HAMILTON ON THE GEOLOGY OF TUSCANY. 287 



It would require a longer examination than I was enabled to 

 bestow on it, to ascertain the exact age of this formation ; from its 

 position it seems to form an intermediate link between the arena- 

 ceous deposits of the marine beds already described, and the post- 

 tertiary beds to which I am about to allude, being connected 

 with the former by the arenaceous beds containing concretionary 

 calcareous nodules, and with the latter by its immediate juxta- 

 position in the vicinity of Colle. On the other hand, both in its 

 mineralogical character, and in the appearance of its fossils, it 

 closely resembles the freshwater limestone of Cetona, which is 

 supposed to underlie the blue marl formation. 



3. Post- Tertiary Formations. 



The principal rocks which come under this denomination, are the 

 deposits of calcareous tuff, assuming a concretionary and at times 

 a spongy and tubular appearance, extending for several miles on 

 both sides of the valleys of the Staggia and the Elsa. This form- 

 ation has been alluded to by Mr. Lyell.* It is best developed 

 in the valley of the Staggia. Near the town of that name is a 

 horizontal formation of travertine, filling up the valley between the 

 two ranges of sand hills, and forming a narrow lacustrine plain, 

 through which the river has subsequently washed itself a bed, ex- 

 posing the horizontal layers of tuff in the bed of the river. The ex- 

 tent of this deposit, probably derived from one spring or source, is 

 very considerable, and may be traced from the town of Staggia to 

 within a mile of Poggibonzi, a distance of nearly seven or eight 

 miles. It is, however, so modern, that the shells found in it 

 appear to be quite recent, and the stems of plants and roots, round 

 which the calcareous tuff has been deposited, still retain, however 

 slightly rotten, their wooden fibre. Impressions of leaves are very 

 abundant in it. 



The beds are more than 100 feet thick in some places ; they are 

 both calcareous and arenaceous, sometimes mixed. The variety of 

 form resembles what Mr. Lyell has said "f respecting the travertine 

 deposited from the spring near the baths of San Vignone, and is 

 probably owing to a similar cause. At times the calcareous particles 

 are collected in concretionary and botryoidal masses, sometimes 

 almost stalactitic. Occasionally the beds are soft and friable, while 

 others acquire the hardness and compactness of Scaglia limestone. 

 Some of them are slightly ferruginous, and sometimes they present 

 a strange conglomeration of cylindrical tubes. In places they are 

 much contorted ; in others, horizontal or slightly undulating. 

 Species of Paludina and Valvata abound in some of the softer and 

 more friable beds. This singular formation must have been de- 

 posited by a stream highly charged with calcareous matter, which, 

 at no very distant geological period, flowed down the valley of the 

 Staggia. A similar formation occurs in the adjacent valley of the 



* Principles of Geol. vol. i., p. 398., 6th ed. f lb. p. 400. 



