9« 



OK spumous CRYSTALS. 



phate of barytes*. The origin of these foims is by no means 



equivocal. 

 P eud6<?r-stils * ^^ *^® colllttionof the Council of Mines are several quartzose 



of quartz. 



ofcak 



of quaHz. 



pseudomorplioses, of which 1 shall content myself with mentioning the 

 most remarkable. The first is borrowed from the metastatic carbonate of 

 lime, and was found at Montbrizon, in the department of the Loire, by 

 Mr. Laverriere, engineer in chief. The origin of this accidental form is 

 by no means enigmatical. It is even necessary, in order to account for it, 

 to have recourse to a sort of cementation, by which the particles of 

 quartz would gradually have taken the places of those of the carbonate of 

 lime, which before occupied the situation ; it is sufficient, that a cavity 

 left void by the calcareous spar, destroyed by any cause, served for a 

 mould to the matter of the quaitz, A piece of calamine, from Somerset- 

 shire, which is in the systematic collection of the Council of Mines, ex- 

 hibit a pseudomorphosis similar to that of the qviartz of Montbri;'.on. 

 The pseudocrystals of this ore of zinc are of a reddish brown colour, three 

 inches long, and hollow within, a circumstance in which they differ from 

 the preceding, those bemg full and compact. The crystals of metastatic 

 calcareous spar, which are sometimes found in the interior of those of ca- 

 lamine, and certain groupes of similar calcareous spar mentioned by 

 Rome de Tlsle, part of which is still in the state of carbonate of lime, 

 while the rest is in that of o-sidft of zinc, leave no doubt respecting the 

 origin of this pseudoinorpliosi*. 



The department of the Saone and Loire, and that of the Nlevre, visited 

 by Mr. Champeaux, have aftorded a variety of psoudomorphoses of a 

 quartzoss nature. These forms, all borrowed from atidiferous substances, 

 derive their origin in some instances from fluate of lime, in others from 

 sulphate of barytes. The regular forms borrowed from fluate of lime are* 

 the octaedron and the cube. These octaedrons are either hollowed out, 

 or in re^ef. TIic faces of the first are plane, or convex : the second ex- 

 hibit sometimes a regular octaedral summit, at others a simple equilateral 

 triangle. The cubic forms, which are more numerous, are either solid 

 or hollow. All these forms exist with the same appearances in the 

 fluor spars found in the same place. The forms originating from the 

 sulphate of barytes are the primitive form of that sulnhate, with the 

 trapezoid, the pointed, the laminar, the concrete, and the radiated vari- 

 eties. The pseudomorphic quartz crystals originating from sulphate of 

 barytes are not accompanied with this sulphate, as those indebted lO 

 fluate of lime for their form are with this Huate ; whether because the 

 sulphate of barytes has been subsequently destroyed, or because the pseu- 

 domorphic quar'z has been removed from its place; which must have 

 happened sometimes, since it is found not only in veins, but in ravines, 

 and on the surface of the ground. However, on proceeding but a little 

 ■wav from tlie places where these pseudocrystals of quartz are fouftd, wa 

 soon meet with veins of sulphate of barytes, and this in sufficient abun- 

 dance, to leave no doubt of the origin of ih^ise psoudomorphoses. 



