342 Mr. PouLETT Scrope on the Volcanic District of Naples. 



feldspathose lavas, trachytes, or graystone lavas approaching to trachyte. 

 These (probably owing to their very inferior fluidity compared with the ba- 

 saltic lavas*,) seem rarely to have flowed in thin sheets or narrow streams 

 down the slopes of the parent mountain, but have usually taken the form of 

 bulky excrescences, projecting like promontories from its flanks or summit; 

 and they have moreover often burst out from lateral openings at no great 

 height from its base, throwing up parasitic cones, consisting of fragmentary 

 trachyte, pitchstone, spongiform scorias and pumice, and usually containing 

 an internal basin. The mountain has thus assumed a deformed and irregular 

 figure f , though stiU preserving a general tendency to the conoidal ; and this 

 irregularity has been no doubt much increased by the convulsive elevatory 

 shocks which seem to have rent and displaced its component beds on many 

 points, as well as by the vast degradation which the tufas making up the 

 greater part of its bulk have suffered from atmospheric and aqueous erosion. 



Of these tufas, some are loose and incoherent, others indurated, apparently 

 by intimate mixture with water at the time of their deposition, in the manner 

 of the trass of the Rhine volcanoes. The indurated have of course resisted 

 the agents of destruction far more effectually than the incoherent ; and these 

 therefore, together with the solid lava-rocks, form the most prominent masses 

 of the visible framework of the mountain. The tufa of Ischia has a very preva- 

 lent tinge of green, occasioned apparently by the intermixture of chloritic par- 

 ticles, or of augite under some casual modification. The trachytic lavas of the 

 island vary much in mineral character. The generality are dark coloured, 

 hard, and crystalline, resembling the rock of the Solfatara. Some are remark- 

 able for very large agglomerations of glassy feldspar crystals ; others for their 

 zoned, ribboned, and brecciated appearance, enveloping rounded as well as an- 

 gular fragments of very different varieties of trachyte, melting into their matrix ; 

 which cannot but recall the similar nodules that occur in the porphyries of the 

 old red-sandstone, and in some granites. The trachytic hummock called 

 Monte Tabor, which has been protruded from the foot of a large cone com- 

 posed of pumice and ashes, called Montagnone, is, in part, of this variegated 



* The universality of the fact, that under similar circumstances of slopes, &c., the feldspathose 

 lavas have possessed a very inferior degree of fluidity to the ferruginous, or basaltic, has been fully 

 shown in another publication (Considerations on Volcanoi, pp. 90-95), and attributed to the 

 inferior specific gravity of the component crystalline particles ; which when the comminution of 

 these particles, and consequently the liquidity of the mass, was the same, must solely determine 

 the degree of its Jluidity, or the tendency of the liquid to move in any or all directions in 

 obedience to the force of gravity. 



t See PI. XXXIV. fig. 5. 



