284 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



all probability, derived. On most of the hill tops, oyster beds 

 and pectens of large size are found. In some of the deep ravines 

 and glens, beds of blue marl are perceived underlying the sands 

 and gravels, but not developed to any great extent or thickness. 

 Near the 21st mile from Florence, extensive beds of blue marl are 

 seen in the ravines N. W. of the road, with a perfectly horizontal 

 line of separation between them and the overlying sandstones. In 

 general, however, the whole surface of the hills, the summits, and 

 the valleys are covered with the sands and conglomerates ; and it 

 is still doubtful whether or not the valleys were already hollowed 

 out in the blue marl previous to the deposition of the sands and 

 gravels on its undulating surface. I am rather inclined to adopt 

 this view, as in the deep valley of the Pesa the great masses of 

 overlying gravel and conglomerate reach to the bottom of the 

 valley, and much lower than where the blue marl occurs in other 

 places. At all events, we may trace from San Casciano to Poggi- 

 bonzi, and perhaps even to Volterra itself, a gradual change in the 

 form of the various elements deposited during the tertiary epoch ; 

 indicating a greater facility of being held in suspension and trans- 

 ported by water : although, perhaps, this will only apply to that 

 portion of the formation which overlies the blue marls. Near San 

 Casciano, beds of conglomerate abound. As we proceed S., the 

 pebbles become smaller, and gradually disappear, being replaced 

 by a fine compact arenaceous tuff, which occurs in the neighbour- 

 hood of Poggibonzi and Colle ; and further S. and S. W., the 

 upper beds become still finer and of a more comminuted character. 



d. Sienna District. — The fourth district of the tertiary marine 

 formation commences a little to the north of Sienna, and extends, in 

 a S. E. direction, through the country watered by the Arbia and 

 the Ombrone by Buon Convento to beyond San Quirico and Pienza. 

 Its southern limits 1 did not ascertain ; but to the N.E. it is bounded 

 by the secondary hills which separate it from the Yal di Chiana 

 and the district of Monte Pulciano. It consists, for the most part, 

 of blue clay or marl, remarkable for its sterility and bleak appear- 

 ance. It contains, here and there, a few crystals of selenite, but 

 a most careful search, in numerous localities, did not produce a 

 single fossil shell. In many places it is capped by beds, varying 

 in thickness, of yellow arenaceous limestone and sand, some of 

 which are full of marine testacea, constituting entire masses of 

 Ostrea and Pecten. 



These elevated table lands, wherever I observed them, have in 

 early ages been invariably made use of by the inhabitants, as 

 affording safe foundations for their cities, which would be sought 

 for in vain on the soft and yielding masses of the subjacent marl. 

 On such table lands are built the towns of Pienza, San Quirico, 

 Montalcino, Sienna, and probably many others. Pienza is situated 

 near the extremity of a narrow, rocky, peninsula, of which the 

 upper bed consists of a compact, agglomerated, calcareous sand- 

 stone, having a slight inclination to the S. W. Immediately below 



