KiXAHAX — Oil Graniiic and other lugenite Rocks. Ill 



other schist is not even analogous to giits or sandstones in a group of 

 sedimentary rocks, for all grits and sandstones are not nietamoi'phosed 

 into quartz-schist, many changing into mica or other schists. In this 

 area the qnartzyte has no true boundary, as both in depth and length 

 it graduates into other kinds of schist, and no matter how cha- 

 racteristic some portions of a series may be, other parts will be so 

 micaceous or felsitic, &c., &c., that except for theii" associates they 

 would never be classed as qnartzyte.*' 



The quartzytes generally are pale-gi'eenish or white in colour, but 

 some are greyish, blueish, reddish, or blackish ; compact, hard, but 

 brittle ; when fi-actiu-ed, often have a saceharoid aspect, and usually have 

 a glimmering vitreous lustre. Few of these rocks are without flakes or 

 plates of mica, generally white, on the surface of the stratification or 

 lamination planes ; some, however, are more or less massive, and in 

 them the mica is not veiy conspicuous. ]y!any of the quartzytes are 

 affected by irregular joint system, on which account, when exposed to 

 atmospheric influences, they break up readily into angular shingle. 

 Some are ribaned, bands of different colours alternating ; many of them 

 seem capable of being split into flags, which is rarely practicable, on 

 which account they are locally called JacTc-avrea, i. e. tangled flags. 



In places in this area, a not uncommon variety of tliis schist is 

 felsitic-quarUyte, an aggregate of quartz andfelsite. Tliis often occurs 

 as a massive or thick-bedded rock, and sometimes, as will hereafter be 

 more fully stated, it may possibly not be a metamorphosed derivate- 

 rock (see page 130). relsitic-quartzyte may graclilate into fehityie, 

 while ordinary qiiartzyte generally merges into ([uartzytic-micasi/te. 

 It has been pointed out that usually a series of quartzytes graduates 

 into one or other of the different groups of schist ; there is, however, 

 one remarkable exception, as lenticular masses and subordinate beds of 

 Iwrnllendyte are not uncommon among the quartzytes of Tar-Con- 

 naught, and between these two kinds of rock there is nearly always a 

 hard abrupt boundary. These subordinate beds of homblendyte will 

 again be mentioned. 



Sornblendyte or AmpMlole-scMst . — Homblendic-micasyte graduates 

 into homblendyte, while the latter often merges into chlorityte 

 or ialcyte. The most interesting of all the schistose rocks in Tar- 

 Connaught are the homblendytes, as they for the most part must 

 originally have been tuffs or tuffose rocks, as they are intimately asso- 



* There are some rocks called quartzyte (the Quartz-rock of Jukes) such, as those 

 at Bray Head, Co. "Wicklow ; to these, ho\rever, I do not refer, as I believe that they 

 and ordinary quartz-schist belong to quite different classes of rock. From my above 

 remark I do not mean to say that in mai^puig a countrj^ the quartzj'te ought to be 

 ignored ; on the contrary, I al'ff'ays advocate that any remarkable beds, no matter 

 ho^ insignificant, ought to be traced out if the geology of the area is to be properly 

 ascertaiued. But I do object to hji^othetical boimdaries being laid doAvn; and if 

 quartzyte is to be given a hard boundary and a separate colom- on geological maps, 

 so ought micasyte, homblendyte, felsityte, &c., and every bed of sandstones or 

 grits among sedimentaiy rocks ; the absurdity of which must strike everyone. 



