206 On the Vents of Hot Vapor in Tuscany. 
are still converted into sulphate of lime by the action of sulphu- 
ric acid fumes ; and the schistose calcareous shale is baked by 
the intense heat into brittle porcelain rock of a red color. But I 
would here observe, that in these recent and partial metamorpho- 
ses 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 
remarks on the nature and origin of the “gabbro rosso” of the 
- Tuscans ; for after an attentive examination of this rock throngh- 
out the tract immediately to the north of the boracic acid coun- 
try, | feel compelled to express my dissent from the opinion of 
Professor Paul Savi, in which my friend Mr. W. J. Hamilton in 
his description of the geology of Tuscany has coincided. The 
chief masses of “gabbro rosso” lie in the tract south of Pisa, 
and east and southeast 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 exhib- 
ited in the ridges of alberese and macigno, which form the east 
and west sides of a longitudinal depression oceupied by subapen- 
nine marls, that extend from Salveti near the -Pisan valley on 
the N. by W., to the valley of the Cecina on the S. 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, par- 
ticularly along its eastern face, a good deal of the “ gabbro rosso,” 
which, as Professor Pilla informed me, obtained this name from 
the village of ‘““Gabbro,” a few miles southeast of Leghorn, 
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. 
be rah coolemntes Cour ale ih, 9) oe ie ores ek ON 
