344 GEOLOGY OF THE BRISTOL COAL-FIELD. 



It is certain therefore that where we now find the Coal, was once 

 a magnificent forest of Conifers and Treeferns 50 or 60 feet high, 

 with a dense nndergrowth of gigantic club mosses, growing in a 

 dark and muddy swamp near the sea, and subject to an influx of 

 the waves at high tides. 



When a tree therefore was destroyed instead of lying exposed to 

 the atmosphere and suffering the usual decomposition, it was at 

 once buried in the mud and sand. 



At the close of the carboniferous shales, as we before stated, the 

 land was gradually rising, and after a very long period had passed, 

 and sand had accumulated to the thickness of the millstone grit, a 

 hollow creek appeared formed by a sluggish river, causing a 

 kind of estuary. On this spot grew the rich growth of trees to 

 which allusion has been made. After many years of luxuriance 

 the land subsided a few feet and allowed the mud and sand to cover 

 up the ground and destroy the vegetation. The leaves and broken 

 bits of wood ground to pieces by the force of the water, got mixed 

 with the sand, just as we now find them. The land again rose and 

 and another forest was the result, which in its turn became 

 engulfed. This is believed to be the explanation of the formation]of 

 the alternating beds of Coal and clays. 



We always find that every Coal seam reposes on a bed of under 

 clay. The Coal seam represents the vegetation while the under 

 clay was the actual soil in which the plants grew. What was once 

 a soft, oozy mud has been^hardened by the lapse of ages, till now it 

 is a hard stone and used for making the best firebrick. 



After these changes had gone on for a long period of tima, a 

 great silting up with sand took place, forming what we know as the 

 pennant sandstone. After a long continuance of sandy waste and 

 another change of level, a second series of forests made their 

 appearance in like manner until they in their turn were buried, in 

 order to form a future storehouse of fuel beneath the conglomerates 

 and sandstones of the Trias. 



Coal is formed from slow oxidation of wood or cellulose aided by 

 warmth and moisture, and the consequent evolution of marsh gas 



