220 GEOLOGY. 



ticed from year to year. The impurities came in part 

 from the woody substance from which the coal was 

 made. There was silex, which is one of the chief of the 

 impurities, in some of the plants of the Coal age. But 

 impurities came chiefly from the detritus which was laid 

 down upon the coal-beds, for there would, of course, be 

 more or less admixture of this with the coal. There is 

 more or less iron pyrites (sulphuret of iron) in coal, and 

 to this is owing the sulphur smell which a coal fire gives 

 out. When there is much of it in coal it detracts much 

 from its value" causing it to crumble on exposure to the 

 air, and to emit strong fumes when burned. 



318. Rate of Formation of Coal. Some calculations 

 have been made in regard to the time required for the 

 formation of beds of coal. Taking some observations 

 of Liebig as to the rapidity of vegetable accumulation, 

 it has been calculated that it would require 1 70 years to 

 make one inch thickness of anthracite coal, and there- 

 fore 122,400 years to accumulate a stratum of 60 feet. 

 This is based upon the ordinary growth of the present 

 time ; but, as you will soon see, the growth of the Car- 

 boniferous age was probably much more rapid than it is 

 now, and this, of course, must reduce the*time. But, at 

 any rate, the time must have been very long for the ac- 

 cumulation of even a bed of ordinary thickness. And 

 when we come to take into the account the formation 

 of the rocky strata as well as the coal-beds, our arithme- 

 tic must give out, for the whole system, from the sub- 

 carboniferous upward, through all the strata, has a thick- 

 ness, in some cases, of from 12,000 to about 15,000 feet 

 that is, nearly three miles. 



319. Plants. The remains of plants are found in great 

 abundance in the coal-measures, though none appear in 

 the great sub-carboniferous limestone floor which is be- 

 neath them. The plants were of various sizes. Many 

 of them were prodigious in size, compared with similar 

 kinds of plants at the present day. The vegetation of 



