Mr. PouLETT ScROPE oti the Volcanic District of Naples. 349 



respective mineral particles of the lava, will have been the facility afforded to 

 the minute augitic matter to agglomerate, by a sort of drainage through the 

 coarse, porous, and loosely aggregated mass of felspar crystals*. 



The rock which formed the Acropolis of that most ancient Greek city, 

 Cumae, has in the wear and tear of ages, lost the characteristic figure, but 

 still retains the structure, of a volcanic cone. Prom the midst of the tufa of 

 this hill a massive rock of trachyte emerges, which varies very much in mi- 

 neral character, being in some places identical with the rock of the Solfatara, 

 in others a complete clinkstone. Parts are zoned with concretionary blotches 

 and stripes of very different texture and mineral composition, like the piperno; 

 and others pass into a breccia from the quantity of what appear to be enveloped 

 fragments and nodular masses of variously coloured trachyte, resembling pre- 

 cisely on these points the rock of Lacco in Ischia. 



To the south of Cumae rises the Monte di Procida, a long ridgy hill com- 

 posed of a trachytic conglomerate in irregular strata, which, on the sea-ward 

 side, are observed to rest upon a thin bed of trachytic lava under peculiar cir- 

 cumstances. The colour of this rock is nearly black, notwithstanding its feld- 

 spathose nature, and that it fuses before the blowpipe into a light-coloured 

 glass ; its texture is earthy and porous, fracture uneven ; it contains nume- 

 rous very brilliant crystals of glassy felspar, and the interior of almost every 

 block or division of the rock consists of porphyritic pitchstone, appearing to 

 proceed from its partial fusion. No well-defined limit separates this lava from 

 the conglomerate above, which, on the contrary, appears to pass into it by the 

 fusion of its finer particles, and the torrefaction of the larger. The conglo- 

 merate envelops large loose fragments of granite and syenite. It would seem, 

 that this bed, or rather horizontal dyke, has either been originally of great 

 extent, or that similar circumstances have produced the same peculiar appear- 

 ances on three or four neighbouring points ; for, at about three hundred yards 

 distance, a rocky ledge rises from the sea, (called Scoglio delle pietre arse. 

 The rock of burnt stones), composed of a pitchstone coated with earthy lava, 

 and enveloping half-melted fragments of various kinds, exactly like that of the 

 Monte di Procida ; and the eastern angle of the island of Procida, immediately 

 opposite and more than a mile distant, exhibits a similar bed, which is here 

 seen to rest upon as well as to support the conglomerate, the whole being- 

 covered by stratified tufa. 



The same island presents traces of three or four distinct craters, which have 



* For other examples of this concretionary process in lavas, see (he Paper on the Ponza Isles. 

 Ante, page 195, 



