63 Sir R. I. Muvchison o?i ^^e Vents of Hot Vapour in Tuscamj. 



logy which I supposed were long ago set at rest. We may in that 

 way be led to abandon many conclusions at which we had arrived, in 

 refuting the doctrine respecting certain rocks of Cornwall, Norway, 

 and other tracts which were believed by some authors to prove trans- 

 itions from granites to slates, and thus to indicate a common origin 

 of these two classes of rock ! If this method of reasoning be again 

 entertained (as it seems to me it is by M. Savi), then many of the 

 inferences which geologists have drawn concerning the posterior in- 

 trusion of granite and other igneous rocks amid depositary strata 

 will be invalidated. For, although there are numerous examples of 

 such phfenomena, which no scejitic can assail, still there are frequent 

 cases where it is impossible to define the precise limit between the 

 erupted molten matter and the altered rock. It is indeed in the very 

 nature of the phcenomenon that such should happen, and the time of 

 practical geologists can be better employed than in disputing upon 

 such points. Some persons may indeed argue, that many varieties 

 of traps and amygdaloids were to a great extent evolved from the 

 melting of the pre-existing strata in the crust of the globe, and I am 

 quite ready to admit that such may have been the case. But this 

 admission by no means removes them from that class of true erup- 

 tive rocks which, in the eye of the geologist, have acted mechanically 

 and chemically upon the strata they have penetrated ; for even some 

 of the lavas of Vesuvius may be, in great part, fused and melted 

 materials, formerly accumulated as marine sediment, which have 

 been transmuted by intense heat under pressure. The practical j)oint, 

 therefore, for which I contend is, that the amoi-^jhous and sphe- 

 roidal " gabbro rosso" of the Tuscans is from its composition, and 

 still moxe from the geological part it has played, a true plutonic and 

 eruptive rock ; whilst the red jaspidified schists, which have been 

 also termed " gabbro," are nothing more than sedimentary strata 

 altered by the heat attending the eruption of tlie adjacent masses. 



Lines of former and j)reseHt disturbance. — As it is along the 

 lines of eruption of the serpentines, greenstones, and gabbro, i. e. 

 from N. and by W. to S. and by E., that nature has been repeatedly 

 labouring to evolve heat in the west of Tuscany, so also have the se- 

 condary rocks been alineated and altered in this direction. It is on 

 the same line that the gi'anitic rocks of Piombino have subsequently 

 uprisen, the average direction of the whole of the coast of this part 

 of Italy being parallel to it * . 



Further, it is on this line that the various Soffioni or vapour vol- 

 canos issue, and that earthquakes still most affect the surface. Those 

 who would wish fully to comprehend the phsenomena attendant upon 

 the earthquake which last agitated the west of Tuscany, and parti- 

 cularly the tracts south of Pisa, should consult the descriptions of 

 Professors Savi and Pilla. In acconlpanying the latter from Pisa 

 to Ci\ata Castellina, and in thus passing from N.N.W. to S.S.E. 

 along the depression in the subapennine marls, which lies between 



* Pareto, Pilla, and the Italian authors show that the granite of Piombino and 

 Elba cuts through the serpentinf;. 



