depending on their Internal Changes. 89 



by the observation of the mechanical composition of the mas- 

 ses. Of itself, the hypothesis is plausible enough that such 

 was originally the case, and that the cohesion among the par- 

 ticles was so slight, as to be afterwards overpowered by the 

 greater crystalline attraction of the same particles in hemipris- 

 matic crystals, subsequently formed, and as they now apppear ; 

 in a manner analogous to the decomposition of the common 

 hydrous sulphates of zinc or magnesia by heat, as described 

 above. The other hypothesis, that the lime in the original 

 species has been subsequently replaced by the oxides of iron 

 and manganese, is rendered more likely by the fact that there 

 are crystals which in part consist of the scheelium-baryte, 

 while near the surface, but within the planes of the original 

 crystals, and where portions of them seem to be wanting, we 

 observe an aggregate of crystals of the scheelium-ore. A spe- 

 cimen of this kind I saw at Schlaggenwald, its native place. 



Here we must also consider Haytorite, a substance newly 

 discovered, which has already given rise to various and con- 

 tradictory hypothesis, and in connection with it some of the 

 pseudomorphoses of rhombohedral quartz in general. Hay- 

 torite has been ascertained by Mr I^evy to have the shape of 

 the species to which he gives the name of Humboldtite. All 

 those mineralogists who have examined it agree in pronoun- 

 cing the substance of it to be Calcedony^ which is itself a gra- 

 nular compound of exceedingly minute individuals of rhom- 

 bohedral quartz ; so much appears from its physical charac- 

 ters. Dr Brewster obtained the same result, by ascertaining 

 its action on light. He has also directed the attention of na- 

 turalists to the circumstance, that the planes of composition be- 

 tween the different individuals, and which are always so very 

 . distinct in Datolite, are as distinct as possible in Haytorite ; 

 and hence he draws the correct inference, that they cannot 

 have been formed in a mould, like the pseudomorphoses. — (See 

 this Journal, No. 1 ^, p. 297 and 301.) Datolite contains a not- 

 able quantity of silica, 36.5 per cent, according to Klaproth's 

 analysis. The successive exchange of its contents of lime and 

 boracic acid for an additional quantity of silica, if it goes so 

 far as completely to destroy the original species, will transform 

 the substance of the crystals into a mass of calcedony. There 



