24 The Rev. S. llaughtou on the Chemical Composition of 



Such being the state of geological knowledge on this subject, 

 it occurred to nie that it might be useful to direct attention to a 

 distinction of a chemical character which appears to exist between 

 these two groups of granites, — a distinction to which I have been 

 led in the course of a series of analyses of Irish granites, in 

 which I have been for some time engaged. The distinction to 

 which I have alluded is the following. The granites of the 

 main chain contain more potash than soda ; and vice versa, the 

 granites to the east of the chain, which are isolated from it and 

 from each other, contain more soda than potash ; showing that 

 the circumstances, unknown to us, under which the isolated 

 granites were formed, were such as to yield to the molten mass 

 a quantity of soda greater than that possessed by the granites of 

 the principl chain. 



In illustration of the foregoing generalization, I offer analyses 

 of granites from the following localities : — 



Granitic chain. j Isolated granites. 



1. Dalkey, Co. Dublin. 1. Ciishbawn, Co. Wicklovv. 



2. Foxrock, Co. Dublin. | 2. Crogban Kiushela.Co.Wicklow. 



3. Three Rock Mountain, Co. j 3. Ballyraotymore, Co. Wexford. 



Dubbn. 4. Ballynamuddagh, Co. We.xford. 



4. Enniskerry, Co. Wicklow. | 



5. Ball5'knocken, Co. Wicklow. 



6. Killballyhugb, Co. Carlow. 



7. Blackstaii's, Co. Wexford. 



8. Ballyleigb, Co. Wexford. 



It will be seen, on reference to the Ordnance Map, or any 

 good map of Ireland, that the localities selected extend from the 

 north to the south of both the granite series ; and on reference 

 to the geological maps of Wicklow and Wexford, it may be ob- 

 served by those unacquainted with the geological structure of 

 this part of Ireland, that the granites of the second group ex- 

 amined are taken from the distinct and distant isolated patches 

 of granite. 



In order to investigate the relative proportions of quartz, fel- 

 spar and mica, of which these granites were composed, I used 

 the following method, which appears to be as simple as any that 

 has been proposed for such a purpose. 



Let the per-eeutage of silica in the granite be divided by the 

 atomic weight of silica, and let the quotient be denoted by a. 



Let the per-centages of alumina and peroxide of iron be divided 

 by the atomic weights of alumina and peroxide of iron respect- 

 ively, and let the sum of the quotients so found be denoted by b. 



Let the per-centages of lime, magnesia, potash, and soda be 

 divided by the atomic weights of these elements, and the sum 

 of the quotients called c. 



