212 DESCRIPTION OF A NEW CRYSTALLINE 



Description of a new Crystalline form of Quartz, 

 By Dr. G. Troost. Head June 4th, 1S22. 



Among the mineral productions of our country, the 

 beautiful quartz crystals of Lake George are much 

 admired : their apparent irregular shape has often 

 embarrassed those, who, though acquainted with geo- 

 metrical figures, yet not being in the habit of examin- 

 ing crystals, expected to find in those of Lake George 

 the hexahedral prism terminated by the hexahedral 

 pyramid with isosceles triangular faces ; or the do- 

 decahedron, and such little modifications as the rhom- 

 bifere, plagiedre, pentahexaidre and co-ordonne des- 

 cribed by Haiiy. In some of these isolated crystals, 

 (as the Abbe Haiiy in his treatise has remarked res- 

 pecting the Variete prisme >bis- alt erne,) the prism has 

 sometimes entirely disappeared ; even the smallest 

 faces are nearly invisible, so that the solid beiug the 

 result of this apparent anomaly, is a rhomboid not dif- 

 fering much from the cube. In fact such crystals are 

 found occasionally at Lake George : a remarkably 

 beautiful specimen, in the collection of Mr. J. Lukens, 

 is upwards of one inch long. These varieties are 

 however not new, they have been found elsewhere; 

 but this is not the case with a variety, the description 

 of which, I will proceed to lay before the Academy. 

 These crystals which we will call JLnnulaire, are 

 hexahedral prisms with the edges of their bases 

 bevelled. This new variety is formed by the de- 

 crement of one row of molecules parallel to the 



