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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



tiary blue marls ; the total thickness of this capping is about 80 or 

 100 feet. This elevated plateau is nearly 2000 feet above the 

 level of the sea. To the W. and N.W. it presents a long extent 

 of steep escarpment, 80 or 100 feet high, near the northern extre- 

 mity of which is a deep and precipitous ravine, called Le Baize, in 

 which extensive land-slips are constantly taking place, and where, 

 during the last 150 years, many houses and churches have been 

 engulphed by the gradual working back of the cliff. A spring of 

 water issues at its foot, between the sandstone and the blue marl, 

 which, acting on the lower arenaceous beds, combined with the 

 effect of weathering and heavy rains on the blue marl itself, has 

 undermined the cliff, and caused its fall into the hollow below. 

 The following is a section of this part of the formation : — 





Shelly limestone. 



Sand. 

 Limestone. 



Sandy limestone. 



Sand, limestone concretions. 



Arenaceous tuff. 

 • Blue marl and sand. 



Blue marl. 



The upper bed of shelly limestone is full of Cardium, Pecten, 

 Ostrea, &c, but in a very comminuted state. In the arenaceous 

 beds they are less abundant, but the nodules or calcareous concre- 

 tions are full, and in fact are sometimes quite composed of them. 

 In some places the separation of the limestone from the sand is 

 not complete, and the whole bed consists of an arenaceous lime- 

 stone. At a short distance from Volterra to the E. S. E. is another 

 hill called Poggio alia Rocca, consisting, like the former, of blue 

 marl capped with limestone and arenaceous beds, but not rising to 

 the same height. Marine fossil shells are very abundant in many 

 parts of this formation. 



This upper formation or capping is evidently a portion of that 

 which, occurring on the summit of many hills in this district, has 

 received from Savi the name of Panchina, and which he considers 

 as the result of local submarine springs depositing calcareous mat- 

 ter in the neighbourhood of their sources. But it is of so general 

 a character, and occurs in so many districts under the same cir- 

 cumstances, that I cannot agree to such a partial explanation. 

 Many of the hills S. of the Cecina, and particularly in the neigh- 



