NOME I. COMPACT LAVA. 



often belong to the latter division ; but are some- 

 times so intermingled with siderite, that they ap- 

 pear delicately dotted or punctuated. Vesuvius 

 presents lava of this kind, which, in spite of the 

 interspersion of mica, receives an admirable 

 polish. 



Faujas, in his general classification of volcanic 

 products, has denominated this kind Laves feld- 

 spathiques ; and mentions one which is black, yet 

 melts under the blow-pipe into a white amel. 

 Some, on the contrary, belong to the white com- 

 pact lavas, about to be described*. 



The grey sorts are, " Felsite lava, of a clear Grey lavas of 



Faujas. 



grey, sometimes bluish, sometimes rather greenish, 

 or white a little inclined to red, of a fine paste, 

 rather disposed in little plates than in grains, with 

 mica more or less black, and a multitude of irre- 

 gular grains of a felspar, whiter or a little yel- 

 lowish, which infringes on the base, and whose 

 parts have a contexture and a direction different 

 from that of the base of the lava. 



* In his ideas, trap resembles felsite ; but he forgets that iron, 

 always a most predominant and characteristic substance, is wanting 

 n felsite. 



His classification of volcanic substances was first published in the 

 Annales du Museum ; and latterly, with great variations, in his 

 Geologic, tome ii. The extracts here given are generally from the 

 former, which is more ample and instructive, on some topics, than 

 his last revision. 



VOL. II. Y 



