68 THE EARTH. 



"coal measures" were forming. This coal, preserved in 

 the depths of the earth, now forms the greatest treasure of the 

 mine, and ironstone (from which iron is procured) would be 

 almost useless but for the occurrence of these two minerals 

 together with limestone (used as a flux) in the same locality, 

 and it is this fortunate circumstance which enables England 

 to produce such vast quantities of iron at such a cheap rate. 

 The quantity of coal consumed in the iron- smelting works 

 and for fuel generally, is beyond what could have been 

 imagined a generation or two back, being somewhere about 

 50,000,000 tons annually, the coal brought to London 

 alone in 1856 being 1,271,800 tons, yet there is such a 

 plentiful supply of this valuable fuel in Great Britain alone 

 that, supposing the annual consumption to rise to 70,000,000, 

 it would serve (according to computation) a thousand years. 

 "Who shall say from whence fuel will then be obtained? 

 probably from some other source provided by the fore- 

 knowledge of God, as was shown in the formation of the 

 coal itself ; for who could have imagined, a thousand years 

 ago, when England possessed such immense forests, and 

 wood was the universal fuel, that this very wood would 

 become too scarce and valuable to be used, and that a 

 substitute would be dug out of the earth ! 



The coal-shales (thin layers of claystone found in the 

 eoal seams) furnish beautiful specimens of ferns and other 

 plants turned into coal (fig. 19), or leaving their perfect im- 

 pressions in the clay. The coal formation occurred during 

 the latter part of one of those long eras of tranquillity which 

 supervened upon the contraction and breaking-up of the 

 older strata; but the laws of nature are immutable, and these 

 days of comparative quiet again came to a close. The 

 same phenomena before described again occurred, and there 

 is hardly a square mile of these strata but shows evidences 

 of the terrible convulsion which desolated the earth. Some 

 of the strata 'were raised, others depressed, and some lost 

 altogether, the cracks and flaws being filled with liquid lava 

 or basalt, which in many cases rose upwards through them 

 and overflowed the surface. Many of the cracks of this, the 

 "carboniferous system," are filled with sulphuret of lead 



