VIII.] COMPOSITION OF NATURAL WATERS. 121 



vert them into mineral matter. TIius, at Matlock, in Derby- 

 shire, the water flowing through the Carboniferous Limestone 

 is caused to deposit its carbonate of lime upon various 

 objects exposed to its action, and in this way the so-called 

 petrified bird's nests and other curiosities are produced. 

 Thick deposits of carbonate of lime are frequently formed 



Fig. 30. — Bridge of travertine at Clermont, Auvergne (ScropeJ. 



by calcareous springs where they issue into the air. Fig. 30 



represents a natural bridge of carbonate of lime formed 



by calcareous water at Clermont in the Auvergne, described 



many years ago by the late Mr. Poulett Scrope.^ Here, the 



^ The Geology and Extinct Volcanoes of Central France. By G. 

 Poulett Scrope, M.P., f'.R.S. Second Edition, p. 22. 1858. (By 

 permission of Mr. Murray.) 



