174 DOMAIN X. TRANSILIENT. 



" Petrosilex, as I have already said, unites 

 itself by gradual shades with all rocks, in whose 

 composition some of the free earths enter, or 

 compound particles which may assist in the 

 formation of the masses which it chiefly consti- 

 tutes. Combined with pure quartz, in which it 

 seems to dissolve, it gradually assumes all- the 

 characters of quartzose rocks ; by a progressive 

 augmentation of talcous earth, it proceeds to 

 unite itself to steatites and serpentines, forming 

 in its progress a kind of fusible jad, which has 

 not the weight of common jad: it acquires the 

 earthy smell, as it approaches the roche de come; 

 the schistose tissue, in uniting with argillaceous 

 schisti. But it is when it approximates traps, 

 that the shades of its transitions are most insen- 

 sible: and an infinity of rocks placed* between 

 the two, leave the greater uncertainty concern- 

 ing the species in which they should be classed, 

 as the composition is scarcely ever the same in 

 all the parts of the same mass : one portion shall 

 incline to trap, while the other is affected by the 

 fire like petrosilex. The base of many porphy- 

 ries is found in this intermediate situation; as 

 well as most of the ancient grey and green ba- 

 salts which come from Egypt, when it happens 

 that the fineness of their paste no longer allows 



