130 DESCRIPTION OF THE STRATA. 



unctuous, especially in the upper part of the beds. The cross fracture 

 is dull, and somewhat rough and hackly. The rock is of a dark slate 

 colour; but when pounded has a lighter hue, resembling grey clay. The 

 cross seams, as in some varieties of coal, are often filled with a kind 

 of coating, and occasionally with ealc spar. The coating is generally 

 an efflorescence of sulphate of alumine, mixed with oxide of iron, which 

 last gives it a rusty colour. The efflorescence sometimes increases by 

 exposure to the air, and the rusty coating tends, under favourable 

 circumstances, to preserve the rock from further decomposition. 



The schistus of this bed is found to contain alumine, silex, 

 magnesia, lime, oxide of iron, bitumen, sulphur, and water: the 

 proportions of which vary considerably in different parts of the bed. 

 Alumine, or pure clay, is of course a principal ingredient in every 

 part of it ; the proportion being often 30 per cent, or upwards. It 

 also contains a large proportion of silex. The sulphur is most copious 

 in the upper part of the bed, and the bitumen in the lower part. A 

 considerable portion of linte is also contained in the schist, being not 

 only found in seams or veins in the state of calc spar, but diffused in 

 some degree through the mass of the rock. The minute shining 

 specks generally discernible in the surface of the laminae, are proba- 

 bly crystals of selenite rather than mica. Selenite is often found in 

 the seams, in fine diverging crystals, crossing one another. The same 

 mineral also occurs in considerable quantity among decomposed 

 ^hale, particularly in the old alum works. The formation of crystals 

 of this kind is a pi'ocess still going forward ; for we often find them 

 among rubbish at the alum works, in circumstances which clearly 

 prove, that they must have been formed since the rubbish was depo- 

 sited. — Thespecificgravity of the schist is, on an average, about 2.48. 



It is from the upper half of this bed that alum is extracted. The 

 dogger and other covering strata being removed, the rock is heM^ed 

 >vith picks, &c., and the broken fragments are laid in a heap, and 



