324 ON THE NATURAL HISTORY 



" moon-stone." But the mineral is to be carefully distinguished: 

 from the "moonstone" of the jeweller, which is an opalescent 

 adularia, and therefore much harder than gypsum, Gypsum, or 

 selemte, is indeed so soft as to be readily scratched, even by the 

 finger-nail, and hence when fibrous gypsum, or satin spar, is cut 

 and polished as an ornamental stone, for sake of its sheen, the 

 material suffers from the slightest abrasion. 



Selenite crystalhzes in what is called the monoclinic, or 

 singly-oblique, or monosymmetric system, and the crystals 

 present perfect cleavage parallel to a certain form which is 

 termed the clinopinacoid. In the most common type of crystal, 

 the plates which are cleaved parallel to this form are diamond- 

 shaped. It has been suggested that the small lozenge-sliaped 

 plates of glass so common in old-fashioned casements represent a 

 survival of the cleavage-plates of selenite used in days before 

 glass was common. Even after glass came into general use for 

 windows, selenite continued to be used in certain religious 

 houses. Such plates, by reason of their softness, soon lost 

 transparency and behaved rather like -ground glass — a decided 

 advantage in places which required a " dim religious light." 

 The mineral is still called in Germany Mavienglas or Fraueneis. 



Probably the largest known crystals of selenite are those 

 from the cave, or goede, in Utah, whence Dr. J. E. Talmage 

 obtained the specimens which he has so generously distributed: 

 in this countr}'. Much nearer home there is the well-known 

 Marienglashohle of Friedrichroda, which presents an example 

 of a grotto with selenite forming the sparry garniture of roof and 

 wall. This cave is situated near, the great ducal castle of 

 Reinhardsbrunn, not far from Gotha, surrounded by the pine-clad 

 hills of the Thiiringer Wald. A cottage near the entrance offers 

 the miners' greeting, " Glnck Auj ! " but let no visitor be tempted 

 by the beautiful crystals to use his hammer, for over the doorway 

 hangs the not unnecessary notice, " Das Abbrechen von 

 Marienglas ist streng' verboten ! " 



Magnificent specimens from this locality may be seen, how- 

 ever, in the British Museum, and smaller examples in the Museum 

 of Practical Geology — the specimens in both cases having been 

 presented by the Prince Consort. It is curious to note how some 

 of these fine crystals are bent and contorted as though the}- had 

 been at one time plastic as sticks of warm sealing-wax. 



