GREEN SLATES. 117 



The slate quarries of Carnarvonshire are the largest in 

 the world, and give employment to 3,000 men ; the whole 

 number employed in slate quarrying throughout Great Britain 

 being about 15,000. 



The "green slates " of Cumberland are composed ot 

 volcanic dust and ashes, which often contain large quan- 

 tities of felspar, and thus form a very tenacious mud. Vol- 

 canic dust, converted into mud, has been found, more 

 or less, wherever the bed of the ocean has been explored, 

 vast quantities, as we have seen, are ejected during erup- 

 tions, and the lighter part is carried hither and thither by 

 the wind. Dust from Mount Hecla has at times been con- 

 veyed to Denmark, and since much larger quantities have, 

 no doubt, been dropped by the way, there are probably 

 large accumulations in the German, as well as in the 

 Indian, Ocean, which may be converted into shales or slates 

 according to circumstances. 



Ancient mud is found, however, in various other con- 

 ditions besides ; some as soft clay, some mixed with lime, 

 when it becomes marl, some as hard clay, and some so 

 altered by heat as to be crystalline, and much harder than 

 even the hardest and oldest of the Welsh slates. 



Many of the most valuable clays occur in a semi- 

 hardened state, and are blasted in rock-like masses ; but 

 whether hard, or soft and sticky, all clays are essentially 

 hydrous /.<?., watery silicates of alumina, and though al- 

 ways containing other minerals, are chiefly compounds of 

 silicon oxide (silica) with alumina, the oxide of the silvery- 

 looking metal called aluminium ; neither silicon, which is a 

 black crystalline substance, nor aluminium, occur in the free 



