Mr. PouLETT ScROPE OH the Geologi/ of the Ponza Isles. 207 



lumnar division, which has without doubt been assumed subsequently to the 

 occupation of its actual position, proves it to have been protruded in a state 

 of liquidity, or at least of softness. 



This liquidity as well as its elevation could only have been the result of 

 intense heat, and it may therefore easily be believed to have effected the 

 partial fusion of the fine and loose base of the feldspathose conglomerate, 

 and of the imbedded fragments, of this rock, (already in a semi-vitreous 

 state, and therefore more easily fusible,) with which it came in contact. 

 The compactness of texture resulting- from this imperfect fusion, as well as 

 the local varieties of concretionary structure, are probably owing to the pres- 

 sure and friction sustained during the forcible protrusion of the prismatic 

 trachyte, and locally modified by accidental circumstances. The porphy- 

 ritic character of the pitchstone will not prevent our recognising it to 

 proceed from the fusion of an earthy conglomerate rock, since both the 

 fragments dispersed through the unchanged rock are themselves porphyritic ; 

 and the earthy pulverulent basis incloses broken crystals of felspar and mica, 

 which in the pitchstone porphyry have only been rendered more visible by 

 the semi-fusion and discolouration of the finer and less refractory matter that 

 envelops them. 



The semi-vitreous conglomerate appears on some few spots to inclose irre- 

 gular and indistinct beds of a very earthy trachyte of an ash-gray or yellow- 

 ish white colour, with numerous felspar crystals and plates of mica. Its hard- 

 est parts are identical with the prismatic trachyte, but it does not ever like 

 this assume a columnar division. It is often slightly zoned with brownish 

 yellow streaks, and is fissile in their direction. From the earthiness and loose 

 texture of its base, it is seldom possible to determine its line of separation 

 from the earthy conglomerate with which it is intimately associated, and which 

 is evidently of contemporaneous formation. For this reason no attempt has 

 been made to distinguish the two rocks in the accompanying profiles. Where 

 this earthy trachyte comes in contact with the prismatic, it suffers the same 

 alteration as the neighbouring conglomerate, being converted to a certain 

 distance into green pitchstone, or more frequently into a pearlstone, which 

 exhibits superficially a kind of bloom similar to that of a plum, and contains 

 some white and cellular parts. 



The alternations and mutual interruptions of the rocks above described will 

 be observed, from the plates, to be repeated throughout the Southern half of 

 Ponza. Towards the Northern extremity the prismatic trachyte almost entirely 

 disappears. With the exception of a pyramidal mass of rock which forms the 

 extreme point called the Punta dell' Incenso, opposite the Islet of La Gabbia, 



VOL. II. — SECOND SERIES. 2 E 



