

SKY. MINERALS. 407 



The next of these minerals is nadelstein, which is found 

 in three states, a compact, a mealy, and a crystallized 

 form. Of these, the compact kinds sometimes recede so 

 far in character from the mineral in its most acknowledged 

 forms, that the names of those which occupy the dis- 

 tant points of this range can only be determined by 

 tracing the gradation of the several varieties. The 

 opaque whiteness, the toughness, and the radiated dispo- 

 sition of those which may be considered as forming the 

 first remove, serve to connect them with the best charac- 

 terized specimens. By a series of gradations the radiated 

 structure disappears, while the mineral acquires additional 

 toughness, verging in 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 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 w6uld on a similar mass of iron. Where the tran- 

 sition into chert is most complete it is scarcely to be distin- 

 guished from those cherts which in other situations occur 

 in trap, and are so frequently to be seen in those traps 

 where nodules of calcareous spar and of chalcedony are 

 found together. Lastly it passes into pure quartz, although 

 there is often a slight boundary discernible between this 

 mineral and the chert that preceded it. 



The causes of this gradual change may be attributed 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 mi- 

 neral species and of definite proportions, since mineralogists 

 are already aware of it, and since many similar cases, 

 attended by the same difficulties, 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 



