1862.] 



HATJGHTON" GRAIWTES OF IRELAND. 



415 



in the accompanying figure (fig. 2), in wliich also the internal lines 

 of structure of the crystal are shown ; from which it seems possible 

 that the optical plane may really bisect the acute angles of some of 

 the component crystals, which would, in this case, have been rotated 

 through 90°. In the specimens from Dooish, the plane of the optical 

 axes is perpendicular to the side of the hexagonal plate of the mica, 

 and therefore joins the acute angles of the primary rhomb. 



This optical peculiarity of the Breezy margarodite distinguishes 

 it from the margarodite of the Leinster granite, in which the plane 

 of the optical axes coincides with the long diagonal of the primary 

 rhomb, joining its acute angles. 



From the preceding analyses the following atomic quotients may- 

 be calculated : — 



Silica 



Alumina 



Iron (peroxide) 

 Tii'me 



Soda 



Potash 



Iron (protoxide) 



Manganese (protoxide) 

 Water 



No. 1. 



0-995 

 0-682 



i. 0-345 



0-222 



No. 2. 



1-005 

 0-644 



0-311 



0-444 



Mean. 



I -000 

 0-664 



0-328 



0-333 



This result gives very accurately the well-known formula of mar- 

 garodite — 



RO, Si03 + 2(A1,03, Si03) + [H0]? 



The mean of four analyses of margarodite, from four distinct 

 localities of the Leinster granite, gave me — 



36-62 



Leinster Margarodite. 



SiHca 44-58 



Alumina 1 



Iron (peroxide) J 



lime 0-78 



Magnesia 0-76 



Soda 0-95 



Potash 10-67 



Iron (protoxide) 0-07 



Loss by ignition 5-34 



99-77 

 The two minerals are plainly identical, and form a well-marked 

 feature of the granite of Leinster and of the granite-veins of 

 Donegal. Margarodite is sometimes, as at Sheskina-roan and Dooish, 

 found in the granite itself. 



6. Hornblende. — The granite of Donegal varies much in texture 

 and appearance, as might be expected from its gneissose character. 

 It occasionally passes into a granitic syenite, composed of hornblende, 

 oUgoclase, and a little quartz and sphene. The composition of the 



