THE GEOLOGICAL RECORD 201 



feet, but are usually from 2 to 10 feet, thick, were formed by the 

 compression and partial decomposition of vegetation which grew 

 on the swampy surface of the delta whenever it was built up to 

 sea level, their thickness depending upon the duration of con- 

 ditions favourable for growth and preservation of the plants. 

 The presence of upright trees in and above the coal seams, resting 

 upon an old clay soil which is penetrated by rootlets and fibres 

 proceeding from their roots, shows that the coal grew where it is 

 now found, and was not as a rule drifted into place. Seams of 

 cannel coal, however, seem to be made of vegetation drifted and 

 waterlogged. Whenever subsidence of the delta recommenced, 

 growth of vegetation was stopped for a time, and the plant-remains 

 were buried up and gradually mineralised and converted into 

 coal. The alternations of plant growth and subsidence were 

 repeated hundreds of times. 



The vegetation of the period was almost exclusively crypto- 

 gamic, but the plants, whose only living representatives are 

 ferns, lycopods, selaginella, and equisetum of lowly size and little 

 importance, then attained great size (Fig. 77). Some few higher 

 plants were present, conifers and cycado-filices, but it is doubtful 

 if there were either monocotyledons or dicotyledons. Microscopic 

 examination of the coal often proves it to consist of bark, epi- 

 dermis, and vascular tissue of the plants, and sometimes entire 

 seams are made up of nothing but spores and spore cases. Air- 

 breathing mollusca and insects of various kinds have been found 

 in the Coal Measures, and remains of the earliest known amphibia 

 have been obtained from the same beds. Fishes and fresh-water 

 shells are abundant, marine shells being more seldom found, and 

 then only in particular bands. The ironstones may have been 

 formed, like the bog iron ores in Sweden, from the growth of ferrug- 

 inous algae. 



The marine fauna of the lower beds is chiefly characterised 

 by brachiopods, corals, and crinoids. There are many foramini- 

 fera, lamellibranchs, gastropods, and cephalopods ; but trilobites 

 are few in number and species, and are on the point of extinction. 

 Volcanoes were active in Scotland, Ireland, and Devonshire. 

 Besides coal and iron, the rocks of this System yield refractory 

 materials such as fireclay and ganister ; clay for pottery and 



