186 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. C, 



4th. The increase in specific gravity ; the mean specific gravity of 

 the two specimens being =2*665. 



In addition to the foregoing peculiarities common to both groups 

 of granites, it may be remarked that the change of character, as 

 respects the iron and lime, is carried further in the third isolated 

 group than in the first. 



Fourth Group of •Isolated Granites. 



In the extreme south-eastern corner of Ireland an outburst of 

 granite occurs, at Carnsore, which appears to be continued to a con- 

 siderable distance under the sea in an E.N.E. and W.S.W. direction. 

 This granite consists principally of grey quartz and reddish-pink 

 feldspar, associated with which in many specimens are green mica, 

 and apparently a variety of hornblende. I selected for examination 

 a specimen consisting almost exclusively of quartz and feldspar, con- 

 taining a few specks of dark-green mica, but no trace of hornblende. 

 This appears to be the prevailing character of the rock ; the horn- 

 blende, when it does occur, appears to be very irregularly distributed. 



The specific gravity of the specimen examined by me was found 

 to be 2-636. 



Analysis of Carnsore Granite. 



Per eent. Atoms. 



Silica 71-80 1-561 



Alumina 11-72 0-2281 



Peroxide of iron. . 3-88 0*048 J 



Lime 2*12 0-076] 



Magnesia trace 



Potash 4-77 .... O'lOl 



Soda 3-06 .... 0*099 



Loss by ignition. . 0-95 



0-276 



0*276 



98*30 



The most cursory examination of this analysis, compared with the 

 average granite of the main chain in Table III., serves to show that 

 we have recovered in Carnsore the original type of potash- granite 

 from which we set out in the main chain. To render this important 

 fact more evident, I shall enter into some further calculations. 



It is plain from the atomic quotients that the quantity of mica 

 present is trifling in amount, and that the rock may be regarded as 

 composed of quartz and feldspar in the following proportions : — 



Quartz =21*50 



Feldspar =78*60 



100*00 



From this analysis we can calculate readily the theoretical composi- 

 tion of the feldspar of the Carnsore granite : — 



