18 RELATION OF PALAEOBOTANY TO BOTANY AND GEOLOGY. [CH. 



to correlate the periods of maximum development of certain 

 classes of plants with definite epochs of geological history. He 

 gives the following classification in which are represented the 

 general outlines of plant development from Palaeozoic to 

 Tertiary times \ 



T -r. . n K (1. Carboniferous epoch. 



I. Keiffn of Acroffens i ^ t^ • i 



" ° (2. Permian epoch. 



( 3. Triassic epoch. 



II. Reign of Gymnosperms j 4. Jurassic epoch (including 



( the Wealden). 



TTT T^ • PA- (5. Cretaceous epoch. 



III. Keie^n 01 An^iosperms i ^ rr, .- i 



° . (6.1 ertiary epoch. 



Since Brongniart's time this method of classification has 

 been extended to many of the smaller subdivisions of the 

 geological epochs, and species of fossil plants are often of the 

 greatest value in questions of correlation. In recent years the 

 systematic treatment of Coal-Measure and other plants in the 

 hands of various Continental and English writers has clearly 

 demonstrated their capabilities for the purpose of subdividing 

 a series of strata into stages and zones ^. The more complete 

 becomes our knowledge of any flora, the greater possibility 

 there is of making use of the plants as indices of geological 

 age'. 



Not only is it possible to derive valuable aid in the correla- 

 tion of strata from the facts of plant distribution, but we may 

 often follow the various stages in the history of a particular 

 genus as we trace the records of its occurrence through the 

 geologic series. In studying the march of plant life through 

 past ages, the botanist may sometimes follow the progress of a 

 genus from its first appearance, through the time of maximum 

 development, to its decline or extinction. In the Palaeozoic 

 forests there was perhaps no more conspicuous or common tree 

 than the genus long known under the name of Calamites. 



1 Brongniart (49), p. 94. 



2 Grand'Eury (77), Potoni6 (96), Kidston (94), &c. 



3 Ward (92), Knowlton (94), Grand'Eury (90), p. 155. 



