114 Proceedings of the Royal Irish Academy. 



namely, the fine compact and the scaly friable. The fine rock has an 

 aspect very similar to qiiartzyte, but clistingTiishable from that schist 

 by its always weathering with a white crust similar to felstone. The 

 scaly or friable felsityte seems for the most part to be an aggregate 

 of minute f elspathic fiakes or scales ; nevertheless, it is very tough and 

 hard to break across, but along the grain can be split with greater or 

 less facihty ; across the grain it breaks with a hackly fi-acture. It is 

 always more or less steatitic. This variety of felsityte sometimes 

 occurs as a stratified rock, but in the country north of Killary 

 Harbour it undoubtedly is in places part of an intnisive mass, being 

 the tuffoid portion of the hereafter mentioned felstone protrusions, 

 which rock having been metamorphosed is changed into these schists. 

 It may be here suggested that all the felsitytes and perhaps also 

 the felsitic-quartzjrtes are probably metamorphosed felsitic tuffs and 

 tuifose rocks. These rocks, however, will hereafter be more fully 

 considered. 



Subordinate schists which claim separate descriptions are pyrrho- 

 tityte or pyrrhotite-schist and rlicntiziiyte or magnesian-schist on account 

 of their remarkable composition, the former also having been worked 

 for the minerals it contains. 



Pyrrhotityte, when typical, is a crystalline aggregate of quartz, 

 pyiThotite, pyi'ite, and chalcopyiite, with a little mica. As the mica 

 increases in quantity the sulphides decrease until eventually the rock 

 passes into mica-schist. In some varieties the mineralogical metals 

 predominate to the nearly total exclusion of all others ; however, it is 

 very variable in its composition, having many accessories, and changing 

 rapidly fi'om a highly metalliferous to a non-metalliferous rock.* 



Rhoitizyte or Magnesian-schist . — This was only observed in one 

 locality (Cannaver Island, Lough Conib). Of specimens of the rock 

 Mr. Forbes says — " Yery identical with one described by me in ]S[or- 

 way."t 



Schistose-limestone, Schistose-dolomyte, Opliiolyte, and Steatyte. — These 

 rocks, although different in aspect and composition to all other schistose- 

 rocks, must be classed among them, as they graduate into some one or 



* Here it may be mentioned that in connexion wdth these schists there are 

 metaUiieroiis lodes e^ddently older than the associated gi-anite-veins, and apparently 

 of metamoi-phic origin. These consist of a crystalline aggregate of pyn-hotite, 

 pyrite, chalcopyiite, quartz, mica, orthoclase, and oligoclase, with marcasite, amphi- 

 bole, &c., as accessories. Adjoining such lodes more recent lodes usually occur. 

 There are also veins of garnet-rock which seem to he connected with the metalli- 

 ferous strata, as they were not noted in any other parts of the coimtry. This vein- 

 rock is a more or less crystaUine mass of bro\^'u garnet ; when very crystalline it 

 usually contains crystals of pyn-hotite, ppite, chalcopyiite, quartz, oligoclase, &c., 

 with the interstices filled witli epidote and ribaned pearl-spar. Some fine crystals of 

 the epidote have been observed. The garnet crystals vary iu size, some being over 

 an inch in diameter. 



t For the description of the Norway rock, see Q,uar. Joui'. Geol. Soc, Lend., 

 August, 18.58. 



