374 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mar. 27, 



gases. Again, these hot gases still produce, though on a small scale, 

 those conversions or metamorphisms of the strata on the sides of their 

 escape, which the geologist can well understand to have operated more 

 largely and powerfully at that period when great masses of serpentine 

 and other igneous rocks were erolyed (or strove to be evolved) under 

 enormous pressure, through the younger secondary and oldest ter- 

 tiary deposits. At Lardarello, for example, small portions of the ordi- 

 nary alberese limestone have been and are still converted into sulphate 

 of lime by the action of sulphuric acid fumes ; and the schistose 

 calcareous shale is baked by the intense heat into brittle porcelain 

 rock of a red colour. But I would here obsen'e, that in these recent 

 and partial metamorphoses by natural causes, as in those of ancient 

 date, traces of the original lamination or stratification are nearly 

 always perceptible in the lumps or masses so affected or altered. 



Gabbro Rosso. — The last observation leads me to offer some re- 

 marks on the nature and origin of the " gabbro rosso " of the Tus- 

 cans ; for after an attentive examination of this rock throughout the 

 tract immediately to the north of the boracic acid country, I feel 

 compelled to express my dissent fi*om the opinion of Professor 

 Paul Savi, in which my friend Mr. W. J. Hamilton in his descrip- 

 tion of the geology of Tuscany has coincided. The chief masses of 

 " gabbro rosso " lie in the tract south of Pisa, and east and south- 

 east of Leghorn, which is bounded on the north by the valley of the 

 Arno, and on the south by that of the river Cecina. The varieties of 

 this rock are instructively exhibited in the ridges of alberese and 

 macigno, which form the east and west sides of a longitudinal de- 

 pression occupied by subapennine marls, that extend from Colle 

 Salveti near the Pisan valley on the N. and by ^^., to the valley of 

 the Cechia on the S. and by E. The direct road from Pisa to the 

 Maremma is conducted along this depression. The westernmost of 

 these ridges, which forms the bold coast, south of Leghorn, containing 

 much granitone, serpentine, and other varieties of eruptive rock, also 

 exhibits, particularly along its eastern face, a good deal of the " gab- 

 bro rosso," which, as Professor Pilla informed me, obtained this 

 name from the village of '^ Gabbro," a few miles south-east of Leg- 

 horn, which is built on the summit of a conical hill composed of 

 such rocks ; — I say rocks called "gabbro," because I shall presently 

 show, that two rocks of entirely different origin have been united 

 under this one name. 



The eastern or inland ridge rises boldly up into the mountains 

 which proceed from the north of Monte Yaso to Castellina Marit- 

 tima on the south, and it is in reference to this group of hills, on the 

 eastern part of which i\lonte Catini is situated, that I specially call 

 attention, as it affords ample materials for settling the question which 

 has arisen between Professor P. Savi and Mr. AY. J. Hamilton on the 

 one hand, and the late Professor Leopoldo Pilla and myself on the 

 other. The two former have endeavoured to show, that whether in its 

 globular and amorphous form, or in its thin-bedded state*, the rocks 

 they call " gabbro rosso " are metamorphic ; whilst Professor Pilla 



* See Mr. Hamilton's translation of Savi's description, Quart. Journ. Geol. Soc. 

 vol. i. p. 288. 



