236 



Poli, of Naples, is in possession of a fragment 

 of rock thrown out by Vesuvius % which I found 

 to be a real mica-slate. Every thing that tends 

 to enlighten us with respect to the site of the 

 volcanic fire, and the position of rocks subject 

 to it's action, is highly interesting to geology. 



It is possible, that, at the Peak of Teneriffe, 

 the fragments of primitive rocks thrown out by 

 the mouth of the volcano w T ere less rare than 

 they appear to be, and are heaped together in 

 some ravine, which may not yet have been vi- 

 sited by travellers. In fact, at Vesuvius, these 

 same fragments are met with only in one single 

 place, at the Fossa- Grande, where they are hid- 

 den under a thick layer of ashes. If this ravine 

 had not long ago caught the attention of natu- 

 ralists, when masses of granular limestone, and 



* In the valuable collection of Dr. Thomson, who resided 

 at Naples till 1805* is a fragment of lava enclosing a real 

 granite, which is composed of reddish feldspar with a pearly 

 lustre like adularia, quartz, mica, hornblende, and, what is 

 very remarkable, lazulite. But in general the masses of 

 known primitive rocks, I mean those which perfectly resem- 

 ble our granites, our gneiss, and our mica-slates, are very rare 

 in lavas ; the substances we commonly denote by the name of 

 granite thrown out by Vesuvius are mixtures of nepheline, 

 mica, and pyroxene. We are ignorant whether these mix- 

 tures constitute rocks sui generis placed under granite, and 

 consequently of more ancient date ; or simply form either in- 

 termediate strata or veins, in the interior of the primitive 

 mountains, the tops of which appear at the surface of the 

 Globe. 



