232 Mr. PouLETT Scrope on the Geology of the Ponza Isles. 



clined plane in a state of imperfect fluidity. The Capo del Arco is the source 

 of this current, its superior elevation and the scorified nature of its rocks at- 

 testing- the proximity of the orifice of production. Whatever accumulations 

 of ejected matter may once have been formed to the west of this cape, may 

 naturally be supposed to have disappeared under the erosive attacks of the 

 sea, whose waves continue still to degrade even the solid lava-rock. The frag- 

 mentary ejections that fell on the surface of the graystone bed, remain there 

 in the form of conglomerate strata (or tufa), which, as far as I could observe, 

 present no indications of having been deposited below the sea-level. The 

 effect of rain-torrents would be sufficient to give them that occasionally waved 

 disposition and alluvial aspect, as well as their partial induration. 



The mass of graystone of San Stefano, though not so decidedly current- 

 shaped as that of the neighbouring island, gives occasion to the same remarks. 

 That of La Botte is reduced to too small a remnant to offer in its figure any 

 indication of its mode of production. 



On the whole, it is worthy of remark, that while the different graystone 

 rocks of this group of islands have the general form and disposition of lava- 

 currents, the subjacent trachytes are on the contrary disposed in a manner 

 wholly different from that of any known body of lava produced by a supra- 

 marine volcano. If we reflect on the rare occurrence of such circumstances 

 as are necessary to the obtaining a view of the internal structure of a tra- 

 chytic formation, elevated by volcanic action from subaqueous vents, we shall 

 be able to appreciate the opportunity afforded in the Ponza group. At the 

 same time it must not be forgotten that the destroying process Avhich has laid 

 open the constitution of these islands to our observation, has without doubt 

 robbed them of a very large portion of their original mass, and, in all proba- 

 bility, of by far the greater part of the fragmentary products by which they 

 were originally enveloped. 



In conclusion it may be as well to offer a few remarks on the connection of 

 the Ponza group with the other congenerous formations of the Italian coast. 

 The respective positions of the points on which the volcanic force has deve- 

 loped itself with the greatest intensity, will be perceived at once from the ac- 

 companying map*. The relations of mineral character between the rocks 

 produced from these different vents are in general extremely close. 



The island of Ischia marks the site of one of the main sources throu<rh 

 which a vast quantity of feldspathose lavas have been disgorged, mostly of the 



* See Plate XXIII. 



