3 



TAB. CIII. 



QUARTZUM calcareum : Far. decompo 



nens. 



Decomposing calcareous Pudding-stone, 



Class 2. Earths. Order 3. Aggregate. 



Gen. 1. Quartzum. Spec. Calcareum. 



Gen. Char. Quartz aggregated by the help of some 

 cement. 



Spec. Char. By the help of carbonate of lime. 



It often happens that siliceous aggregates decompose, espe- 

 cially when their cement is calcareous ; which is the case 

 with the present specimen. It is perhaps now first spoken 

 of, and gives us reason to suppose that the iron or colouring 

 substance is disengaged from the calcareous cement by 

 some agent capable of penetrating the inmost recesses of 

 the stones. This cement filled the cavities of the mass, 

 leaving them hollow, or with the porous remains of the 

 pebbles only filling a part of the old cavity. It has been 

 asked whether these stones are not rather forming than de- 

 composing. We answer, that the cavities are shaped as if 

 each had been filled by a whole stone, and the whole mass 

 seems to be falling to pieces. In these masses some of the 

 stones are whole, and in part soft, and may be scratched 

 by a knife, or even by the nail, while the other part is as 

 hard as a common flint pebble. Some of the pebbles 



b 2 



