The manner of occurrence of plants in sandstones, shales or other rocks is often of considerable importance to the botanist and geologist, as an aid to the correct interpretation of the actual conditions which obtained at the time when the plant remains were accumulating in beds of sediment. To attempt to restore the conditions under which any set of plants became preserved, we have to carefully consider each special case. A nest of seeds preserved as internal casts in a mass of sandstone, such as is represented by the block of Carboniferous sandstone in fig. 19, suggests a quiet spot in an eddy where 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 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 débris may be correlated with definite seasons of growth[130]. [Illustration: 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. ½ nat. size.] 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.