Mineralogy of Sky, 87 



ones. By a series of gradations the radiated structure disappears, 

 while the mineral acquires additional toughness, verging in its. 

 aspect first to chalcedony, and lastly towards chert ; while in some 

 cases it would be difficult to distinguish it, without trial of its 

 hardness, from the white limestone of the north of Ireland : in 

 this state it is not scratched by hard steel, while its toughness is 

 such that a heavy hammer makes no more impression on it than it 

 would on a similar mass of iron. The last transition is into a per- 

 fect chert, scarcely to be distinguished from those which in other 

 situations occur in trap, and which are so frequently to be seen in 

 those traps where nodules of calcareous spar and of chalcedony 

 are found together. 



If we were to reflect on the causes of this gradual change, we 

 should attribute it to the successive diminution of the proportions 

 which the other constituent earths of this mineral bear to the silica 

 which it contains. I need not point out the difficulty of recon- 

 ciling such a supposition to the general theory of mineral species 

 and of definite proportions, since mineralogists are already aware 

 of it, and since many other cases, attended by similar doubts, are 

 well known. It is a question too important to be discussed with- 

 out much more numerous and better established facts than those 

 which we yet possess, and it will hereafter become an object of 

 serious investigation to mineralogists, when their science shall have 

 made further progress. 



That variety which is called mealy is also here presented under 

 different aspects, by which its nature is in some measure illustrated. 

 This condition has, I believe, been generally attributed to the loss 

 of its water of crystallization, the result of decomposition. It is 

 obvious however that this is not the cause, since the specimens of 

 this variety are found in the centre of solid nodules, of the glassy 



