138 Froceedinys of the Royal Irish A('ade)uy. 



If the suggestions that we hare been consideriiig are correct, the 

 granite that iirst cooled ought always to be less siliceous than the 

 granite intruded into it ; while the veins of segregation or cooling 

 {granityte) ought to be more siliceous than the rock in which they 

 appear, and from which they separated ; and this seems to be always 

 the case in West Galway.* 



than, tliose farther away. In the north of Ireland, at the Moume intnisive granite 

 district, coimty Do-mi, the rocks next the granite are only very slightly altered, and 

 apparently this metamoi-phism is much older than the intrusion of these post-car- 

 boniferous granites ; while in the Carlingford district, coimty Louth, the carbonife- 

 rous rocks (limestone), through which a large mass of granitoid elvanyte protrudes, 

 are apparently unchanged. To the north of these districts the older intrusive 

 granite of Xe-sviy and Slieve Croob, as proved by my colleague, W. A. Trail, 

 F.E.G.S.I., has alongside in places rocks more metamorphosed than is general iu the 

 county Down, but this alteration is always very small, and does not appear to be 

 general round the mass. It is also so slight, that all the metamoi-phic rocks in. the 

 vicinity of the Newry and Slieve Croob granite belong to the class called " Sub- 

 mr'tamorphic rocks" by the officers of the Indian Geological Survey. Trail has also 

 pointed out that in those places where the derivate rocks are thus additionally 

 altered, the adjoining granite is foliated ; as if subsequent to the intrusion 

 of the granite a strip of country had been invaded by heat (wet or dry), 

 wliich had developed a foKation in the granite, and slightly increased the meta- 

 morphism of the adjoining derivate rocks ; and this, he thinks, seems to be suggested 

 bv the action decreasing both ways as we leave the junction of the granite and the 

 scliist, at which Hne, naturally, the heat might be expected to be most intense. 

 \_Xote added in the p7-ess.~\ 



* This remark can only refer to a system of granites formed at one time ; for if 

 one system of granites were foimed in the silurian period, and another in the 

 carboniferous period, the basic granites belonging to the latter might occur breaking 

 up through the highly-siliceous granites belonging to the former. Two quite diffe- 

 rent classes of vetns are kno^wn as "veins of segregation;" one kind, which is 

 always more or less regular, is due to a portion of the tiuid rock segregating from 

 the rest, and filli ng the shrinkage iissiu'es and such like vacancies ; to such veins the 

 above remark applies. The other class is always most irregiilar, often lenticular, 

 and appears to be due to minerals in solution, which subsequently crystallized, 

 filling ii-regular vugs or cavities in the rock mass ; to such veins belongs the rock 

 previously described under the name of pegmatyte. Hunt, as previously mentioned, 

 has suggested the name of endogenous for these vein-rocks or " granite vein-stones," 

 to distinguish them from true granitic-dykes. — " Report, Geol. Stuw. of Canada," . 

 p. 192. 



