Lower Palceozoic liocks of the South-East of Ireland. 611 



lated tracts of granite were formed by the irruption of the granite of the main 

 chain, which granite became mixed up with impurities, consisting principally of 

 bases, and thus degenerated into the separate special forms of granite, the ana- 

 lyses of which are given in Table XII. There is much difficulty in determining 

 the nature and amount of the impurities thus added to the granite of the main 

 chain ; but as the subject is one of great importance, I shall make an attempt 

 to exhibit at least the method by which the question should be solved, and trust 

 to further observations to furnish sufficient data for its complete determination. 



Let us take, for example, the Coollattin granite, No. 2, and endeavour to 

 determine the amount and kind of admixture with the main granite sufficient 

 to produce it. 



Let M represent any arbitrary number of tons of main axis granite, C the 

 unknown number of tons of granite of the composition of No. 2, resulting from 

 the admixture of C— ili" tons of unknown substances with if tons of molten 

 main chain granite. 



It is required to determine tlie nature and amount of the substances thus 

 added. 



It is easy to see the truth of the following equations : — 



(22) 



In these equations, silica, alumina, &c., stand for the unknown number of 

 tons added to a given number of tons M of main chain granite, to make C tons 

 of Coollattin granite. There is one unknown quantity more than the number 

 of equations. Adding all together, we find, since the silica, alumina, &c., must 

 be equal to C — M, 



100 (C - il/) = 100 C - 100 if, 



which is an identity. We, therefore, cannot obtain an additional equation from 



