196 PHYSIOGRAPHY. CLASS n. 



rite only by the smallness of its grain. Immediately with 

 those varieties of Chlorite whose composition is impalpable, 

 the Green Earth is connected, from which, however, we 

 must except what has been termed crystallised Green- 

 earth, and which consists of decomposed crystals of para- 

 tomous Augite-spar. The species Talc comprehends the 

 varieties of generally pale green, particularly apple-green, 

 grey and white varieties, and is divided into common, earthy, 

 and indurated Talc. Simple varieties are common Talc ; 

 also such compound ones in which cleavage is transformed 

 into slaty structure, the latter being generally very per- 

 fect ; or such as consist of columnar particles of composi- 

 tion ; earthy Talc, or Nacrite, consists of loose particles, or 

 such as are but slightly cohering ; and indurated Talc refers 

 to imperfect and coarse slaty varieties, in which this kind 

 of structure is more the consequence of composition than 

 of imperfect cleavage. If this structure be sufficiently im- 

 perfect to become coarse and indistinctly granular, Pot-stone 

 or Lapis ollaris is formed, which, possessing the united pro- 

 perties of softness and tenacity, may be easily turned into 

 vessels ;* and it is perhaps only for that reason that it used 

 to be distinguished as a particular species. It must be re- 

 marked, however, that several of the most important natu- 

 ral-historical properties of the substances at present com- 

 prised within the species of prismatic Talc-mica, are too 

 little known to admit of an exact comparison, so that they 

 may possibly require in future to be divided into several 

 species. 



2. Three varieties of the present species, foliated Talc 

 analysed by VAUQUELIN, slaty Chlorite analysed by GRU- 

 NER, and Green Earth analysed also by VAUQUELIN, 

 have yielded : 



* I have been informed by Captain STEWART, that the 

 translucent white variety of common Talc from Almorah, 

 in the Himalayan mountains, is employed for the same pur- 

 poses. H. 



