Cole — On Composite Gneisses in Boylagh^ West Donegal. 220 



DESCRIPTION OF PLATES. 



Plate I, 



Fig. 1. — Composite gneiss, produced by intnision of granite into 

 the schistose series near the base of Trusklieve, on the northern shore of 

 Toome Lough (p. 215). 



Fig. 2. — Pegmatitic granite cutting and sending veins into horn- 

 blende-schist, which has previously become considerably-" granitised." 

 North-east slope of Derkbeg Hill (p. 218, and PI. in., fig. 2). 



Plate II. 



Fig. 1. — Section showing margin of a thin sheet of granite in horn- 

 blende-schist, south-west side of Carbane, Glenties (p. 213). x 9. 



This microscopic section represents in miniature^^the structure of 

 the granitic and schistose country in Boylagh. The granite has pene- 

 trated the schist, after the latter had become foliated and crumpled. 

 A fluidal structure along the margin has converted the granite locally 

 into gneiss ; this is occasionally emphasised by the presence of flakes 

 removed from the schist. Subsequent movements, represented in the 

 section by the faulting, have had some effect upon the joint mass ; 

 but the gneissic structure is connected with original conditions of 

 intrusion. 



Fig. 2. — Section showing junction of phyllite and gneissic granite, 

 south-east side of Carbane, Glenties (p. 212). x 7. 



The coarse granite is seen below, with streaks of biotite, due to 

 inclusion of material from the phyllite. "When seen over a wider 

 field, these give a well-marked gneissic structure to the granite. The 

 phyllite contains numerous crystals from the granite, and resembles 

 a " porphyroide " or a felspathic ash. This intermingling seems due 

 to earth-movements acting after the cryatalHzation of the granite, but 

 in continuation of those under which the igneous rock was intruded. 



