On the Granites of the South-east of Ireland. 23 



differences were discernible. From all bis observations on 

 mineral crystals M. Knoblanch concludes, that with certain 

 bodies of this class calorific rays exhibit both quantitative and 

 qualitative differences dependent on the direction of the rays 

 through the crystal. Only in directions which are perpendicular 

 to the optic axis are the effects the same. If the heat is polar- 

 ized, differences dependent on the position of the plane of 

 polarization are observed in one and the same direction. Here 

 also the radiations perpendicular to the axis are the same. 

 Along the axis alone is the transmission of the heat, and its 

 further deportment dependent on the plane of polarization. 

 The differences spoken of are greater in the case of polarized 

 than of natural rays, when the plane of polarization in the one 

 case has the same direction as the axis, and in the other, en- 

 closes an angle of 90° with it ; the differences disappear alto- 

 gether when the plane of polarization is kept coincident with 

 the crystallographic axis. 



IV. Notes on Miner alorjy. — No. II. On the Chemical Comjm- 

 sition of the Granites of the South-east of Ireland. By the 

 Rev. Samuel Haughton, Professor of Geology in the Univer- 

 sity of Dublin^. 



THE granites of the south-east of Ireland occur in the coun- 

 ties of Dublin, Carlow, Kilkenny, AVicklow, and Wexford, 

 and may be divided physically into two distinct groups : — 



1st. The chain of gVanite hills extending from Booterstown 

 and Dalkey, co. Dublin, in a N.N.E., S.S.W. direction, to Poul- 

 mounty in the south of the county of Carlow, within five miles 

 of New Rosi?. This granite chain has a length of 68 miles, and 

 a breadth varying from 8 to 15 miles. 



2nd. The series of granite hills occurring at intervals in the 

 slate of the counties of Wicklow and Wexford, isolated from 

 each other, and rising like islands through the slate. This 

 group of granite hills lies between the main chain and the sea, 

 and appears to be arranged in lines parallel more or less to the 

 axis of the main chain. These granite hills arc about twenty in 

 number, and extend for a distance of 43 miles from Ballinaclash, 

 county Wicklow, to Camarus Hill, county AVexford. 



Hitherto, so far as I am aware, no decisive proof of difference 

 of geological age has been discovered between these two groups 

 of granites ; they are both newer than the Silurian slates, which 

 they penetrate and metamorphose. 



* Communicated by the Author. 



