HAMILTON ON THE GEOLOGY OF TUSCANY. 277 



1. Tertiary Marine Formation. 



The principal localities in which I observed this formation may- 

 be referred geographically to the following districts : — 



a. The basin of Volterra and its neighbourhood, with the valley of the Era. 



b. Leghorn. 



c. Poggibonzi. including the country from Colle to S. Casciano, with a 



great portion of the valley of the Elsa. 



d. Sienna, and the country watered by the Ombrone, extending to Buon 



Convento, S. Quirico, and Pienza. 



e. The upper portion of the Val di Chiana, and the basin containing the 



lakes of Chiusi and Monte Pulciano. 



a. The Basin of Volterra and its neighbourhood, with the Valley 

 of the Era. — This district commences on the north with the hills 

 which form the southern boundary of the Val d'Arno, near Ponte 

 d'Era, and extends S.S.E. as far as the Cecina, where it consti- 

 tutes a range of hills on the south bank of that river, resting against 

 a confused district of serpentine and scaglia limestone. On the 

 east it is bounded by the hills which separate the valleys of the 

 Era and the Elsa, and which, where I had an opportunity of 

 seeing them, consist of secondary limestone ; and on the west by 

 the hills to the S. E. of Leghorn, of which the highest point is 

 known by the name of M. Nero. In the S. portion of this region 

 rises the insulated mass of hills extending from E. to W., from 

 M. Catini to Castellina, and which is thence prolonged north- 

 wards to M. Vaso. These hills consist of the secondary formation 

 already described, lapping round masses of gabbro rosso and ser- 

 pentine ; this latter rock has protruded itself in many instances, 

 and with it the metalliferous deposits of this district are mainly 

 connected. 



The beds of winch this tertiary formation consists rise gradually 

 from the N.W., from under the alluvial formation of the Val 

 d'Arno, towards Volterra, where they attain a height of nearly 

 1800 feet. By far the greater portion of the whole thickness is a 

 stiff blue clay, called by the inhabitants matajone, throughout 

 which are disseminated many small crystals of selenite. It first 

 appears at the foot of a lofty cliff on the right bank of the Era, 

 called Ripa Bianca, under the village of Peccioli. It is overlaid 

 by a thick bed of sands, limestone, and arenaceous tuffs, varying 

 in thickness from 20 to 100 feet, the lower portions of which are 

 very friable, and contain many small calcareous nodules, arranged 

 in parallel layers. On the left bank of the Era, near Capannoli, 

 I observed in the sand several parallel beds of oysters, thickly 

 matted together, and associated with a few broken Pectens. 



The summit of the lofty hill of Volterra, affording one of the 

 most commanding positions in the country, and remarkable as 

 being the site of an old Etruscan city, gives another interesting 

 section of the limestone and arenaceous beds, which cap the ter- 



T 3 



