364 



Proceeding!^ of the Roi/al iri><h Acddenn/. 



the foliation-layers, and still fui'ther emphasiae its handsome struc- 

 ture. In Castleore, on the other hand, just above the Correagh hamlet, 

 the,quartzose type prerails, and may easily be taken for a true quartzite 

 in the field. In sections under the microscope, however, the rock is 

 seen to be still felspathic, and a good type of that fluidal aplite, 

 affect-ed locally by pressure, "which forms the basis of the gneiss of 

 north-west Ireland. 



In Castleore, the rocky bosses of brown gneiss show a delicate 

 banding, which is mainly due to abundant strings of garnets carried 

 out along the general lines of flow (fig. 1). "Where blocks of amphi- 



Fig.l. 



Microscopic section of fine-grained gneiss (fluidal aplite) witli 

 atundant garnet. Castleore. x IS. The garnet is derived 

 from the included eclogites and amphibolites. 



bolite occur in the gneiss, the banding becomes emphasised, and the 

 flow-surfaces fold round them. At its junction with one large mass, 

 the gneiss sends off dykes into the amphibolite, and cuts into it along 

 a zig-zag surface, the flow -lines following the serrated margin 

 (fig. 2). The appearance of sharp folds thus produced in the gneiss 

 is due to its having worked its way into the amphibolite along 

 joints or planes of weakness. In one place a dyke arose ; in another 

 the amphibolite became deeply notched ; and the crest of the " infold " 

 of gneiss occupying the notch sometimes runs on as a thin sheet into 



