426 Scientific Proceedings, Royal Dublin Society. 



and epidote, and a portion of the silica set free by the decomposi- 

 tion has crystallised as trydimite. 1 We have seen above that the 

 alteration of our felspar also necessitated the liberation of a defi- 

 nite quantity of silica : hence, it is quite possible that the two 

 modifications, quartz and triclymite, have been produced in the 

 case before us. The researches of Friedel and Sarasin prove that 

 both these modifications or forms of silica may be produced in the 

 wet way ; but tridymite requires a higher temperature than 

 quartz. 



Trictymite was recognised by Yon Lasaulx 3 in the liparite of 

 Tardree, Co. Antrim ; and this remains at present the only re- 

 corded instance of the occurrence of this mineral in the British 

 Isles. It is probable that we now have a second locality, and that 

 also in Ireland ; but the infrequency of its occurrence in the rock 

 we are describing does not permit us to express a decided 

 opinion. 



Triclinic felspar is present, but is mostly represented by the 

 micro-perthitic interlaminations. When it occurs separately, it is 

 seen to be albite or oligoclase-albite. Periclinic twinning is visible 

 in addition to the albitic. It undergoes the same alteration as 

 the orthoclase ; but calcite seems to be a bye-product. 



The quartz is corroded : it contains the liquid inclusions, which 

 possess the geometric form of the quartz (so-called " negative 

 crystals ") and enclosures of gas. The inclusions are far behind 

 the granites in quantity. Portions of the groundmass are also to 

 be found in the mineral. 



A greenish hornblende represents the ferro-magnesian consti- 

 tuent. It is not of frequent occurrence, but was one of the first 

 minerals to consolidate, having been enclosed by the felspar, the 

 formation of which was a later proceeding. The hornblende is 

 mostly altered into chlorite, thin threads and streaks of which are 

 also to be met with in the felsitic-looking groundmass. Yellowish 

 epidote was a bye-product of this alteration. The pleochroism 

 of the hornblende is from deep grass-green to light-yellow. A 



1 Miuroscopische Analyse ostbaltischer Gebirgsarten : Archiv fur die Naturkunde 

 Liv— , Est—, und Kurlands, serie 1 ; Bd. viii. ; heft 2, p. 66. 



2 Jour. Eoy. Geol. Soc, Ireland; new series, vol. iv., 1876, p. 227 ; also Min. u. 

 petr. Mitt. (Tschermak's) N. F., vol. i., p. 410. 



