EXPLANATION OF JOGGINS SECTION. 187 



succession is underclay — coal — bituminous limestone. This arrange- 

 ment, so common in other parts of the section, seems to show a 

 connexion other than accidental between the long periods of tei-res- 

 trial repose required for the growth of coal, and those of quiet 

 submergence necessary for the growth of mussel-beds. Probably the 

 peaty areas of coal accumulation were gradually subsiding, and when 

 this process finally caused their submergence, the submerged coal- 

 swamp was the most fitting habitat for Naiadites and its associates ; 

 and these sunken swamp areas may have been so protected by thick 

 margins of jungle as to resist for a long time the influx of turbid 

 waters. 



In the lower part of No. XIII., and immediately above Coal-group 

 21, I observed a very curious association of erect plants. An erect 

 tree, converted into coal, springs from the surface of the shale, and 

 passes through fourteen feet of sandstone and shale. Apparently from 

 the same level there rises an erect ribbed tree, probably a Sigillaria, 

 in the state of a stony cast, which, however, extends only to the top 

 of the sandstone. In the sandstone, and rooted about a foot above 

 the base of the erect trees, are a number of erect Calamites. In this 

 case the forest soil has been covered by about a foot of argillaceous 

 sand, on which a brake of Calamites sprung up. Further accumu- 

 lations of sand buried them, and covered the trunks of the trees to the 

 depth of eight feet. By this time the Sigillaria was quite decayed, 

 and its bark became a hollow cylinder, reaching only to the surface of 

 the sand, and ultimately filled with it. The other tree still stood 

 above the surface until six feet of mud were deposited, Avhen, its top 

 being broken off, it also completely disappeared beneath the accumulat- 

 ing sediment ; and being softened and crushed by the lateral pressure 

 of the surrounding mass, it was finally converted into an irregular 

 coaly pillar, retaining no distinct traces either of the external form or 

 internal structure of the original plant. The structure of similar trees, 

 to be noticed further on, renders it likely that this coaly tree is the 

 remains of one of the Araucariari Pities, which, it appears, flourished 

 in the coal-swamps in company with the Sigillaria;. The surface of 

 the clay which buried this remarkable tree became itself an underclay 

 or soil ; and on the sandstone resting upon it were found casts of two 

 erect trees, one of them five feet in height, and a Sigillaria with 

 distinctly marked leaf-scars. The tops of these trees have been 

 entirely removed, and their hollow stems filled with sand, before the 

 deposition of a bed of mud resting upon them, ami which is now the 

 underclay of a bed of coal. This coal was next submerged under the 

 conditions required for bituminous limestone and mussel-beds. The 



