Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/95

 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 perfect 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 reconciling 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 without 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