COAL 31 



and undergrowth growing thickly together in a salt or 

 brackish marsh supplied a large quantity of debris which 

 fell into the mud or water below them, and were thus 

 shut off from the air and partly preserved. When con- 

 ditions favoured the formation of a coal seam the land 

 level was slowly sinking, and so, though the debris col- 

 lected in large quantities, it was always kept just beneath 

 the water level. Finally the land sank more rapidly, till the 

 vegetable mass was quite under sea water, then mud was 

 deposited over it, and the materials which were after- 

 wards hardened to form the roof rocks were deposited. 

 This was the case in those seams in which " coal balls " 

 occur, and the evidence of the sea water covering the 

 coal soon after it was deposited lies in the numerous sea 

 shells found in the roof immediately above it. 



(d} In salt water, drifted material. Tree trunks 

 and large tangled masses of vegetation drifted out to 

 sea by the rivers just as they do to-day. These became 

 waterlogged, and finally sank some distance from the 

 shore. (Those sinking near the shore would not form 

 pure coal, for sand and mud would be mixed with them, 

 also brought down by rivers and stirred up from the 

 bottom by waves.) The currents would bring numbers 

 of such plants to the same area until a large mass was 

 deposited on the sea floor. Finally the local conditions 

 would have changed, the currents then bringing mud or 

 sand, which covered the vegetable mass and formed the 

 mineral roof of the resulting coal seam. There is a 

 variety of what might be called the " drifted coals", 

 which appears to have been formed of nothing but the 

 spores of plants of a resinous nature. These structures 

 must have been very light, and possibly floated a long 

 distance before sinking. 



If we could but obtain enough evidence to under- 

 stand each case fully we should probably find that every 

 coal seam represents some slightly different mode of 

 formation, that in each case there was some local peculi- 

 arity in the plants themselves and the way they accumu 



