260 The Rev. S. Haughton's Notes on Mineralogy. 

 The mineralogical formulae of the four piinerals are : — 

 I. Leipidomelane : 



II. Black Mica o/Carlow: 



III, Black Mica of Donegal: 



IV. Black Mica of Canton : 



[^(3RO)+^(R^03)]si03^. 



It appears to me that the preceding formulae^ representing 

 black micas from Russia, Ireland, and China, balance around 

 a mean or average formula, which may be regarded as the type 

 species of this mineral j viz. — 



[ir(3K0)+i^(R^03]si03. 



This abstract or theoretical black mica, probably exists only as 

 an idea or conception in our minds, and may not have a concrete 

 development in any place ; but it must be regarded as an essen- , 

 tial constituent of the original granite formed in the astrono- 

 mical epoch by the cooling of our globe. All our researches 

 tend to prove that there is an original or type-granite, charac- 

 teristic of the azoic epoch of the earth's history, marked mine- 

 ralogically by the presence of four important minerals, — 



1. Quartz; 



2. Orthoclase felspar ; 



3. Black mica; 



4. White mica ; 



and marked chemically by the abundance of potash and the 

 absence of lime. 



Trinity College, Dublin, 

 March 10, 1859. 



