MODE XII. TUFA. 50Q 



of carbonic acid*. The chalk-stones which ap- 

 pear at the joints of gouty persons, and greatly 

 resemble that substance, are now said only to 

 consist of uric acid and soda. 



The structures and aspects of chalk, are rather 

 various in minute parts, than in general masses 5 

 so that an excess of precision in this respect 

 might, as sometimes happens, only introduce 

 erroneous ideas. 



Chalk, with various shells ; from many parts 

 of England, France, and Denmark. 



The same, with the shells in flint. 



The same, with balls of pyrites divergingly 

 radiated. 



MODE XII. TUFA. 



The name of calcareous tufa, is most justly Description. 

 applied to a light and porous rock, gradually 

 formed and daily increasing by the depositions 

 of springs and streams much impregnated with 

 stony matter. It is, of course, the newest of all 

 the mineral productions ; and often contains 



* Da Costa, p. 77, says that chalk, called creta, from Crete, 

 where it is found, as it is, among other places, in the archbishopric 

 of Cologne, is used for the heartburn and diarrhoea. It is also 

 employed in manures, to cleanse metals, and in baking sugars. 



