294 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



light to dark yellow near the edges. These are the parts most 

 sought after by the lapidaries of Florence, on account of their 

 beautiful shadings, and the effect they produce in the " pietra 

 dura " works : other portions are more transparent, consisting of 

 agates, cornelians, and opalines of various colours. 



These silicious masses, as I observed, are situated in the soft, 

 decomposing serpentine, in which, on each side of the principal 

 dyke, are several thin, narrow filaments of silicious matter 

 (chalcedony), generally of a reddish colour, and parallel to the 

 principal mass. 



With regard to the origin of this curious variety of chalcedonic 

 formation, my first impression was, that it must have been a quartz 

 dyke, rising up through the serpentine ; but the variety of colour 

 and character which the rock exhibits, incline me rather to at- 

 tribute it to an aqueous origin. It might be considered as the 

 result : -pf a thermal spring, charged with silicious matter which has 

 been deposited in fissures of the serpentine, unless the occurrence 

 of the remarkable sojfioni or vapours of Monte Cerboli, in such 

 close vicinity, and that of other similar phenomena in the same 

 district, render it more probable that it is owing to the escape 

 of vapours charged with silicious and other matters ; when the 

 silex, as the least soluble, would be first deposited on any 

 sensible diminution of temperature. The subject is one of con- 

 siderable interest, as connected with other geological phaenomena 

 in the country. 



Between the village of Monte Gemoii and the high and lofty 

 position of Libbiano, the river Trossa has forced its way through 

 a narrow pass between two masses of serpentine of the same 

 character as that of Monte Rufoli. At Libbiano itself, the ser- 

 pentine and greenstone rocks have been elevated to a considerable 

 height, carrying with them to the summit a conglomerate of the 

 pre-existing j rocks, gabbro rosso and Apennine limestone, min- 

 gled with masses of serpentine ; these cover the north-east flank of 

 the hill, and give a remarkable appearance to the elevated narrow 

 ridge on which this desolate village has been perched. 



Another interesting locality where the serpentine occurs is 

 near the village of LTmpruneta, about six miles south of Florence. 

 The village stands on a hill of green serpentine which crops out 

 even in the market-place, though the principal mass of it is about 

 half a mile further south. It rises up through the secondary 

 rocks, which dip off from it on all sides, more or less altered, and 

 in some places reduced to the metamorphic state of red gabbro, 

 in which all traces of stratification are obliterated. 



A new road has been cut through the solid serpentine, leaving 

 a cliff fifty or sixty feet high on each side. Here the rock appears 

 darker and blacker than on the surface, but equally susceptible of 

 decomposition on exposure to the air. Besides the numerous 

 crystals of a pale green colour, which distinguish the serpentine 

 of LTmpruneta, many portions are traversed by thin veins of 

 calcareous spar ; others are beautifully marked with red veins, and 



