18 Account of the 



They are composed of sand, clay, and marl, and of a solid rock 

 called Travertino. The sand is of a yellowish colour, and is in 

 many places very calcareous. The clay is generally also calca- 

 reous, and may properly be called an argillaceous marl : it is of 

 a yellowish-grey colour, contains scales of mica, and small frag- 

 ments of pyroxene, a common mineral among volcanic products. 

 These deposits all contain lacustrine shells, particularly those 

 found in stagnant waters. 



The travertino, from its importance, deserves a more particu- 

 lar description. 



If water be highly charged with carbonic acid gas, it acquires 

 the property of dissolving carbonate of lime, i. e. limestone, so 

 as to hold a considerable quantity in solution. If water thus 

 charged be exposed to the open air, and especially if the surface 

 be increased, as is the case when streams are broken by a cataract, 

 the carbonic acid gas escapes, and the calcareous matter which 

 it enabled the water to hold in solution is deposited. Most pe- 

 trifying springs are of this nature, and whatever objects are pre- 

 sented to the water, are more or less speedily encrusted. The 

 calcareous matter, in certain cases, forms a solid crystalline mass^ 

 which sometimes cannot be distinguished from statuary marble 

 either in grain, colour, or composition, as is the case with that 

 found in the neighbourhood of Civita Vecchia, which is as close 

 and crystalline in its texture as Carrara marble. 



A vast number of these calcareous springs occur in Tuscany, 

 in many parts of which the whole ground is coated over with 

 the deposit. In other places, in the same country, compact 

 rocks are seen descending the slanting sides of hills, very much 

 in the manner of lava currents, except that they are of a white 

 colour. At St Vignone, near Kadicofani, there is a spring 

 which has deposited a vast series of strata, to the depth of 200 

 feet, and so compact as to form an excellent building stone, of 

 which enormous blocks have been raised. Near the same place, 

 are the celebrated thermal springs and baths of San Filippo, 

 where the water is so highly charged with calcareous matter, 

 that a hard stratum of stone, of a foot in thickness, is obtained in 

 four months ; and there is a mass of stone a mile and quarter in 

 length, one-third of a mile in breadth, and 250 feet thick in 

 some places. But one of the most remarkable places in which 



