HAMILTON ON THE GEOLOGY OF TUSCANY. 285 



it, and overlying the blue marl, is a more sandy bed, of no great 

 thickness, full of broken shells, chiefly two species of Ostrea. 



San Quirico stands upon a similar table land, the only difference 

 being that several beds and layers of conglomerated pebbles, 

 chiefly consisting of the white alberese or Scaglia limestone, are 

 associated with the sandstone tuff. 



e. Vol di Chiana. — The fifth district is the upper or southern 

 portion of the Val di Chiana, and the basin, containing the lakes of 

 Chiusi and Monte Pulciano, extending to the frontiers of the papal 

 dominions, near Citta della Pieve. A portion of this district, low as 

 it is, is remarkable, as constituting, since the great hydraulic opera- 

 tions undertaken by the Tuscan Government, the watershed between 

 the drainage flowing into the Arno and the Tiber. Two miles 

 to the south-east of the town is the Pi an della Biffa, formerly a 

 marshy lake, across the centre of which is a dyke called L'Argine 

 della separazione, on the respective sides of which the waters 

 flow to the Tiber and to the Arno, sluice-gates being placed at 

 each end to regulate the escape of the water in the spring, and 

 to prevent the plains below, particularly on the banks of the Tiber, 

 from being flooded. 



My geological observations in this plain were chiefly confined to 

 the neighbourhood of Chiusi and the north-east flanks of the range 

 of hills on the west side of the plain, from Cetona to Monte 

 Pulciano. The hills round Chiusi, on one of the highest points 

 of which the town is built, consist of numerous alternating beds of 

 gravel, conglomerate, sandstone and blue marl ; some of these 

 beds, particularly the latter, and occasionally the sandstone, con- 

 tain numerous marine tertiary shells. In some, complete oyster 

 beds are still preserved, forming large masses of shells of con- 

 siderable thickness, which are particularly developed near the lake 

 to the east of the town. Here I observed, in an ascending order, 

 the following section : — 



1. Sandy tuff, or friable sandstone, of very great thickness, 

 sometimes containing intercalated beds of gravel. In the so-called 

 tomb of Porsenna, two or three miles north-east from the town, is 

 a band of gravelly conglomerate in this sandstone tuff, which has 

 been made use of to form the flooring or separation between two 

 tiers of excavated tombs or chambers. Most of the well-known 

 Etruscan tombs in this neighbourhood are. excavated in this rock, 

 which does not contain any fossils. 



2. Blue clay, a few feet in thickness ; organic contents un- 

 certain. 



3. Reposing on the blue clay is a very thick, solid bed of oyster 

 shells, in which are some of a most diminutive size. 



4. A thick bed of gravel and conglomerate, strongly cemented 

 together by a calcareous paste. This is, in many places, of very 

 considerable thickness, forming the capping of the hill on which 

 the town of Chiusi is built, as well as of several other eminences 

 in the neighbourhood. On an estate belonging to the bishop, near 



