1823.] On the Geology of Devon and CornwalL 34 



solution, if not too strongly alkaline, will afford a blue precipi- 

 tate when added to a solution of iron. 



Indigo. — This substance when heated in a spirit lamp readily 

 rises in the state of purple vapour. Sulphuric acid, even in its 

 concentrated state, does not destroy its colour, and, therefore, 

 the presence of indigo is very readily ascertained. 



Smalts. — The colour of smalts resembles that of ultramarine itt 

 resisting the action of heat ; but as it is not destroyed by any 

 acid, and as the colouring matter of ultramarine is, any admix- 

 ture of smalts will be easily discovered. 



Colour prepared from Oxide of Cobalt and Alumina, — This 

 compound, which greatly resembles ultramarine in appearance, 

 although its colour is not quite so bright and intense, may be 

 distinguished from it, by remaining unacted upon by acids pre- 

 cisely hke smalts. Heat does not readily change its colour, but 

 if a drop of solution of carbonate of potash be added to it on 

 platina foil in the fiame of a spirit-lamp, it is readily blackened; 

 an effect which is not produced upon ultramarine. 



Article VIII. 



m 



On the Geology of Devon and CornwalL 

 By the Rev. J. J. Conybeare, MGS. , 



(Continued from vol. v. p. 184.) ii^j 



Inferior Slate. — At and near its immediate contact with the 

 granite appears as a somewhat indistinct and ill characterized 

 gneiss, in some beds of which the felspar so predominates that 

 they have been termed slati/ felspar. Judging, however, from 

 their less ready fusibility, from the large proportion of metallic 

 oxides (iron and manganese) which they contain, and from the 

 examination of many specimens from various quarters, we shall, 

 I think, approach nearer to strict accuracy by regarding these as 

 compact felspar intimately mixed with mica (or rather with 

 chlorite) and quartz. To the geologist, who seeks the aid of 

 mineralogy and of chemistry, examples of this intimate penetra- 

 tion of one simple mineral by another (so, as in many cases, 

 to alter very considerably tHe external and empirical charac- 

 ters of that which yet predominates) must be familiar. Many 

 subordinate beds of the earlier greenstone formation exhibit 

 every stage of a similar phenomenon, and an accurate examina- 

 tion would probably show, that most of the substances named 

 petrosilex, corneenne, saussurite, jade, and even flinty slate, are 

 in fact only admixtures of this nature, in which felspar varying 

 from its more compact and semitransparent to its earthy and 

 granular form, is uniformly and intimately penetrated by some 



d2 



