154 ON THE FORMATION OF COAL v 



as coal districts owe their importance to the fact 

 that they were areas of slow depression, during a 

 greater or less portion of the carboniferous epoch ; 

 and that, in virtue of this circumstance, Mother 

 Earth was enabled to cover up her vegetable 

 treasures, and preserve them from destruction. 



Wherever a coal-field now exists, there must 

 formerly have been free access for a great river, or 

 for a shallow sea, bearing sediment in the shape of 

 sand and mud. When the coal-forest area became 

 slowly depressed, the waters must have spread 

 over it, and have deposited their burden upon the 

 surface of the bed of coal, in the form of layers, 

 which are now converted into shale, or sandstone. 

 Then followed a period of rest, in \Yhich the 

 superincumbent shallow waters became completely 

 filled up, and finally replaced, by fine mud, which 

 settled down into a new under-clay, and furnished 

 the soil for a fresh forest growth. This flourished, 

 and heaped up its spores and wood into coal, until 

 the stage of slow depression recommenced. And, 

 in some localities, as I have mentioned, the process 

 was repeated until the first of the alternating 

 beds had sunk to near three miles below its 

 original level at the surface of the earth. 



In reflecting on the statement, thus briefly 

 made, of the main facts connected with the 

 origin of the coal formed during the carboniferous 

 epoch, two or three considerations suggest them- 

 selves. 



