1862.] HAUGHTON GKANITES OF IRELAND. 407 



the deposition-planes of the gneiss, and having its crystalline facets 

 of black or green mica set in the same direction. The gneiss exhi- 

 bits good examples of the development of particular minerals, fel- 

 spar, mica, &c., in nests or pockets. The veins in the granite, up to 

 the summit of the pass, consist of quartz, orthoclase (white), and 

 black mica : beyond the summit, on the slope of the mountains 

 towards Glenveagh, the veins consist of quartz, orthoclase (white), 

 and white mica. At the opposite side of the valley (south of Lough 

 Veagh), the veins are pink orthoclase and quartz ; and the granite 

 itself either grey or pink (with nests of black mica developed in the 

 grey granite), passing, towards Kilmacrenan, into gneiss, in the same 

 manner as at Lackagh Bridge on the north." 



In many places the metamorphic slates are penetrated by coarse 

 veins of granite, as at Pintown Gap, Lough Moume near Barnesmore 

 Gap, the Black Gap near Pettigo, and at Castlecaldwell in the Co. 

 Fermanagh ; and in some of these localities, the granitic material 

 even seems as if formed in the bosom of the rock, without any con- 

 nexion with a vein or dyke of granite. Thus at Lough Mourne I 

 made the following note, in company with the Eev. William Steele ; — 



'^ September 8, 1856. — In a quarry east of Barnesmore Gap, ob- 

 served isolated nodules (1^ ft. in diameter) of white milky quartz, 

 with large crystals of red felspar and micaceous oxide of iron ; also 

 veins of the same composition, which cut across the strata of the 

 rock, which is a fine-grained, hard mica-slate, tending to a gneissose 

 character from the development of felspar-crystals : the plates of 

 mica in the rock are smaU, black, and well de&ied." 



At Castlecaldwell, the granite-veins which penetrate the gneiss 

 are of two totally distinct kinds : — 



A. Yeins composed of quartz, pink orthoclase, white mica, black 

 mica, and schorl ; all the crystals being large. 



B. Veins composed of quartz, pink orthoclase, yellowish-green 

 waxy finely striated oligoclase, black mica, sulphuret of molyb- 

 denum, and copper-pyrites ; crystals moderately large. 



At Fintown Gap and at Black Gap, the veins of granite are of the 

 first kind (A). 



The stratified rocks resting upon the gTanite appear to be divisible 

 into four groups, which, in an ascending order, are as follows : — 



1. Quartz-rock, associated with limestone containing garnets in 

 dodecahedral crystals, and idocrase, and accompanied by sphene- 

 rock. — ^Mamore quartz = 7600 ft. 



2. Slate-rock, formed chiefly of hornblende- or anthophyllite- 

 slate, associated with beds of limestone, potstone, and steatite, with 

 contemporaneous stratified syenite. — Buncrana slate =5400 ft. 



3. Flaggy quartz-rock of Culdaff=3000 ft. 



4. Micaceous slates and gneiss, with numerous beds of blue and 

 white crystalline impure Hmestone. — Thickness unknown. 



Total probable thicknes8=: 16,000 ft. to 20,000 ft. 



IV. Chemical Composition of the Granite of Donegal. — The fol- 

 lowing Table contains the analyses I have made of the granitic 

 axis of Donegal from N.E. to S.W. 



