i486 CLASSIFICATION AND SUCCESSION OF PLANTS. 



of coal by means of thin sections shows that the " mineral charcoal " of 

 an ordinary bituminous coal is woody tissue and fibres of bark. The 

 coarser and more laminated portions of the coal are made up of " a con- 

 fused mass of fragments of vegetable matter belonging to various descrip- 

 tions of plants." The shining and brilliant layers of coal represent the 

 bark of Sigillarioids or Lepidodendroids preserved in a flattened condi- 

 tion. Certain coals, or parts of certain coals may, further, be extensively 

 composed of the macrospores or microspores of various Carboniferous 

 Cryptogams ; but it does not appear that coals are generally made up 

 of such spores for any considerable portion of their mass. Lastly, the 

 microscopical structure and chemical composition of the " cannel-coals," 

 would show that these are " of the nature of the fine vegetable mud 

 which accumulates in the ponds and shallow lakes of modern swamps." 



The flora of the Permian period is, on the whole, nearly related 

 to that of the Carboniferous. The Sigillarioids appear now to have 

 become extinct, but the Lepidodendroids seem to have survived 

 into the earlier portion of the Permian period, while the Calamites 

 are represented by the genus Arthrofiitys, and die out at this stage. 

 No undoubted Permian Angiosperms are known, but Conifers are 

 tolerably abundant. Of the latter the genus Voltzia, which survives 

 into the Trias, is an old type of the Taxodi?tece ; while Walchia 

 connects the Yews with the Araucarias ; and the genus Ulhnania is 

 of interest as producing true cones. 



As regards the Mesozoic period, the predominant forms of plant- 

 life in the Triassic, Jurassic, and Lower Cretaceous rocks are Cycads 

 and Conifers ; and the name of " Age of Cycads " has sometimes 

 been given to the Mesozoic period, as regarded from a botanical point 

 of view. Ferns, however, occur abundantly in the earlier Mesozoic 

 deposits (Triassic and Jurassic rocks), and true EquisetcB make their 

 appearance in the Upper Trias. The first unquestionable remains 

 of Monocotyledonous Angiosperms present themselves in the Meso- 

 zoic period, the Podocarya and Kaidacarpum of the Jurassic rocks 

 appearing to be referable to the Screw-pines (Pcifidanacece). True 

 Palms do not appear till the Middle Cretaceous period is reached, 

 but a few remains of Dicotyledonous Angiosperms have been 

 detected in the Lower Cretaceous deposits of Greenland. 



While the Lower Cretaceous rocks must be associated botanically 

 with the Jurassic and Triassic, the Upper Cretaceous deposits, on 

 the other hand, are characterised by a flora similar to that of the 

 Tertiary period. From the point of view of the palaeobotanist, 

 therefore, the line of division between the Mesozoic and Kainozoic 

 epochs falls to be drawn in the middle of the Cretaceous system. 

 With the coming in of the Upper Cretaceous period, in both the Old 

 and New W T orlds, a remarkable change takes place in the characters 

 of the plant-life of the land, the Cycads now assuming a position of 

 comparative insignificance, while the Dicotyledonous Angiosperms 

 undergo a great development and become the dominant forms. 



