Sir R, I. Mui-chison on the Vents of Hot Vapour in Tuscany. 61 



and they fall to pieces exactly like " basalte en boule." But besides 

 this rock, which is the "gabbro " proper, and plays the same part in 

 relation to the sedimentary strata as the granitone and serpentine and 

 other adjacent rocks of a similar origin, there is a rock also called 

 "gabbro rosso " by Savi and Hamilton, which abounds on the eastern 

 flank of ]Moute Catini and other places, which is not only totally dis- 

 similar in composition and form from that which has been described, 

 but which I admit is clearly a metamorphosed stratum. 



This is a jaspidified red and green calcareous schist, marked by 

 numerous thin laminae of deposit, which is evidently nothing more 

 than the argillo-calcareous portion of the alberese or macigno forma- 

 tions, which happened to be contiguous to the true gabbro when the 

 latter was erupted. For it is plain that the amorphous gabbro (as 

 seen in a very clear natural section) has twisted back these finely 

 laminated jaspideous strata jto„te . ^^^ 



upon themselves at a point of catini. -cig- 5. „ 



eruption, as seen in fig. 5. 

 That in perforatuig, bending 

 back, indurating, and disloca- 

 ting the schist, the intrusive 

 matter should have communi- 

 cated its colour, and to some a b c 



extent its mineral composition, «• Gabbro rosso, a. Altered red schist, c. Alberese. 



to the argillaceous and calcareous strata thus affected by it, is nothing 

 more than must be looked for, and is indeed frequently found to be the 

 case under similar geological conditions. This appearance of transi- 

 tion, from what must be granted to be true altered sedimentary layers 

 into the amorphous spheroidal " gabbro," has led Savi and Hamilton 

 to think that the spheroidal red gabbro is simply a still more highly 

 fused or altered accumulation of the same aqueous matrix. AVhen, 

 however, we recede from the immediate point of contact, we have 

 not only very different forms in the matrices of the altered and the 

 eruptive rocks, but an essential difference of composition and struc- 

 ture. Pilla has indeed cited instances just as notable of the conver- 

 sion or metamorphosis of the strata by gabbro rosso, as by granitic, 

 pyroxenic, and porphyritic rocks*. One of those examples is seen 

 in the spot called Botro del Ribuio near Serazzano, where the sphe- 

 roidal "gabbro rosso" has thrown the strata of macigno into a 

 vertical jjosition, and has changed them into jaspers of blood-red 

 colour, higlily charged with silex and oxide of ironf. 



If, indeed, the argument about transitions from the rock which has 

 been the agent of alteration into the strata which are altered, be ad- 

 mitted, we must re- open elementary questions in the ]jhysics of gep^^ 



* III lii8 ' Uichezza Minerale ilella Toscana,' Pilla unites the " gabbro rosso" 

 with tlie otlier ophiolitic or serpentine rocks, which having acted as partial cen- 

 tres of elevation and eruption, rise up as conical, elongated, and rugged mounts, 

 detached from one another (p. .30). He describes the copper of Monte Catini as 

 lying in a true vein, which has tiie peculiarity of being contemjioraneous with tlie 

 associated gabbro, both of which are posterior to the sedimentary strata (p. 40). 



t Trattato di Geologia, Part I. p. 510. 



