2 Professor Hull — On Irish Granites. 



at various angles corresponding with the planes of cleavage. The 

 cases here observed are also referable to the same cause ; but the 

 cellular structure is scarcely so regular and well defined as in the 

 case above referred to. 



TricUnic Felspar (Oligoclase ?). — The fine parallel lines charac- 

 teristic of the triclinic felspars are observable in several instances, 

 and are well brought out by polarized light. • The crystals, however, 

 are not nearly so numerous, nor are their forms so well defined as in 

 the case of the orthoclase ; sometimes, indeed, there is no crystalline 

 form whatever, the felspar occurring as an amorphous grain. An 

 unusually well-formed crystal is represented in Fig. 5. It is slightly 

 clouded. 



Silica.' — The silica, as usual in granites, forms the basis in which 

 the other minerals (felspar and mica) are imbedded. It is itself with- 

 out crystalline form, receiving only the forms given to it by the sides 

 of the felspar crystals ; and with polarized light the boundary edges 

 are often seen lined by narrow parallel bands of different prismatic 

 colours. The interior portions exhibit, on rotating the analyzer, the 

 usual gorgeous shades of colouring, one colour sometimes imper- 

 ceptibly shading off into another over the fields of view, like the 

 blending of the colours in a rainbow. 



With the ^-inch object-glass the silica is seen to be highly cellular, 

 and fluid bubbles in some of the cells come into view. With the 

 No. 2 eyepiece, magnifying 350 diameters, the bubbles are well 

 developed, and appear to occur in most of the cells (Fig. 6). Some- 

 times the cells are exceedingly irregular in form, sending out angular 

 projections in various directions. The bubbles seem generally to 

 occupy a lai"ge proportion of the cells, about one-third or one-fourth 

 of the entire space, from which it might be inferred that the vapour 

 from the condensation of which the bubble has been formed was not 

 originally highly rarefied. Sometimes the cells occur in long slightly 

 curved lines. Tubes and trichites are rare in the silica of this slice. 



Mica. — With a high magnifying power the mica has a green colour, 

 and shows a wavy structure. It sometimes incloses black grains, 

 which I have little doubt are those of magnetite. 



Magnetite. — Some grains of magnetite undoubtedly occur, one of 

 which is represented in Fig. 3. They are as usual opaque, and 

 generally imbedded in the mica flakes. A few individuals, of which 

 that above referred to is an example, are imbedded in the felspar. 



II. — Microscopic Structure of Irish Granites. 



No. 3. Granite of Ballyknockan, Co. Wicklow. 



By Professor Edward Hull, M.A., F.E.S., President E.G.S.I. 



(PLATE I.) 



THE granite of Ballyknockan belongs to the great mass of the 

 South-east of Ireland, which extends from Booterstown, near 

 Dublin, to Poulmounty in Co. Carlow. 



The Ballyknockan granite is considered by the Eev. Dr. Haughton, 

 F.R.S., to be the best building-stone near Dublin. It has, according 



