804 Dr Brewster on the Structure and 



bility, the edges of the crystal thus formed would readily be- 

 tray its unnatural origin. 



The only supposition, therefore, which can be for a mo- 

 ment admitted, is, that crystals of Humboldtite, (or some un- 

 known mineral of the same form as Haytorite,) were imbed- 

 ded in some matrix or substance capable of receiving smooth 

 and perfect impressions from their crystalline faces ; that these 

 crystals of Humboldtite have been subsequently decomposed ; 

 that their ingredients had escaped so entirely, as to leave their 

 moulds free of any material substance ; and that fluid chal- 

 cedony, or chalcedony in a state of solution, had found its way 

 by injection, infiltration, or otherwise, into these moulds, and 

 filled them up. In this way there might be formed crystals 

 of chalcedony like Haytorite, having the form of Humboldtite, 

 or some unknown mineral. But as the chalcedony must have 

 been introduced into the mould by some orifice of sensible 

 magnitude, through which also the Humboldtite had escaped, 

 it is obvious that this imperfection ought to appear at some 

 point of the crystal. We shall not, however, avail ourselves 

 of this objection, nor of other difficulties which very readily 

 present themselves. We shall freely admit that the Humboldt- 

 ite escaped from the small orifice, and entirely disappeared ; 

 that the chalcedony insinuated itself, without leaving any tra- 

 ces of its passage ; that Humboldtite, or some unknown mi- 

 neral of the requisite form, though now extinct in Devonshire, 

 had once existed, and, consequently, that isolated crystals of 

 Haytorite may thus have been modelled on the forms of de- 

 parted minerals. 



But how will this admission, ample and liberal as it is, ac- 

 count for the formation of the crystals of Haytorite as they 

 actually occur in nature ? In the specimens which you have 

 sent me, the crystals of Haytorite do not exist in an insulated 

 state, surrounded by any matrix in which the Humboldtite 

 could have formed a mould. They are actually aggregated 

 closely together, and when they are detached, the faces which 

 Jiave been in contact are perfectly crystallized, without a trace 

 of the interposition of any foreign matter. Such crystals 

 could not possibly be formed in the manner above-mentioned, 

 nor, indeed, upon any hypothesis however extravagant ; and 



