Sept.. 1S93. STRUCTURE OF IGNEOUS ROCKS. 219 



Campanian plain, and even remnants of which are found filling 

 depressions in the limestone as far as Bari. This tuff much resembles 

 the piperno, except that the grey interstitial matter gets finer and 

 lighter the further we extend outwards radially from the true piperno. 

 Coincident with this, we find that the blacker cakes become more and 

 more spongy, not at first rapidly decreasing in size, but decreasing in 

 bulk specific gravity. After a certain distance, we find they also 

 decrease in size, until at distances of 20 to 30 kilometres they are 

 almost invisible. The flattening out of the cakes, parallel to bedding, 

 soon disappears as the black inclusions get lighter and more 

 scoriaceous. 



All geologists who have attempted to explain these principal 

 peculiar characters, have utterly failed to do so, and had I space to 

 enumerate many minor ones, the difficulty would be still greater. 

 Unfortunately, most of these conclusions have been jumped at, as the 

 result of that useful instrument though unfortunate misleader of 

 geology, the microscope, which has caused investigators to forget 

 that it is only one means to an end, and that field investigation is 

 of far greater importance. 



Some have supposed it to be a metamorphosed tuff, but I have 

 shown that subjacent beds of pumice are as fresh almost as the day 

 they fell ; others, have imagined it metamorphosed by infiltration or 

 some internal reaction ; others, again, maintain it to be a lava. It was 

 only after years of careful investigation that the true explanation 

 suggested itself to me. 



In my researches on the Geology of Monte Somma and Vesuvius,- 

 I was able to bring forward conclusive evidence that the volcanic 

 paste that occupied the duct of a volcano was nearly, if not entirely, 

 vapour-free, and that it dissolved H.^O out of the upper stratum of 

 aquiferous rocks of the earth crust. I showed that in any explosive 

 eruption, the uppermost part of the paste in the canal was the most 

 gaseous (H^O), and that, as the eruption progressed, the pumice ejected 

 became more compact, and cooled slower by a less conversion of its 

 potential into kinetic heat for the evolution of less gaseous contents. 



The Geological Society pubhshed my facts, refusing to do so 

 with my conclusions, but the Royal Dublin Society was more charit- 

 able and put them before the public. 3 Those facts and conclusions 

 have never been assailed, but have carefully been passed over by 

 subsequent writers on volcanoes, although they are, I maintain, the 

 only scientific and generally applicable explanation of volcanic action 

 offered to the public — not a romantic hypothesis, but one borne out 

 in every region I have yet examined. 



Now, what appears to have occurred with the pipe i)io eruption is 



2 0. jf. Geol. Soc, 1884, vol. x\., pp. 35-112. 



•'' Q. J. Gcol. Soc, 1885, vol. xli., pp. 103-106; and Scientif. Proc. Roy. Dublin Soc, 

 vol. v., n.s., pp. 112-15G. 



