -++Suecession implies time; the lower 
members of a series of sedimentary rocks are certainly older than 
*Read before the Geological Section of the American Association for 
the Advancement of Science, Washington meeting, August 21, 1891. 
+Les terrains contemporains ou formés i la méme ¢poque renferment 
des fossiles identiques. Réciproquement; les terrains qui contienent 
des fossiles identiques sont contemporains.” Traité de Paléontologie, etc., 
par F. J. Pictet, 2d ed., Vol. I, Paris, 1858, p. 100. 
{“Les terrains contemporains ou formés i la m@éme epoque renferment 
des flores, sinon complttement identiques, du moins homologues, et par 
cons¢quent: Les terrains qui renferment des flores identiques ou homo- 
logues sont contemporains.” Traité de Paléontologie Végé¢tale, etc.; par 
W. Ph. Schimper. Vol. I, Paris, 1869, p. 100. : 
