450 SECONDARY LIMESTONE. 



The nature of such admixture is only ascer- 

 tained by chemical analysis ; but it is accompa- 

 nied by certain external characters by which it 

 is afterwards recognized without that assistance. 



A. Calcareous carbonat, with magnesia, chiefly. 

 Magnesian limestone. 



a. Massive. 



b. Laminar, and flexible when moist. 



Sunderland. 



The proportion of magnesia amounts to 

 twenty per cent., or more. The lustre exceeds 

 that of common limestone : the fracture is granu- 

 lar and somewhat arenaceous. That of England 

 (well known) is of a dull ochry yellow colour. 



B. Calcareous carbonat, with a conspicuous pro- 

 portion of clay intermixed : contains also some silica. 



Aberthaw limestone. Lias limestone. Oc- 

 curs in beds among the series of the same name. 



a. Massive: sometimes in laminae divided 

 so minutely by clay or shale as to be 

 nearly schistose. 



As the proportion of clay is sometimes very 

 considerable, it often effervesces with difficulty, 

 flies to pieces in the fire, and will not burn so as 

 to slack in water: if the heat is excessive, it 

 forms a slag. The lime produced from some of 



