IDEBT. PRISMATIC TALC-MICA. 197 



Silica 62-00 29-50 52-00. 



Magnesia 27'00 21-39 6-00. 



Oxide of Iron 3-50 23-39 23-00. 



Alumina 1-50 15-62 7'00. 



Water 6-00 7 '38 4-00. 



Potash 0-00 0-00 7'50. 



Lime 000 1-50 0-00. 



These analyses, as well as those of several other varieties 

 of the species, shew that our information also in respect to 

 its chemical constituents is still very defective. Before the 

 blowpipe some of them lose their colour, and are difficultly 

 fusible, others are changed into a black scoria, still others 

 are infusible. In these phenomena also there is so little 

 agreement between the varieties employed, that we are 

 forced to draw the conclusion, either that the varieties 

 were not pure enough, or not simple ones, which they al- 

 ways should be if we wish to arrive at a correct result, or 

 that they belonged to different species. 



3. Common Talc, indurated Talc, Potstone and slaty 

 Chlorite, constitute themselves beds in primitive mountains. 

 The latter frequently contains imbedded crystals of octa- 

 hedral Iron-ore ; some of the former contain rhombohedral 

 Fluor-haloide, several species of Lime-haloide, of Augite- 

 spar, &c. Common Chlorite in particular is found in beds in 

 primitive rocks, consisting chiefly of ores of iron, rhombohe- 

 dral Lime-haloide, and several species of the genus Augite- 

 spar. Other varieties, and among these the small scaly crys- 

 tals of foliated Chlorite and earthy Chlorite, occur in veins of 

 various descriptions, and in the crystal caves of the Alps. 

 Green-earth, and sometimes also foliated Chlorite, occur in 

 amygdaloidal rocks, where they are found either lining the 

 vesicular cavities, or as imbedded nodules in the body of the 

 rock itself. Earthy Talc or Nacrite, of whose natural his- 

 torical properties but little is known, has been found in 

 lead veins. 



4. Those varieties which by themselves form mountain 

 masses, are met with in the primitive districts of several 

 countries, as in the Tyrol, in Salzburg, Switzerland, Swe- 



