84 [Assembly 



the crevices of the limestone rocks, by the carbonic acid which 

 it contains, dissolves the lime ; and on coming again to the light 

 and air in springs, re-deposits it in the form of tufa, a drab- 

 colored mass which is nearly pure lime. This, as it gradually 

 forms, incrusts the leaves, sticks, etc., with which it comes in 

 contact ; and often, as they decay, replaces them in such a man- 

 ner as to present the same form and structure ; pieces of wood 

 being thus replaced or substituted by a stony mass closely re- 

 sembling the original substance. 



