IV] 



CONDITIONS OF PRESERVATION. 



91 



seeds were deposited in the sandy sediment. Delicate leaf 

 structures with sporangia still intact, point to quietly flowing 

 water and a transport of no great distance. Occasionally the 



Fig. 19. Piece of Coal-Measures Sandstone with casts of Trigonocarpon seeds, 

 from Peel Quarry near Wigan. From a specimen in the Manchester 

 Museum, Owens College. | nat. size. 



large number of delicate and light plant fragments, associated 

 it may be with insect wings, may favour the idea of a wind 

 storm which swept along the lighter pieces from a forest- clad 

 slope and deposited them in the water of a lake. In some 

 Tertiary plant-beds the manner of occurrence of leaves and 

 flowers is such as to suggest a seasonal alternation, and the 

 different layers of plant debris may be correlated with definite 

 seasons of growths 



The predominance of certain classes of plants in a particular 

 bed may be due to purely mechanical causes and to differential 

 sorting by water, or it may be that the district traversed by the 

 stream which carried down the fragments was occupied almost 

 exclusively by one set of plants. The trees from higher ground 

 may be deposited in a different part of a river's course to those 

 growing in the plains or lowland marshes. It is obviously 

 impossible to lay down any definite rules as to the reading of 

 plant records, as aids to the elucidation of past physical and 

 botanical conditions. Each case must be separately considered, 

 and the various probabilities taken into account, judging by 

 reference to the analogy of present day conditions. 



Various attempts, more or less successful, have been made 

 1 Heer (66). 



