ancient Gypsum of the Alps. 67 



at least, they only present some brilliant points, even with 

 a magnifying glass ; but if they are coarsely powdered, the 

 brilliant points are multiplied to the eye, and the powder ob- 

 served with a microscope shews, even in its smallest parti- 

 cles, only crystals of sulphate of lime in rhomboidal tables. 



Besides the small crystals, which appear to form the mass, 

 larger crystals of sulphate of lime are seen imbedded in it. 

 There are nevertheless some gypsums which appeared to me 

 decidedly compact, but their colour was grey ; they are 

 however associated with crystalline gypsum. 



Sonie varieties are a little laminated, but the greater num- 

 ber do not shew this structure in a decided manner. Their 

 masses break indifferently in every direction. 



These gypsums are generally of a snow Mhite colour. 

 When considerable masses are met with, free from mixture, 

 they are advantageously employed in sculpture, as is the 

 gypseous alabaster of Tuscany. (That of St. Leonard has, it 

 is said, been used to make statues for the churches at Fri- 

 bourg). I have found the grey varieties only in Savoy, in 

 the Val d'Arbonne near St. Maurice, and the yellow near 

 Brides ; this last colour appears to be owing to subsequent 

 alterations ; all the surrounding gypsum is white. 



Many different mixtures are seen in it. 



1st. Carbonate of lime, which is compact, sometimes of a 

 dull grey colour, at others and most frequently of a dark 

 blackish grey. It exists under the remarkable form of 

 noduks, most commonly angular, and sometimes rounded. 

 The greatest number, of mineralogists, who have observed 

 these mixtures, have considered them as conglomerates or 

 arenaceous rocks, which gives the nodules an existence an- 

 terior to the paste of gypsum containing them. 



I had myself embraced this opinion, and do not pretend 

 to regret it yet ; but the workings at Sarran have led me to 

 doubt the pre-existence of some of these nodules. Small 

 veins of compact limestone are found in this gypsum quite 

 parallel with the beds, and having all the characters of con- 

 temporaneous origin. I should be led to suspect, especially 



E 2 



