Cole— r//6 Intrusive Gneiss of Ttrerrill and Drumahair. 367 



1895 (p. 52), thougli important remarks on the structm-e of the Ox 

 Mountains appear on p. 42 of that valuable work. Here it is stated, 

 however, that the amphiholites penetrate the gneiss in the region to 

 the west. This is contrary to my experience elsewhere. It is 

 of interest also to observe that the geological map of Sir E. Griffith, 

 edition of 1855, shows a patch of "gneiss passing into granite" on 

 Benbo, near Manorhamilton. This gives us no clue, however, as to 

 whether the granite was regarded as intrusive ; we may almost safely 

 presume that the metamorphic view was then adopted. 



ng.3. 



Microscopic section of gneiss, with inclusions of light and dark mica, 

 garnet, and pyroxene, derived from the adjacent eclogites. 

 Stonepark, near Drumahair. x 18. 



A variation on the prevalent type of composite gneiss is seen in 

 the strongly banded masses east of Castleore. A granulite with pale 

 pp'oxene and biotite has here arisen, with obvious residual inclusion- 

 flecks containing both these minerals. I have not been able to trace 

 the original pyroxenic rock in this instance ; but coloui'less pyroxene 

 occurs in many of the amphiholites and eclogites of southern Donegal. 

 Such basic crystalline rocks arise as products of metamorphism from 

 very different materials, when these become invaded by and immersed 

 in a gi'anite magma ; and the variety of mineral constitution in the 



K.I. A. PKOC, VOL. XXI-V., SEC. B.] 2 S 



