322 GEOLOGY. 



through them in comparatively small quantities, all 

 the substances comprehended within the limits of the 

 mineral kingdom. 



Rocks deeply seated below the earth are converted 

 into clay. 



Coal. 



In the west of England trials have been made to 

 reach coal, by shafts sunk through strata now known 

 to occupy invariably an inferior position to the coal 

 measures where both are present. Where we find in 

 the map greywacke, transition limestone, mountain 

 limestone, or old red sandstone, depicted as the su- 

 perficial rock, it would be absurd to sink for coal. 



That coal is of vegetable origin seems now gene- 

 rally admitted. 



Chalk. 



Chalk has not been found in any part of South 

 America or Africa yet explored. It seems confined 

 to the North of Europe and the Crimea. 



The formation of flint has been much speculated 

 upon, but no plausible theory has as yet been in- 

 vented to account for it. 



One opinion entertained of flint is, that possibly it 

 may have been deposited stalactically in strata of 

 chalk, by aqueous infiltration of the siliceous parti- 

 cles ; another opinion is, that it has been forcibly in- 

 jected while ignited and in a fluid state between the 

 chalk strata. 



The decomposition of pyrites in chalk produces 

 sulphate of lime, and in aluminous slate, alum. 



London Gravel. 



By exposure to the weather flint loses its dark 

 grey or black colour, and becomes ochry, probably 

 in consequence of further oxidation of the iron ori- 

 ginally contained in it, and hence the colour of those 

 beds of gravel met with in the vicinity of London, 



