GEOLOGY OF THE BRISTOL COAL-FIELD. 343 



also subjected to enormous side pressure, for the Coal Measures in 

 maDy places, as for instance from Patchway to Portskewet, assume 

 the most extraordinary contortions and wave-like forms. The 

 subterranean force that produced this, most probably happened 

 when the Clifton chasm appeared. The great Avon fault shews 

 great diagono-lateral pressure, and twisting the strike of the beds 

 from N.E. and S.W. to J^.W and S.E., thus turning the dip of the 

 beds nearly a fourth of the compass. 



A singular bed of Conglomerate 60 feet in thickness, intervenes 

 between the "Polecat^" and " Doxall " veins. It is composed of 

 water- worn flint and other pebbles, and is cemented together with 

 carbonate of lime. Among these pebbles are some curious light green 

 ones, the exact nature of which is not quite clear. The most curious 

 part of it is, that the very Fame kind of conglomerate occurs in the 

 Worcestershire and Shropshire coal-fields. In each place the 

 peculiar green pebbles occur, and the whole conglomerate is so 

 closely alike, that it is quite impossible to distinguish the several 

 specimens. A hard strong bed lies between this conglomerate and 

 the " Doxall " seam, forming a good roof. 



FORMATIOX OF CoAL. 



The microscopical and chemical examination of the several 

 varieties of Coal shew most conclusively that it Jhad a vegetable 

 origin, and that it is the result of slow decomposition. That the 

 trees grew where we find them is evident because on opening a 

 Coal seam we find them in situ with the roots penetrating the 

 ground in every direction. 



In many places at the present day we find the counterpart of the 

 old coal forest. 



The well known Pern Creek at Dandenong in Australia is in a 

 hollow and it generally happens in sub-tropical climes that these 

 beautiful cryptogams prefer sheltered cavities where the decadent 

 leaves falling from the stem, leaving the '' scar," and perish on the 

 damp ground ; and all our carboniferous Coalfields are in the form 

 of a series of hollows. 



