Sollas — On Soda-Granites and Associated Dykes of Wicklow. 269 



supposing the anorthite and albite molecules united in the same 

 felspar, this would give an albite felspar of approximately the com- 

 position, Abn An ; or if the orthoclase be included, a potash, con- 

 taining albite, having the composition Ab 24 An 2 Oi*i results, and the 

 total quantity of felspar in the rock becomes 53*9 per cent., and of 

 quartz, 46 - 10 per cent. 



In thin slices crystals of felspar and rounded granules of quartz 

 are revealed, the latter inosculating with the former in a micro-peg- 

 matitic fashion ; patches of chlorite, with associated epidote and 

 magnetite are also visible, and point to the original presence of 

 biotite, which has since become converted into chlorite by 

 weathering ; from the patches of chlorite thin yellowish films and 

 streaks extend outwards, running between the adjacent minerals, 

 and filling whatever cracks and revealed cleavage planes may be 

 present. 



The felspar crystals never show zonal structure, but they pre- 

 sent other features of great interest. Many of them show very 

 fine frequently-repeated twinning on the albite plan. When symme- 

 trical extinctions can be observed, the angle of extinction, measured 

 from the twinning trace, amounts to about 15° : this confirms the 

 suggestion that the principal felspar present has a composition 

 approaching Abi 2 Am. 



Other crystals present a microperthite structure, and the angle 

 of extinction for the interrupted lamellae of these agrees closely 

 with that of the albite crystals. This suggests the idea that the 

 perthitic structure is simply a phase of synthetic twinning on the 

 albite type ; and this is clearly the case, for in some instances a 

 large felspar plate will show the usual repeated albite twinning 

 over one-half of its area, and over the other microperthitic struc- 

 ture, the optical orientation of the lamellae in the two halves being 

 identical. In a few cases twinning on the pericline plan is super- 

 added to that on the albite type, and the microperthite then 

 becomes microcline, or rather albite with the structure of micro- 

 cline. Some of the felspar present does not show twinning ; this 

 usually extinguishes at from 20° to 21°, and is possibly albite or 

 soda-microperthite, cut parallel to the face OIO. In some cases a 

 crystal may be observed in which twinning is present over the 

 greater part, but suppressed over the remainder of the section. 



The quartz is present in rounded granules, penetrated by occa- 



