116 Proceedings of tlie Boyal IrisJi Academy. 



Gneiss. — Gneiss, as previously stated, ranges from scliistose to 

 gTanitoid. in structure. Thajm.&.j'hQ fine, 2)laty, rihaned, compact, coarse, 

 Jihrous, granitoid, porphyritic, nodular, or conglomeritic. The four first 

 varieties usually are characteristic of the schist-series, in which they 

 occur as subordinate rocks. In the rihaned all the constituents are of 

 small sizes and form thin even layers ; the fibrous has a structui^e some- 

 what similar to fibrous schist ; the granitoid and porphyritic varieties 

 seem generally to occur together, while most of the nodular and con- 

 glomeritic gneiss seems to -be metamorj^hosed agglomerates. 



In composition, gneiss may vary from highly siHceous to basic. 

 Earely is it typical, that is, being composed only of plates or layers 

 of quartz, felspar, and mica, as various other minerals will also be pre- 

 sent, sometimes only as accessories, but generally as essentials in addi- 

 tion to or in part replacing the typical constituents. On this account 

 the varieties and sub- varieties are numerous, the following being the 

 most remarkable — qtiart%itic, felsitic, micaceous, liornhlendic, cMoritic, 

 and pyritic, the rock being named after the mineral that is more 

 developed or more conspicuous than the others, giving a character to 

 the mass. 



Sornhlende-roclc. — Tliis rock is found as a bedded rock also in dykes 

 and intrusive masses. Associated with it are schist and gneiss, the 

 rocks from which it was formed evidently having been intruded con- 

 temporaneously with the deposition of the original sedimentary rocks. 

 Also associated with these rocks are granites that are supposed to be 

 metamorphosed felstones ; consequently, it would seem that some of 

 these granite veins are older than some of the hornblende rock. This, 

 however, is more apparent than real. Both kinds of rock (whin stone 

 and felstone) seem to have been ejected at different intervals during 

 the accumulation of the sedimentary rocks, but all were metamorphosed 

 at the same time, the basic igneous rocks being changed into horn- 

 blende-rock, and the highly siliceous rocks into granitic or granitoid- 

 rocks, on which account it is evident such granites cannot be older 

 than the hornblende-rock, while the rocks from which the latter were 

 formed must have been intruded prior to the granite that was intruded 

 during or subsequent to the metamorphism of all the rocks. 



Hornblende-rock varies much both in structure and composition, 

 apparently according to the nature of the original rocks ; still, how- 

 ever, all the varieties may conveniently be grouped under this old 

 name of Macculloch.*' 



Large protrusions of plutonic rocks in other localities, especially 

 if in part bedded masses, vary much in composition, part being 

 eurytes (basic felstones), and part whinstones, both often occurring 

 associated together. Such would also seemingly have been the original 

 condition of the igneous rocks of Yar-Connaught, in part occurring 

 as outbursts and dykes, in part as bedded masses, and the latter 



* See Handy Book of Rock Names, by the writer (Hardwicke, 1873), p. 61, 

 et seq. 



