448 DOMAIN XII. VOLCANIC. 



under it in immense plates; which covering, 

 though in many places it but feebly adheres to 

 it, since it may be removed by simply washing 

 with water, in others is so closely united that 

 it forms the last rind or outermost part of the 

 glass, which induces me to believe that it is a 

 superficial decomposition of it. Beneath this 

 earthy coating the glass appears, which is ex- 

 tremely perfect, and as if it had just come out of 

 the volcano. If we except a few pieces, in 

 which its structure is spungy, it is extremely 

 compact and solid, and therefore much heavier 

 than either of the other three kinds. It is of an 

 olive colour, and transparent when in thin scales, 

 examined by a bright light ; but in the mass it 

 appears opake. It gives sparks rather plenti- 

 fully with steel. Pieces of perfect glass, it is 

 well known, when broken, have their fractures 

 striated, waving, and curved. In this glass some 

 of the fractures are the same; but in general 

 they are conchoids, like those of flints. Its con- 

 sistence is not perfectly homogenous, as it con- 

 tains many felspathose points. Its aspect is not 

 lively and brilliant, like that of glass, but some- 

 what unctuous and dull ; from all these qualities, 

 this product appears to be more properly an 

 enamel than a glass j unless we are willing to 



