PLANT KECORDS OF THE ROCKS ^ 



By A. C. Sewasd 

 Professor of Botany in the University of Cambridge, England 



From an examination of the remains of plants preserved as fos- 

 sils it is possible in some degree to reconstruct scenes from the 

 remote past and to follow the changing vegetation on the earth's 

 surface through many periods of geological history. Students of 

 the history of the plant world turn to the records of the rocks in 

 the hope of obtaining light on the origin and relationship of the 

 various classes and groups represented in existing floras; their aim 

 is to trace the development of the plant world through the ages, 

 from the earliest age of which any records are available to the 

 present day ; to visualize the procession of floras " foreshortened in 

 the tract of time." 



The sources available to the botanical historian are the fossils 

 preserved in the earth's crust, that is, the relatively thin film which 

 is the only part of the earth accessible to human investigation. 

 Rocks are in part consolidated gravels, sands, and muds upraised 

 from ancient lakes and seas, sediments essentially similar to those 

 now being carried by rivers and ocean currents and deposited in 

 deltas, on sea beaches, and on the floors of shelving coasts. Other 

 rocks are igneous in origin ; some formed far below the surface under 

 pressure and intense heat by the crystallization of molten magmas ; 

 others, such as lavas and layers of ash derived from volcanic sources 

 and spread over the surface of the land or on the sea floor. 



The present is the key to the past; a turbid river charged with sand 

 and finer particles of clay carries to its delta, with these scouring- 

 from the rocks, floating stems and branches of trees which were grow- 

 ing on banks undermined by the stream; on its surface float twigs, 

 leaves, and other wind-borne fragments. The sediment comes to rest 

 as the velocity of the river is checked and in it are included samples of 

 the contemporary vegetation. Forests growing near an active vol- 

 cano are overwhelmed by lava streams or showers of ash, and it is not 

 uncommon to find remains of plants of former ages preserved in 

 material which caused their death. 



1 Sixth Hamilton lecture, illustrated by lantern slides, delivered at the Smithsonian 

 Institution on Mar. 30, 1932. 



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