EXPLANATION OF JOGGINS SECTION. 181 



of these fossil soils support coals, others Buppoii erect trunks of trees 

 connected with their roots and still in their natural position. 



Believing the underclays to have been soils, we find similar reasons 

 tu conclude thai the coal-seams wore originally vegetable matter, 

 which accumulated in the manner of peat ; and on examining the coal 

 minutely, we often find distinct evidences that it is composed in part 

 of woody fragments, sometimes retaining their .-tincture in sufficient 

 perfection to enable the kind of wood to which they belonged to be 

 ascertained. These appearances arc most distinctly seen in the 

 coarser and more impure coals, and in the hands of clay and ironstone 

 which occur within the coal-seams. In the # more pure coals, the 

 vegetable matter has sometimes been reduced by chemical change and 

 pressure into an almost homogeneous mass. It will be observed that 

 in the section I have indicated the kinds of vegetable matter which 

 may be observed in the several coals; and I shall have occasion to 

 return to this subject in the sequel. 



The lowest coal-bed in Group 44, Subdivision I, Division 4, of the 

 above section, has an underclay or soil four feet in depth, and sup- 

 porting a layer of vegetable mould which has been compressed into 

 half an inch of coal. Above the coal rests a very different description 

 of rock, one of those hard dark-coloured limestones already referred 

 to. It is filled with innumerable little shells of minute crustaccous 

 animals of the genus Cythere, the modern representatives of which 

 reside in countless numbers in ponds and river estuaries and in the 

 sea, and are most voracious devourers of dead animal substances. 

 Our coal-bog therefore became, from some cause, probably subsidence, 

 a pond or lagoon, in which Cythere and other aquatic animals must 

 have existed for some time before their remains could accumulate in 

 sufficient quantity to form these two inches of hard bituminous lime- 

 stone. The (TyMere-inhabited waters, however, were dried up, and on 

 the rich marly soil grew another forest, Avhose rootlets may be seen 

 finely preserved in the limestone ; and the result was a thicker seam 

 of coal than the first, succeeded by other limestones and coals, and then 

 by a considerable thickness of. shales and bituminous limestones, in 

 which we find not only the Cythere, but the scales of small fishes, 

 bivalve shells (Naiaditea)* allied to the common mussel, and a small 

 whorled shell (Spirorbis ccirbonarius) resembling those now found 

 adhering to the seaweeds of the shore (the common Spirorbis 

 spirillum) Fig. 31. The bituminous limestones containing these 

 remains, alternate with shales indicating that irruptions of mud 

 partially filled up, at different times, the waters in which calcareous 



* ArUhracomya and An&racoptera of Salter. 



