PEOCEEDINGS OP THE GEOLOGICAL SOCIETY. 9$ 



come under the definition of accidental ejectaraenta. They are only- 

 ejected when the apex of the crater-cavity, formed by an explosive 

 eruption, extends below the platform of the volcano into the under- 

 lying rocks. He then traced the history of the eruptions of Somma- 

 Yesuvius through divers phases, showing that it was only at a com- 

 paratively late period that limestone-fragments were blown out, 

 though this had taken place long before the Plinian eruption. The 

 stratified limestones have been chosen for the first part of this paper, 

 because their original lithological structure acts as a guide as we 

 proceed from a normal limestone to its extreme modifications. 



Part I. — The character of the limestones which underlie the plat- 

 form of Vesuvius may be studied in the peninsula of Sorrento, 

 where the mass attains a thickness of 4700 feet. They are mag- 

 nesian in varying proportions. A table was given showing twenty- 

 seven analyses, made principally by Ricciardi, the amount of MgO 

 ranging from 1 to 22 per cent. Silica rarely exceeds 2 or 3 per 

 cent., whereas in the greater number of limestones it is absent. 

 The bituminous matter, though a powerful colouring agent, usually 

 exists in quantities too small for estimation, but sometimes reaches 

 3 per cent. Such are the materials out of which the extraordinary 

 series of silicate-compounds have been developed, and as these 

 materials of themselves could not form peridotes, micas, pyroxenes, 

 &c., it is clear that the silica, alumina, iron, fluorine, &c. must have 

 been introduced from without, viz., from the neighbouring igneous 

 magma. The Author then discussed the question of the probable 

 methods, being inclined to favour the notion of vapour in combi- 

 nation with acid gases. 



The bulk of the paper was occupied with a detailed description 

 of the microscopic structure of these stratified limestones and 

 their derivatives. The Author remarked that the same meta- 

 morphic changes may be traced on a much grander scale amongst 

 the ejected blocks, and hinted at the similarity of these changes to 

 those of contact-phenomena as seen elsewhere, and even of regional 

 metamorphism, the two main factors to be considered being the 

 composition of the rock to be acted upon and that of the magma 

 acting. 



The changes which ensue in an impure limestone are, in the first 

 place, the carbonization of the bituminous contents, which are con- 

 verted into graphite ; and a kind of recrystallization, approaching 

 the saccharoidal structure, seems to have taken place, although the 

 stratification &c. is preserved. A few grains of peridote now begin 

 to make their appearance, chiefly as inclusions within the calcite 

 crystals, and thus by degrees the results already recorded are 

 effected. In the early stages only is the metamorphism selective. 

 The order in which the new minerals seem to develop is the fol- 

 lowing : — 



(1) Peridote, Periclase, Humite. 



(2) Spinel, Mica, Fluorite, Galena, Pyrites, Wollastonite. 



(3) Garnet, Idocrase, Nepheline, Sodalite, Pelspar. 



