§ 34. INCRUSTING SPRINGS. 75 



portion of carbonic acid be added, carbonate of lime is 

 formed, and precipitated in an insoluble state. If, however, 

 the carbonic acid be in such quantity as to supersaturate 

 the lime, it is again rendered soluble in water ; and it is 

 thus that carbonate of lime, held in solution by an excess 

 of fixed air, not in actual combination with the lime, but 

 contained in the water and acting as a menstruum, is com- 

 monly found in all waters. An absorption of carbonic acid, 

 or a loss of that portion which exists in excess, will there- 

 fore occasion the calcareous earth to be set free, and pre- 

 cipitated on any substances in the water, such as stones, 

 twigs, and leaves of trees, &c. Some springs contain so 

 large a portion of calcareous earth when they first issue 

 from the rocks, and so speedily throw it down in their 

 course, that advantage has been taken of the circumstance 

 to obtain incrustations of various objects, as leaves, branches, 

 baskets, nests with eggs, and even old wigs. The incrust- 

 ing springs of Matlock and other places in Derbyshire are 

 celebrated for such productions.* The substance deposited 

 is termed tufa, or travertine; and in Italy, and many other 

 countries, it constitutes extensive beds of concretionary 

 limestone, which is often of a crystalline structure. The 

 Cyclopean walls and temples of Paestum, are formed of 

 travertine. At the baths of San Filippo, in Tuscany, 

 where the waters are highly charged with tufa, this pro- 

 perty is applied to a very ingenious purpose. The stream 

 is directed against moulds of medallions and other bas- 

 reliefs, and beautiful casts are thus obtained. 



34. Incrustations. — As specimens of this kind are 

 commonly, but erroneously, termed petrifactions, it will be 

 necessary to explain their real nature. We have before us 

 several incrustations from various places : baskets of shells, 



* An account of the incrusting springs at Matlock is given in 

 " Medals of Creation," vol. ii. p. 940. 



