306 Br. Johnston-Lavis — Vesuvius and Monte Somma. 



eruptions are of the trachytic type or phonolitic at the most, 

 "whereas those of more permanent activity, as Etna, Stromboli, 

 Vesuvius, Eoccamonfina, and some of the Eoman ones, have a 

 much more basic character.^ Whatever may really be the cause, in 

 this variation of volcanic ejectamenta in this district, it would be 

 unsafe to dogmatize ujDon, nevertheless the above facts are worthy 

 of thinking over and carefully considering. 



We have somewhat erred from our way in endeavouring to show 

 that a fluid igneous matter might be modified by the strata it 

 traverses ; it now remains to study the igneous fluid upon the 

 neighbouring solid rocks. All geologists who have studied meta- 

 morphic rocks have always recognized that the changes they had 

 undergone, had been the result of heat, pressure, and chemical 

 replacement. Perhaps one of the most interesting of such examples 

 are the series of erratic blocks ejected during paroxysmal eruptions 

 from Monte Somma. In some cases we may see that the rock 

 ejected is indistinguishable from what may be collected in the 

 Castellamare limestone quarries. From this we may trace all 

 gradations to finding the structure and composition entirely changed, 

 having its cavities lined by such minerals as leucite or pyroxene 

 that are normal elements of the igneous rock, or anorthite, meionite, 

 and many others that may result from the combination of leucitic, 

 augitic, or other material with the lime and magnesia of the Apennine 

 limestone. That such changes take place with considerable rapidity 

 is proved by the pyroxenic or micaceous crust to fragments of lime- 

 stone enveloped accidentally in lava streams, specimens of which 

 are not rare to find. 



Another fact, at first somewhat difficult to grasp, is the difi'erent 

 proportion or variety of minerals volcanic rocks may contain, yet 

 meanwhile have the same general composition. 



That simply temperature is not the sole element in determining 

 the fusion or crystallization of a mineral is well demonstrated by the 

 species leucite and sanidine. Thus leucite, according to the 

 experiments of Messrs. Fouque et Levy, crystallizes at a white heat, 

 whereas the felspars only did so at a red. Such order is also borne 

 out by many leucitic rocks, yet it is not an uncommon thing to find 

 a sanidine crystal partly or entirely enveloped in one of leucite, 

 not only as micro- but also as macroscopic crystals. This certainly 

 shows that the two minerals may follow each other in their separa- 

 tion or may increase contemporaneously, and therefore must be due 

 to some other variation than that of temperature. Is it not possible 

 that the combination of certain temperatures, pressures, presence of 

 water, etc., may favour the formation of some minerals and retard 

 others ; for why should amphibole and pyroxene replace each other 

 with so much facility ? 



Let us suppose that the conditions are suitable to the crystalliza- 

 tion of a mineral from a fused mass of mixed silicates, which differs 

 from it in the proportion of its elements. This first mineral, which 

 we will call A, will continue to crystallize until it has exhausted to 



^ Scrope seemed inclined to accept a somewhat similar idea. — Volcanoes, 1825, 

 p. 145, 



