TEA VERTINE. 297 



mortar or cement, and BO truly flattened that in walking 

 round the building and carefully prying, I could find no 

 crevice into which a slip of ordinary writing paper, or the 

 blade of a pen-knife could be inserted. Yet this temple 

 was an antiquarian monument in the days of the Roman 

 emperors. 



The rough natural surface of the stone is exposed, and 

 at first sight appears as though weathered, but this appear- 

 ance is simply due to its natural sponge-like structure. It 

 appears to have been coated with some sort of stucco or 

 smoothing film, which, either by forming a thin layer, or 

 possibly by only filling up the pores of the travertine, 

 gave a smooth surface upon which the coloring was ap- 

 plied. This is now only indistinctly visible here and 

 there, and if I remember rightly, some have disputed its 

 existence. 



But this travertine, though so familiar to the Italian, is 

 such a rarity here that some further description of its 

 structure and composition may be demanded. It is a lime- 

 stone formed by chemical precipitation. Most limestones 

 are more or less of organic origin, are agglomerations of 

 shells, corals, etc., but this is formed by the same kind of 

 .action as that which produces the stalactites in limestone 

 caverns. It has some resemblance to the incrustation 

 formed on boilers by calcareous water. Although the ma- 

 terial of so many ancient edifices, it is, geologically speak- 

 ing, the youngest of all the hard rocks. Its formation is 

 now in progress at some of the very quarries that supplied 

 Imperial Eome. 



On the Campagna, between Rome and Tivoli, is a small 

 circular lake, from which a stream of tepid water, that 

 wells up from below, is continually flowing. Its local name 

 is the "The Lake of Tartarus. " The water, like that of 

 Zoedone, or soda-water or champagne, is supersaturated 

 with carbonic acid that was forced into it while under pres- 

 sure down below. This carbonic acid has dissolved some 

 of, the limestones through which the subterranean water 

 passes, and when it comes to the surface, the carbonic acid 

 flies away like that which escapes when we uncork a bottle 

 of soda-water, though less suddenly, and the lime losing its 



