200 On the Vents of Hot Vapor in Tuscany. 
Federigo, and St. Ippolito. These places are all situated in that 
elevated northern portion of the Tuscan Maremma which lies on 
the left bank of the Cecina. Thence the affluents of that river 
(the Pavone, Posera, Trossa, and Sterza) flow northwards ; whilst 
the Cornia and its feeder the Melia run down to the Mediterra- 
nean in a westerly and southerly direction. The tract, penetra- 
ted at intervals by the hot gases, hasa length of about eight geo- 
graphical miles from N.N.W. to 8.S.E., and a breadth of about 
five miles from W.S.W. to E.N.E.; the whole being comprised 
within 43° 8’ and 43° 16’ N. latitude. 
Subtended generally on the E. and N. by the Cecina, this 
hilly tract, which is much fissured from N. 15° W. to 8. 15° E., 
is separated on the east from the deep valley in which that river 
runs by a lofty ridge extending from Monte Castelli on the N.N.W. 
to the Gerfalco mountain on the S.S.E.; whilst another but 
lower ridge parallel to the above, is seen upon the western side 
of the gaseous district passing from Monte Rufoli to Lustignano, 
whence it slopes down to the sea-coast between Leghorn an 
Piombino. The gaseous vents occur therefore in an elevated and 
broken trough, on lines more or less parallel to the older flanking 
ridges. The general character and age of the sedimentary de- 
posits of this region have been recently explained by myself.* 
It is enough then, for my present purpose, to state, that although 
the adjacent and undulating hills and valleys abound in marls 
and sands of tertiary subapennine age, and that to the south the 
lowest member of these accumulations is charged with coal of 
miocene age, the upland tract now under consideration, and from 
which the boracic acid fumes issue, is chiefly composed of the 
rocks called Alberese and Macigno. ‘The latter containing Num- 
mulites, represents, in my opinion, the eocene, and the former be- . 
longs to the cretaceous system. Professor Pilla enumerates, 
_ indeed, cretaceous fossils found in these hills, whilst the still 
higher ridge on the east of the tract which terminates southwards 
in Monte Gerfalco, as well as the ridges of Monte Calvi and Cam- 
piglia on the west, are both of jurassic age, the Ammonites Co- 
nybeari, Sow., and A. costatus, Schith., occurring in them. 
_ All these sedimentary rocks, from the jurassic to those of the 
eocene group inclusive, have been penetrated, and for the most 
part much altered, by igneous or plutonic rocks, the greater num- 
ber of which have a serpentine character, their prevailing direc- 
tidh being equally N.W. by N., and S.E. by S. Upon entering 
1 elevated tract from the north, I found that its chief tow”, 
cia,t was situated on a plateau of shelly, tufaceous, yel- 
ae a 
‘aa 
ae rterly Journal of the Geological Society, vol. v, p. 276, et seg. . 
__ + Pomarancia is the chief residence of Count Lardarel, the spirited and hospita- 
ble proprietor of the boracic acid establishments. 
