No. I. — Account of Mount Vesuvius. 203 



having imbedded crystals, but sometimes it is nearly homo- 

 geneous, and highly crystalline. In this condition it much 

 resembles the rocks of basalt so well known in this country, 

 and which may give those who have not visited active volcanos 

 an excellent idea of this, one of their most remarkable pro- 

 ducts. Of this kind, as nearly as I recollect, is the lava quar- 

 ried for the pavement of the streets of Naples and the road to 

 Portici, which is a splendid causeway. Some approach very 

 nearly to the characters of the coulee of ancient lava at Capo di 

 Bove, near Rome, which Daubeny considers as an intimate 

 union of augite and leucite. The specimens in my possession 

 are amazingly compact, and resemble perfectly some varieties 

 of basalt, as I have noticed in my paper on the materials used 

 by the Romans, in the last Number of this Journal. * Ano- 

 ther variety of the compact lava, and very common, has a gray- 

 ish ground, with imbedded black crystals of augite. That of 1822 

 is dark and compact, though sometimes scoriaceous. The true 

 partridge-eyed lavas belong principally to the extinct part of 

 Vesuvius, — the Monte Somma. They owe their name to the 

 beautiful crystals of whitish leucite, bordering on pink, thick- 

 ly studded on a dark ground. It must not, however, be con- 

 founded with another variety of lava from which the fine spe- 

 cimens of this beautiful zeolite are taken, which is so porous 

 in its structure as hardly to be considered a compact lava. 

 Its colour is dirty brown, and it is very tough. The lavas of 

 the Somma are generally far the most crystalline, and well 

 suited for polishing. The variety is very considerable, owing 

 to the different parts of that compound formation. The dikes 

 already alluded to are quite different from the general mass of 

 the strata, though I cannot speak as to their generic difference. 

 Breislak enumerates no less than twenty-two varieties of the 

 lava of the Somma, but his descriptions are too undefined to 

 give us very precise ideas on the subject. We must not sup- 

 pose that the partridge-eyed lavas are peculiar to the ancient 

 eruptions, and this part of the mountain in particular. I have 

 very pretty specimens of this kind, with a basis reddened by 

 iron from the lava of 1760, which flowed from the very oppo- 

 site side of the volcano. 



* No. xvii. p. 38, &c. 



