SECONDARY LIMESTONE. 441 



are not sufficient to enable us to decide on the 

 class of any such rock from an examination of 

 its mineral characters alone. Where organic 

 remains are not present, that difficulty can only 

 be removed by investigating the geological con- 

 nections of the rock in question; and it is from a 

 mixture of theoretical opinions on this subject 

 with superficial observations, that so many secon- 

 dary limestones have been placed in the imagin- 

 ary transition class. 



It ought not to be omitted, that among the 

 peculiar characters of this limestone, that of flex- 

 ibility has been found to exist, as it is also known 

 occasionally to do in some rare varieties of gneiss 

 and of sandstone. A peculiar magnesian limestone 

 of Sunderland, mentioned in the catalogue of 

 varieties, offers, however, as I believe, the only 

 example which has yet occurred. 



The simple minerals which enter into the 

 composition of the secondary limestones, are, 

 principally, carbonat of lime, with occasional, 

 but far inferior proportions of quartz sand, and 

 of clay. Charcoal, or carbon under some modi- 



