SECONDARY LIMESTONE. 451 



the varieties, hardens suddenly when wetted, like 

 tarras cement. The stone used in lithographic 

 engraving belongs to this place. The colours 

 are generally various tints of grey, from pale to 

 nearly black. It is also occasionally of a dull 

 white. The texture is more or less minutely 

 granular, or uniform and smooth. 



b. Concretionary : small spheroidal : bo- 



try oidal. 



c. Concretionary : large spheroidal : ge- 



nerally oblate, and sometimes at- 

 tached in pairs by a cylindrical bar. 



Occurs imbedded in the massive : in Rasay 

 and Sky. 



C. Calcareous carbonat witb a considerable pro- 

 portion of oxyde, or rather rust of iron, as well as of 

 clay and 'silica. Spheroidal, flattened. Often divided 

 internally into prisms by calcareous spar : septaria. 



This is imbedded in the blue clay of England. 

 Produces water cements, like some varieties of 

 the preceding. 



D. Calcareous carbonat with notable proportions 

 of clay, silica, and oxydes of iron. 



Occurs in the lowest red sandstone. Is fre- 

 quently red, yellow, or mottled of various colours, 

 or grey, or even white, and scarcely burns to lime. 



G G 2 



