26o PROCEEDINGS OF THE COTTESWOLD CLUB 



granite even on the tops of hills 1, 600 feet above the 

 present river plain. Recent deposits are, however, grad- 

 ually filling up and obliterating the older features of the 

 country, and it is with these deposits that we are at present 

 chiefly concerned. 



They are of two kinds : — 



I. — Calcareous: consisting of calcium carbonate in the 

 form of travertine, corresponding to the well-known 

 deposits in Auvergne and the Campagna. 



2. — Siliceous : composed of almost pure silicon hy- 

 drates, to which, so far as I know, there is nothing of 

 similar chemical character in Auvergne. 



The origin of each is easily explained. The strata 

 underlying the sheets of trachyte and basalt consist of 

 cretaceous and Jurassic limestones; and the water perme- 

 ating these strata is highly charged with carbon dioxide, 

 one of the principal gases emitted by volcanoes. Such a 

 solution readily acts upon all calcareous rocks, the calcium 

 carbonate which they contain being converted into calcium- 

 hydrogen carlionatc a substance which reacHly dissolves in 

 water. 



Experiment. — Solution ol calcium h^'drate poured into a bottle of 

 carbon dioxide ; a white precipitate of calcium carbonate (chalk) is 

 thus formed. This precipitate, suspended in the liquid, is transferred 

 to another bottle of carbon dioxide and shaken up. The calcium 

 carbonate dissolves, and a dear solution of calcium-hvdroi^en carbonate 

 is obtained. 



Thus the water which comes to the surface and issues 

 in the form of springs contains large quantities of this 

 calcium compound, which on exposure to the open air is 

 decomposed with evolution of carbon dioxide and re- 

 formation of insoluble calcium carbonate. This decom- 

 position goes on much more quickly when the solution is 

 heated. 



Experiment. — The clear solution obtained in the last experiment is 

 poured into a flask and heated to boiling. It becomes turbid owing to 

 the precipitation of insoluble calcium carbonate. 



