220 NATURAL SCIENCE. Sept., 



as follows : A trachytic paste occupied a volcanic duct towards the 

 surface. During a considerable period of repose the upper portion 

 absorbed much water, its temperature fell, and its tension rose in 

 proportion ; lower down, less change of this nature had taken place. 

 When the final outburst occurred, the vent must have been of 

 tremendous size, as proved by the enormous quantity of material 

 ejected. In consequence, the lower part rose almost coincident with 

 the upper, as shown by the arrangement of the deposit. The upper 

 cooler, more aquiferous part, was soon reduced to a solid dust by vesicu- 

 lation, and fell as such, while the hotter, lesser aquiferous, was only 

 separated into cakes. The heaviest and most compact of these would 

 fall amid the dust as hot masses, close to the vent where they would 

 spread out by their own weight and by the pressure of other material 

 that immediately buried them. They were squeezed out by a movement 

 hardly amounting to flow, but just sufficient to elongate them where 

 they rested on an inclined surface. The greater the inclination, the 

 more are they pulled out, so that in the buttress under Camaldoli, 

 which shows the bed fairly inclined, the flackers are reduced to that 

 of the thinnest paper. 



The top and bottom of each piperno bed are less compact, and have 

 less of the enclosed flackers of lava, for very obvious reasons, too 

 lengthy to detail here. Even in a single bed of piperno, the variation 

 in size and numbers of the flackers varies greatly with the horizon, 

 but such variations are continuous over considerable distances. 

 These differences of structuie really record the fluctuations in the 

 force of the eruption and the temporary predominance of one or other 

 of the two kinds of ejecta. 



Near the vent little dust fell, because it reached great altitudes 

 and was carried away by the wind. The heavier lava cakes, 

 however, fell by preference near the vent, especially the more com- 

 pact and heavier ones ; more and more scoriaceous, and therefore 

 lighter were they, the farther they travelled, and the colder were they 

 when they fell, so that at a short distance they were too hard to 

 flatten out. At still greater distances, however light and scoriaceous 

 they were, the large, then smaller and smaller ones, fell, until finally 

 the deposit is practically composed of the dusty part only. 



The piperno at Pianura consists of several beds intercepted by 

 brecciated fragments of trachyte of the same composition, which, no 

 doubt, mark an interval during which a crust solidified, to be 

 re-broken up by the fresh outburst that formed the next bed. 



I have spoken of one vent, but I do not deny the possible exis- 

 tence of a long fissure that reached the surface at several points. 

 Neither should I dare to attribute certain pipernoid tuffs, south of 

 and near Rome, to the vent, or the main one if several, that opened 

 between Monte Spina and Camaldoli. At Monte Spina the blobs of 

 lava were so large and so slow in cooling that they actually flowed 

 as a stream. 



