1850.] MURCHISON VENTS OF HOT VAPOUR IN TUSCANY. 369 



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 Ger- 

 falco, as well as the ridges of Monte Calvi and Campiglia on the west, 

 are hoth of Jurassic age, the Ammonites Conybeari, Sow. and A. cas- 

 ta tus, Schlth. 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 number of which 

 have a serpentinous character, their prevailmg direction being equally 

 N.W. and by N., S.E. and by S. Upon entering this elevated tract 

 from the north, I found that its chief town, Pomarancia*, was si- 

 tuated on a plateau of shelly, tufaceous, yellowish, sandy marlstone 

 — in parts a travertine. This band clearly overlies the subapennine 

 marls of the adjacent hills and valleys on the north, in which the 

 rock salt and springs of Volterra occur, and is probably of the same 

 age as the uppermost yellow marine ' panchina ' of Tuscany, or as 

 the lacustrine deposit in the valley of the Elsa, which I have alluded 

 to in a previous memoir f. Charged with land and freshwater shells, 

 this rock is disposed in horizontal masses, and denuded into abrupt 

 escarpments, which in the middle ages formed the natural defences 

 of the old feudal town. This tertiary deposit occupies the tract 

 between the picturesque heights of Rocca Sillana on the east, the 

 hot springs of S. Michele on the west, and Monte Cerboli on the 

 south, where rocks of serpentine and gabbro rise up through strata 

 of whitish grey alberese limestone and some contiguous schists and 

 sandstone. It is near the junction of the intrusive rocks of serpen- 

 tine with the depositary strata, which are there much contorted and 

 broken, that certain hot springs appear, four of which, at Monte 

 Cerboli, have recently been made known and their contents analysed 

 by Professor Targioni Tozzetti|. His observations and analysis 

 are of geological importance, inasmuch as they show that those 

 springs which appear at intervals between Monte Cerboli (Mons 

 Cerberus ?) and Lardarello, where the vapours issue, define a line, as 

 he says, from N. to S. (but accurately N. and by W., S. and by 

 E.), and that essentially the springs contain, though in different 

 proportions in each, the same ingredients as the lagoni or vapours 

 to the south of them. Thus, exclusive of organic and bituminous 



* Pomarancia is the chief residence of Connt Lardarel, the spirited and hospi- 

 table proprietor of the boracic acid establishments. 



t Quart. Journ. Geol. Soc. Lond. vol. v. p. 294. 



t Delle acqne-termo minerah de Monte Cerboli. Firenze, 1846. Estratta della 

 Gazzetta Toscana delle Scienze Medico-fisiche, An, 4. 2. 1. In this memoir the 

 reader will find indications that the vapours of boracic acid had no issue in the 

 beginning of the sixteenth century. See also notice of this tract by the early 

 geological traveller Targioni Tozzetti (Viaggi), whose descendant, the living Pro- 

 fessor of Chemistry in Florence, has also published analytical descriptions of the 

 waters of Mont Alceto, ilapolano, Monte Catini, Castrocasa, Cimiano, and Casale. 



Since writing the memoir I have been informed by Dr. Daubeny that the 

 boracic acid vapours contain nitrogen gas — thus sustaining his views on the 

 origin of volcanic action. 



