147 



TAB. CCCLXXVIII. 



ARGILLA electrica. 

 Stellated Tourmaline, 



The different appearances that various substances assume 

 is the greatest stumbling-block in Mineralogy : it becomes 

 therefore necessary to learn them; and although forms under 

 the same terms may belong to various substances, they are 

 nevertheless often characteristic of particular species. The 

 present variety, although not common, is pretty well known 

 among the Cornish miners, who call it Cockle, which is 

 generally recognised as Schorl or Tourmaline by mineralo- 

 gists. See tab. 209 and 210- We may find specimens thai 

 lead from the more crystallized form of those of the fila- 

 mentose structure through a great variety of arrangement 

 to the present stellated one, where the crystals shoot as it 

 were from a centre, are thin, and interrupted in their forms 

 by pressing among each other; so that externally we guess 

 at them from a similarity in appearance to those of a more 

 conspicuous size, generally angular, and mostly shining and 

 longitudinally striated on all sides. If experience does not 

 determine them to our satisfaction in this, we must proceed 

 to hardness, electricity, &c, to find their correspondence. 

 Our specimen is rather remarkable, being in a gangue 

 chiefly composed of Mica. I have a specimen from Aber- 

 deen nearly like it ; but the Schorl is chiefly imbedded in 

 Quartz, and a mass of Mica attached to the specimen has 

 only a few scattered bits in it. 



M 2 



