DECREMENTS. 75 



of carbonate of lime by the abstraction of secondary 

 molecules similar in form to that secondary rhomboid. 



These secondary molecules would consist of certain 

 numbers of primary ones arranged in the same order 

 as they would be in the production of the entire 

 secondary crystals, and they would in fact be minute 

 secondary crystals. 



There is an interesting paper on this subject by 

 the Abbe Haiiy, in the 14th vol. of the Annales du 

 Museum, p. 290, where, in order to express the laws 

 of decrement in as low numbers as possible, he has 

 in several instances conceived the decrements to take 

 place on secondary forms. 



Another circumstance apparently influenced by a 

 cause in some degree similar to that which produces 

 decrements, is the colour which occasionally covers 

 some of the modifying planes of a crystal while the 

 other planes remain uricoloured. 



A specimen of carbonate of lime, from St. Vincent's 

 rocks near Bristol, which now lies before me, affords 

 an instance of this. 



Fig. 109. 



In this specimen the planes , , of some of the 

 crystals and those planes only^ are covered with par- 

 ticles of, I believe, oxide of iron, upon which no 

 molecules of carbonate of lime appear to have been 

 subsequently deposited; but a thin plate of that sub- 

 stance is observed on some crystals as at c, to cover 



