472 DOMAIN V. CALCAREOUS. 



of the atmosphere. The manner in which the 

 effect is produced in the latter instance may be 

 understood by a reference to what was said re- 

 specting that superficial disintegration which takes 

 place occasionally in calcareous free-stone. 



" This variety of limestone is employed very 

 generally for the purpose of covering the roofs of 

 houses ; whence it has been called lapis tegularis. 

 The property of being thus easily separated into 

 laminae depends partly upon the proportion of 

 clay contained in it (for this property is in general 

 more remarkable in proportion to the quantity of 

 clay contained), and partly upon the nature of its 

 original deposition; for the stone of some parts of 

 the quarry contains a considerable quantity of 

 minute shells, resembling millet seeds; and it 

 seems worth observation, as connected with the 

 schistose property of the stone, that the depo- 

 sition of shells is more abundant on the surface 

 than in the substance of the laminae. 



" In some instances a singular arborescent ap- 

 pearance is observable on the contiguous surfaces 

 of adjoining laminae : the colour of this is for the 

 most part black; and, from some experiments 

 that were made for the purpose of ascertaining its 

 nature, appeared to be principally manganese. 

 The same appearance is observable in some varie- 

 ties of Florentine marble. Sometimes the colour 



