KiNAHAN — On, Granitic and other Lujenite Rochs. 137 



however, it appears that granite is the result of metamorphism, and not 

 metanioi'phisni of gi'anite, and that the rocks associated with granite 

 formed in situ must be metamorphosed ; but that rocks, even in con- 

 tact with intrusive granite, may only be slightly altered, not more so 

 than often happens to rocks in contact with large dykes or masses 

 of plutonic rocks. "^ 



In the islands north of the mouth of Galway Bay (Gorumna and 

 Lettennullan), the rocks are very little altered, yet they are adjoiriing 

 or in close proximity to a large tract of intrusive granite. ]^rorth of 

 Killaiy Harboiu', the rocks forming the hills north of Doolough are 

 scarcely changed, so that if met with in any other locality the altera- 

 tion would be passed over unnoticed ; yet immediately to the north of 

 them is a large tiact of intrusive granite, while in the neighbourhood 

 of the Oughterard tract of granite, the sedimentary rocks are less 

 altered than in many other places in West Galway. Here limestone is 

 found in contact with the granite, and often caught up in it, yet the 

 limestone is not much changed ; that caught up in it, or in immediate 

 contact, has lost all its colour and become highly crystalline ; but 

 these changes only exist for a few jards, as limestone not 200 feet 

 from it was found to be scarcely altered, compared with other places 

 in the district. On the other hand, an observer can always know 

 when he is approaching granite formed in situ, by the remarkable 

 changes that take place not only in the aspect, but also in the structure 

 of the rocks. f 



* In the coimtry nortli of Killaiy Harbour there are shales adjoining dykes of 

 felstone altered into homstone. At Curraghrevagh, on Longh Corrib, there are Silu- 

 rian rocks micacised and indurated by a protrusion of diabase, and in many other 

 places in Ireland similar alterations could be pointed out ; the rocks in each case being 

 more metamorphosed than any of the rocks that I saw associated -nath, or even in 

 contact with, the Dartmoor gi-anite, Devonshire. From the cuxumstances associated, 

 when derivate rocks in contact with phitonic rocks are altered, I am inclined to 

 heUeve such alterations are not due to the plutonic rocks, but to heat, either wet or 

 diy, that came up in the divisional plane or the fissure between the different classes of 

 rocks. 



t In some places, as for instance Eilliney, the rocks in contact with the mass of in- 

 trusive granite of Dublin, "VTicklow, Carlow, and Kildare, are miich altered ; but in 

 other places, such as the neighbourhood of Castledennot, Co. Kildare, they are not. 

 To account for this, I would suggest that some of the rntnisive masses of granite 

 cooled under such circumstances that the heat from them had to percolate the 

 associated rocks, while in such masses as that at Daiimoor, Devonshii-e, where the 

 rocks in contact are scarcely changed, the rock must have cooled under such 

 circiunstances that the heat all escaped "without affecting the associated rocks. This 

 pent-up heat, if it escaped along the walls of a plutonic or volcanic dyke, would more 

 or less alter the rocks it passed. Every fresh locality for granite that I visit, the 

 more I am con-^-inced that gi-anite is the result, not the cause of metamorphism. Since 

 this essay was read I have more carefully examined the rocks in the vicinitv of the 

 gjanite exposure north of Doolough, county Mayo, and find some of the rocks so 

 little altered that their fossils are quite perfect ; while at the extreme S. E. end of 

 the county Wexford there is granite due to metamoi-phism that graduates through 

 gneiss into schists, while to the X. E. of the same county, and in the county 

 Wicklow, the rocks in juxtaposition with the intrusive eranite are less altered 



