70 Dr. Blum on Pseudomorphous Minerals, 



Thai, a bed of this mineral in mica slate consists of gypsum 

 wherever it is exposed. At Bex in Switzerland the same fact is 

 observed ; the unaltered anhydrite is invariably obtained by dig- 

 ging down sixty or one hundred feet from the surface. The 

 same has been observed by Alberti in the muschelkalk of Ger- 

 many. This change consists merely in the addition of water, 

 and the facts cited evince that it takes place principally through 

 atmospheric agency. 



Chalcedony with the form of Datholite. — According to Levy 

 the crystallized chalcedony from Hay-Tor, Devonshire, called 

 Haytorite, is altered Datholite. The change consists in the re- 

 moval, from the compound, of lime, boracic acid, and water, and 

 the addition of silica. 



Cimoliie with the form of Augite. — These pseudomorphs oc- 

 cur near Bilin in gneiss along with kaolin, silver-white mica and 

 quartz, and the basalt near by is changed to a clayey rock. The 

 gneiss when unaltered contains brown mica and fresh crystals of 



augite. Rammelsberg's analysis gives the formula Al Si^-f 3H, 

 which is identical with that of Cimolite. The specific gravity 

 after alteration is 2-21 or less ; before, 3-3. Three atoms of 



the original augite, 3R3 Si^, with 6H, would make, on substf* 



luting Al2 for the bases, 2(AlSi=*4 3H), or 2 atoms of Cimo- 

 lite. Dr. Blum suggests that carbonic acid may have been the 

 agent that carried off the lime, yet remarks that no trace of car- 

 bonate of lime has been found in the rock. 



Lithomarge with the form of Topaz. — Topaz altered to litho- 

 marge occasionally occurs in Brazil in a talcose slate, and a Si- 

 berian specimen has been observed with a milk white exterior, 

 owing to a similar change. The change consists in the removal 

 of fluorid of aluminium and the addition of water. 



Lithomarge with the form of Feldspar. — Numerous instances 

 of this change are cited by Dr. Blum. In explanation of it, it is sta- 

 ted, that from the feldspar, (KSi+Al Si^,)KSi is removed, and H, 

 (or water,) added. Carbonated water is supposed to be the prin- 

 cipal agent in effecting this change. 



Kaolin with the form of Feldspar. — This very frequent change 

 requires only the most common agents. Forchhammer has 

 shown that heated water, even when pure, will cause the altera- 



