324 DOMAIN XII. VOLCANIC. 







silex, that is telsite*. There are some white spots 

 of undissolved felspar, and some specks of siderite, 

 which occasionally appear rusty and earthy from 

 the oxygenation of the iron. He also describes a 

 grey homogenous lava, of a very fine grain, with 

 very small dots of a clearer colour, which, ex- 

 amined with a lens, present a looser texture than 

 the other parts, and have often pores in their centre. 

 His laves silicees also belong to this kind, being as 

 compact as porcelain, with spangles of black mica, 

 while sometimes there are long fibres, as in melted 

 glassf. 



Breislak says that the grey lava, which issued 

 from Vesuvius in the noted eruption of 1794, is in 

 some parts so compact that the grain resembles 

 flint. It has a faint interspersion of micaj. 



Grey compact lava, with very small pores, 

 abounds at Volvic in Auvergne, where it is used in 

 building : it chiefly reposes on a fine grained grey 

 granite. 



Micronome 2. While compact lava. 



This kind is uncommon, and must arise from 



* Dolomieu Etna, 240. See afterwards Breislak's account of the 

 eruption of Vesuvius, 17p4. 

 f Ponces, 104. 

 i. 222. 



