CH. IV] OLD SURFACE-SOILS. 55 



Seeing that the greater part of the sedimentary strata have 

 been formed in the sea, and as the sea rather than the land has 

 been for the most part the scene of rock-building in the past, it 

 is not surprising that fossil plants are far less numerous than 

 fossil animals. With the exception of the algae and a few 

 representatives of other classes of plants, which live in the 

 shallow-water belt round the coast, or in inland lakes and seas, 

 plants are confined to land-surfaces; and unless their remains 

 are swept along by streams and embedded in sediments which 

 are accumulating on the sea floor, the chance of their preserva- 

 tion is but small. The strata richest in fossil plants are often 

 those which have been laid down on the floor of an inland lake 

 or spread out as river-borne sediment under the waters of an 

 estuary. Unlike the hard endo- and exo-skeletons of animals, 

 the majority of plants are composed of comparatively soft 

 material, and are less likely to be preserved or to retain their 

 original form when exposed to the wear and tear which must 

 often accompany the process of fossilisation. 



The Coal-Measure rocks have furnished numberless relics 

 of a Palaeozoic vegetation, and these occur in various forms of 

 preservation in rocks laid down in shallow water on the edge 

 of a forest-covered land. The underclays or unstratified 

 argillaceous beds which nearly always underlie each seam of 

 coal have often been described as old surface-soils, containing 

 numerous remains of roots and creeping underground stems of 

 forest trees. The overlying coal has been regarded as a mass 

 of the carbonised and compressed debris of luxuriant forests 

 which grew on the actual spot now occupied by the beds of coal. 

 There are, however, many arguments in favour of regarding the 

 coal seams as beds of altered vegetable material which was 

 spread out on the floor of a lagoon or lake, while the underclay 

 was an old soil covered by shallow water or possibly a swampy 

 surface tenanted by marsh-loving plants^ 



The Jurassic beds of the Yorkshire Coast, long famous as 



some of the richest plant-bearing strata in Britain, and the 



Wealden rocks of the south coast afford examples of Mesozoic 



sediments which were laid down on the floor of an estuary or 



' Discussed at greater length in vol. ii. 



