264 Scientific Proceedings, Royal Dublin Society. 



by opaque white felspar, partly altered into muscovite (possibly 

 also into meionite, but this is a point I am still engaged in investi- 

 gating) ; plagioclase felspar continued to fall from 2*756 down to 

 2*616, between 2*66 and 2*64 it was accompanied by a large quan- 

 tity of quartz; finally, from 2*56 to 2*523 soda-bearing microcline 

 and orthoclase were precipitated. 



In thin slices the rock presents a noncrystalline granitic tex- 

 ture. By weighing camera lucida sketches of its component minerals 

 as seen under the microscope, its mineral composition was estimated 



to be approximately as follows : — 



Per cent. 



Microcline, 9.129 



Other kinds of Felspar, . . 51*354 



Biotite, 7*096 



Quartz, 31*814 



Epidote (sp.), -607 



100*000 



In describing the constituent minerals, I shall take them as 

 nearly as possible in the order of their consolidation. 



Zircon. — This occurs in small crystals of more than one form ; 

 the commonest presents the combination (IOO), (OIO), (III), the 

 next in frequency the same pinnacoids, but with a more acute 

 pyramid, probably (311) ; the form (110) (III) is only occasionally 

 met with. Minute rod-like crystals of a mineral (rutile ?) whicli 

 polarizes in brilliant colours occur as inclusions within the zircon ; 

 fluid cavities are also present. The zircon itself occurs as an enclo- 

 sure, and this in all the other constituents of the rock, except 

 microcline. The mineral which it chiefly infests, however, is biotite, 

 and this presents the usual pleochroic aureole around its guest ; 

 not unfrequently also it occurs within crystals of apatite, with a 

 coaxial orientation. 



Apatite. — Hexagonal prisms of this mineral are somewhat 

 abundant as inclusions within the biotite, and when not actually 

 inclosed within it they usually occur in its vicinity. 



Magnetite. — Irregular crystals and grains of magnetite are 

 present, some of primary consolidation, others the result of altera- 

 tion of biotite. 



Epidote. — An interesting species, probably of epidote, of which 



