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



ments are water-worn^ and they all have evidently been enveloped in the 

 base immediately upon their ejection by the volcarsic impetus. They cohere 

 strongly to the matrix, but are separated from it by decided fracture lines, 

 which, as well as their variety and number, make it impossible to doubt that 

 the rock is essentially a conglomerate, in spite of the remarkable passages it 

 exhibits by insensible gradations into rocks of a different character. 



The inclosed fragments, from their superior solidity, protrude from the 

 weather-worn surface of the cliffs, and protect the line of rock, immedi- 

 ately beneath, from the rain by which the more exposed parts on either side 

 are corroded, giving rise to pyramidal or buttress-shaped masses, each capped 

 by one of the larger blocks, and standing out from the front of the escarp- 

 ment in parallel rows. When these are in considerable number, an effect is 

 produced which may be easily mistaken at a distance for the result of a co- 

 lumnar division*. The rock is on the contrary amorphous, with the exception 

 of an obscure appearance of irregular stratification, which shows itself in some 

 inasses, the larger fragments being arranged in parallel lines like the flints of 

 our chalk-beds, a character far from uncommon amongst the volcanic conglo- 

 merates. 



It has been observed, that the prismatic trachyte and this conglomerate 

 are associated confusedly throughout the greater part of the island, interrupt- 

 ing each other alternately with an irregularity of direction of which the ac- 

 companying profiles will convey a clearer idea than any verbal description. 

 The general impression arising from an examination of all their relations, is 

 that the prismatic trachyte has broken through the conglomerate from below, 

 and forms irregular intruded masses or dykes within it. On some few points, 

 however, the former rock appears to have overspread the surface of the latter, 

 and to have been in turn covered by other beds of conglomerate. 



Change observable at the point of contact of the Conglomerate with the 



Prismatic Trachyte. 



It is remarkable that throughout the numerous sections afforded of the com- 

 mon boundary of these two rocks, in the precipitous shores of the island, the 



* The process by which these buttresses are worked out is the same which produced and con- 

 tinues to augment the number of the celebrated pyramids formed out of alluvial soil near Botzen 

 in the Tyrol. The spiry and pinnacled forms assumed by conglomerates of all ages are probably 

 owing to the influence of the same causes. Even where the protecting fragment has at lenvth 

 fallen, from the gradual mining of its support, that support still continues to wear away in apyra. 

 midal form^ from the equal action of the vertical fall of rain upon all its sides. 



