NOME II. VESICULAR LAVA. 



the floating trees beins^ stopped by some obsta- 

 cle. Similar caverns in Iceland, especially near 

 Hecla, are described by Von Troil. 



The vesicular lava, like the compact, may be 

 divided into two principal kinds : those with a 

 base of siderite, and those with a base of felsite. 



HYPONOME I. OF SIDERITE. 



This is the most common of all the lavas, and 

 covers the sides and skirts of every volcano*. 

 The colour is black or grey, derived from the 

 melted siderite. The vesicles are generally round; 

 the larger, of two or three lines in diameter, being 

 interspersed with many smaller pores. It is often 

 spotted with white spangles of felspar; and the 

 vesicles sometimes contain crystals of the same 

 substance, and sometimes of zeolite. Those of 

 Vesuvius, once itself an extinct volcano, and of 

 the extinct volcanoes of Italy in a more northern 

 direction, often contain leucite, a white stone crys- 

 tallised like a garnet. This last may be said to 

 form the base of some lavas, comparatively more 

 abundant in cabinets than in naturef . 



Homogenous vesicular lava, from Etna, Vesu- 



* Saussure, 1?8, concludes that his roche de come forms the 

 base of all the black lavas. 



f Dol. Etna, 441, says that pyrites are formed in decomposed lava, 

 in the humid way, by the union of the iron with the sulphur. 



2 B 2 



