288 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Elsa near Colle. Other formations, referable to this same recent 

 epoch, are the extensive alluvial and the now drained lacus- 

 trine deposits of the Val d'Arno, particularly that portion known 

 as the Val d'Arno di sopra, extending from the rocky defiles of 

 Incisa, once evidently the barrier of these inland lakes, through 

 S. Giovanni, Montevarchi, and Levane, to Arezzo. This forma- 

 tion consists chiefly of denuded hills of gravel, sand, and yellow 

 marl, resting against the secondary sandstones, and in which have 

 been found the abundant fossil remains, chiefly of mammalia, 

 which now form the riches of the inuseums of Montevarchi and of 

 Arezzo. They include two varieties of Elephant, two species of 

 Mastodon, remains of Rhinoceros, Hippopotamus, Felis, Cervus, Bos, 

 Antilope, Equus, Ursus, and also some long plates of a Tortoise. 

 They are so abundant in places, that the peasants have used them 

 to mark the boundaries of their property, or to line the banks of the 

 streams. The tusks of the elephant are enormous ; the fragment 

 of one at Montevarchi is 12 feet long, and nearly 2 feet in circum- 

 ference. 



Another recent deposit remains to be described in the neigh- 

 bourhood of Chianciano. This place is famous for its hot baths, 

 which are visited by strangers from all parts of Italy ; near the 

 source of the thermal spring is a curious natural basin of tra- 

 vertine, about 8 or 10 feet deep, and of an oblong shape. The 

 natives believe it to have been excavated in the travertine, and then 

 crusted over ; but it is clearly the result of a natural deposit, formed 

 by some abundantly calcareous spring, which has successively raised 

 the walls of the basin or reservoir in which it rose. It has thus 

 formed a perfectly level bank all round, on which circular lines or 

 ripple marks are distinctly visible, showing the process of the 

 formation. An irregular mass of the same 'matter extends some 

 way down the hill where the water escaped, deposited by the 

 overflowing of the pool : it is rather remarkable that no deposit 

 whatever is left by the present hot spring, in which the ther- 

 mometer rises to 31° Reaumur. 



B. Metamorphic Rocks. 



I now proceed to describe the metamorphic rocks which came 

 under my notice in those portions of Tuscany which I visited. 



The only rock of this character is that called gabbro rosso 

 by Savi. The origin and nature of this rock has been a matter 

 of great uncertainty and dispute amongst geologists, both as to the 

 period to which it belongs, and as to the source from whence it 

 has been derived. It is here almost invariably of a reddish- 

 brown colour, and is found in immediate contact with the ser- 

 pentine or ophiolitic rocks, which abound in some districts. 



The definition of this rock given by Savi in his memoir on the 

 physical constitution of Tuscany is so correct, that I cannot do 

 better than give an English version of his statement : — 



" We understand by gabbro rosso, or red gabbro, a class of 

 " rocks produced by a particular alteration of the soil of the macigno, 



