v.] ON THE FORMATION OF COAL. 105 



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 which 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 themselves. 



In the first place, the great phantom of geological time 

 rises before the student of this, as of all other, fragments 

 of the history of our earth springing irrepressibly out 

 of the facts, like the Djin from the jar which the fisher- 

 man so incautiously opened ; and like the Djin again, 

 being vaporous, shifting, and indefinable, but unmis- 

 takably gigantic. However modest the bases of one's 

 calculation may be, the minimum of time assignable to 

 the coal period remains something stupendous. 



Principal Dawson is the last person likely to be guilty 

 of exaggeration in this matter, and it will be well to 

 consider what he has to say about it : 



" The rate of accumulation of coal was very slow. The climate of 

 the period, in the northern temperate zone, was of such a character 

 that the true conifers show rings of growth, not larger, nor much less 

 Ustinct, than those of many of their modern congeners. The Sigil- 



