Lower PalcEozoic Rocks of the South-East of Ireland. 593 



give for the average ratio of potash to soda in tlie felspathic paste of the granite 

 of that mountain, picking out the mica and orthoclase — 



Potash, .... 403 

 Soda, 4-74 



1 shall compare this result with the composition of the felspar deduced by 

 myself from calculation, a method which admits of very considerable accuracy. 



I entertain grave doubts as to whether the felspathic paste of the granite is 

 entitled to be considered as a distinct mineral, as it contains, as appears from 

 the preceding estimate of alkalies, an insufficient quantity to form a tersilicated 

 felspar, without the aid of the lime and magnesia, which exist in it in con- 

 siderable quantity. It must be remembered that it is only by an arbitrary 

 fiction of the mineralogists that we consider granite as composed of three, four, 

 or two minerals. I shall show that the granite of Leinster unquestionably 

 contains /cwr minerals, viz., Quartz, Orthoclase, White Mica, and Black Mica, 

 together with a confused felspathic paste, the composition of which may be 

 ascertained, but which, being uncrystallized, except in a confused manner, we 

 are at liberty to consider as a mineral or not, just as we please. It is as much 

 a mineral as the chance slag of an iron furnace would be. No doubt, we may 

 ascertain its chemical composition, and form a formula therefrom, but until it 

 is found crystallized in a definite form we have no right to call it by the name 

 of any recognised mineral ; it is neither orthoclase, nor albite, nor oligoclase, 

 but a confused mass of felspathic paste, having a composition that is constant 

 for many miles of superficial extent. 



5. White Mica. — In every specimen of granite that I have seen from the 

 main chain, white mica is abundant, and as it contains 5 or 6 per cent, of water, 

 it becomes highly interesting, as its occurrence is a proof that during the for- 

 mation of the granite, water must have been present, in order to be chemically 

 combined with the silicates composing the mica. This water must have occurred 

 at a temperature between cherry red and the melting-point of silver, between 

 which limits the granite was formed. 



The white mica is often found in lozenges (^ in. by \ in.) in the body of 

 the granite, and attains in the granite veins the dimensions of 2 in. by 1 in. 

 and even 3 in. ; but, in general, it occurs in plates not exceeding ^th of an 



