194 THE WORLD'S LUMBER ROOM. 



Similar lumps are found at the foot of the Brazilian copal 

 tree. 



It may not at first sight be obvious what connection iron 

 pyrites has with vegetable refuse ; but being a compound 

 of iron and sulphur, both of which enter into the composi- 

 . tion of plants, it may clearly be derived from them. 



The ash of beech-wood, for instance, contains enough 

 sulphuric acid and peroxide of iron to form pyrites to the 

 amount of ^siyyyth of the weight of the wood ; and twenty- 

 three times as much as this might be made if during its de- 

 composition it should come in contact with water containing 

 sulphuric acid. Many seaweeds contain a much larger 

 proportion of sulphuric acid than this, and if, instead of 

 escaping into the air, it were to come in contact with 

 peroxide of iron, pyrites might well be formed. 



The sulphur contained in the seaweed thrown up every 

 year near Helsingors is enough to make 332,000 pounds 

 of pyrites. 



Probably, therefore, the large quantities of pyrites found 

 in chalk are derived from seaweeds ; and this, too, is likely 

 to be the origin of the small percentage contained both in 

 the blue limestone of the Jura (to which it is said to owe its 

 colour) and in the bluish marl constantly deposited on the 

 coast of St. Malo. 



