182 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 6, 



sitioii of all the isolated granites, which present greater differences ot 

 mineral constitution than occur in the main chain. Topographically 

 considered on the large scale, it has been suggested to me by Dr. 

 Griffith, who is well acquainted with this district, that we may refer 

 the isolated granites to four parallel broken chains, as follows : — 



1. From the north-east of Rathdrum, A, passing south-west 

 through Ballinaclash, to the village of ilughrim, in the county of 

 Wicklow. 



2. From Croghan Kinshela, B, south-west to Conna Hill, in the 

 county of Wexford. 



3. From the south of Oulart, county Wexford, C, through Ballin- 

 amuddagh to Camarus Hill, midway between Wexford and New Ross. 



4. The extreme south-east of Ireland, at Carnsore, D, extending 

 to the Saltee Islands, in the same general direction as the other 

 outbursts. 



First, or Western Group of Isolated Granites, 



The granites of this district extend in a broken manner from near 

 Rathdrum to Aughrim, a distance of ten miles north-east and south- 

 west, and attain in Cushbawn Hill an altitude of 1318 feet. In the 

 northern part of this chain, the granite contains red feldspar and 

 black mica, at West Acton ; but the general character of the granite 

 is best exhibited in Cushbawn Hill, which is composed of a fine- 

 grained granite, containing grey quartz, white feldspar, and minute 

 particles of grey and dark-green mica. An average specimen from 

 this hill, selected with care, gave the following analysis : — 



Granite of First Isolated Group. 

 Specific gravity =2"671. 



Per cent. Atoms. 



Silica 70-32.... 1-529 



Alumina 11-241 n-^^ft 



Peroxide of iron 4-80 J * * 



Lime 3-01") 



Magnesia 0-73 I a-ina 



Potash 2-27 f • " '^^^ 



Soda 3-39J 



Carbonate of lime 1*34 



Loss by ignition 1-62 



98-72 



The carbonate of lime in this granite is present accidentally, and 

 has been introduced by infiltration from the limestone-gravel of the 

 drift which covers the district. On comparing the preceding analysis 

 with the average granite of Table III. we may observe the following 

 differences : — 



1 . In the granite of Cushbawn there is an increase of peroxide of 

 iron replacing alumina, and attended with an increase of specific gravity. 



2. The quantity of lime and soda is increased, so as to invert the 

 relative proportions of potash and soda, rendering the granite of 

 Cushbawn a soda-granite, and showing that it has proceeded from 



