128 Proceedings of the Royal Irish Acadenvj. 



Galway-type-gi'anite. In some places it is possible, although not 

 veiy probable, that ontburst of granite may hare occiuTed in these 

 places ; this, however, appears rarely, if ever, to have happened, as in 

 all cases one kind of rock seems to graduate into the other. This 

 granite has the same essentials and accessories as the G-al^vay-type- 

 granite, is similar in texture to the coarse varieties, and to me it appears 

 probable that similarly, as the sedimentary rock, change into granite, 

 so may also the Trhin stones and emytes. If such may happen, these 

 masses of olic,o:-lasic-gi'.anite may only be extremely metamorphosed 

 portions of the original outbursts of plutonic rock. 



Intrusive-oligoclasic-granite (Omey- type-granite). — ^Although some 

 of the oligoclasic-gi'anite may have been fonned in sitti, part is 

 undoubtedly intrusive, such as the masses at Omey, Eoundstone, &c., 

 which appear as protmsious. To the large tract near Galway, and 

 also to the small tracts since mentioned, there are regular bound- 

 aries, while to these protrusions the boundaries are more or less irregu- 

 lar ; besides, instead of the granite gTaduating into gneiss, it may 

 be associated with either gneiss or schist, breaking up through them ; 

 consequently the metamoi'phic sedimentary rocks may dip at or 

 oblique to it, but can never lie conf oiTaably on it. ^Moreover, it sends 

 veins into those rocks, proving that this variety must be a true intru- 

 sive rock. 



In some places this oligoclasic-gTanite is in juxtaposition with the 

 gneiss and schist, but in many cases, more especially if the boundary 

 is very iiTeg-ular, veins and masses of the orthoclasic, or highly-sili- 

 ceous-gi'anite, inteiwene. In such cases the adjoining gneiss and 

 schist are usually cut up, displaced, and ti-aversed in all directions by 

 regular and iiTegular veias of the latter granite, fi-om mere strings to 

 yards in width, sometimes foiming such a close irregular network 

 that it is impossible to di'aw an exact boundary between the granites 

 and the metamorphosed sedimentary rocks. 



The intiTisive-oligoclasic-gTanite is very similar in aspect and com- 

 position to the oligoclasic-gTanite of metamorphic origin ; nevertheless, 

 after the rocks are studied and known, certain peculiarities will be 

 learned, that are always distinctive marks between the two varieties. 



Oligoclasic-granite. OUgocIasic-granife. 



(Metamorphic.) (Intinisive.) 



Amphibolc, titanite, and such Amphibole, titanite, and such 



like minerals, are locally essen- like minerjils, seem ever to be es- 



tials, while often they ai'e acces- sentials, and sometimes are not 



sories. accessories. 



Gneissoid, or schistoid layers Gneissoid, or schistoid layers or 



or coiirses, sometimes occur in the courses, never occiu", while lentels 



mass of the rock, while lentels and irregular patches ai'e rare, 

 and irregular patches are not un- 

 common. 



