406 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 18, 



granite, indicated by its great central valley, differs by 11° 20' from 

 the principal system of cleavage-planes developed in this granite, 

 which lie 31° 40' S. of W., while the geographical axis lies 43° 

 S. of W. ^ 



III. Geological Belations of the Granite of Donegal. — The granite of 

 Donegal appears to be interstratified with the quartz-rock, mica-slate, 

 and limestone with which it is associated; but it is probablysubsequent 

 to them in age, and in its central portions is perhaps of igneous 

 origin, originally deriving its cleavage-planes and gneissose character 

 from the pressure exercised upon it by the stratified rock, which has 

 been lifted, to the north and south, to a nearly vertical position. 



On the boundary of its outcrop, both north and south, it is inti- 

 mately interstratified with the aqueous rocks. For example, at 

 Glenlehen, between Fintown and Doocharry Bridge, near its southern 

 boundary, but still well within the granite-border, I made the fol- 

 lowing note, in company with Mr. Eobert H. Scott and Mr. Athel- 

 stane Blake : — 



"August 5, 1861. — Glenlehen, summit-level. Observed vertical 

 beds of quartzose mica-slate and of g-neiss lying in granite, with 

 which they are interstratified, also beds of sphene-rock, and one bed 

 (3 ft. thick) which seemed to be an altered quartzose limestone, con- 

 taining garnet and chalcedony. The sphene-rock is composed of quartz 

 and orthoclase, with crystals of sphene, running in veins and 

 scattered. [At Anagarry Hill, where this rock is abundant, it con- 

 sists of a white paste of felspar (orthoclase), with crystals of horn- 

 blende and large crystals of sphene : it is said always to lie next to the 

 altered limestone.] " 



The joints of the granite bear N. b°l^. 



Its gneissose structure bears E. 10° N., vertical. 



The stratified rocks bear E. 10° N., „ 



Still nearer to the southern boundary of the granite, a thin bed of 

 limestone has been traced, lying nearly vertically for several miles. 

 At the northern edge of the granite, it passes by equally insensible 

 gradations into stratified rocks. 



At Lackagh Bridge and Ballygihen, in company with Mr. James 

 "Wood and Captain Montgomery, I made the following notes : — 



" August 19, 1856. — North of Lackagh Bridge, observed fine 

 alternations of micaceous quartz-rock (flaggy), hornblende-slate, and 

 gneiss, with pockets of black mica and nests of large felspar- and 

 quartz-crystals. The pink felspathic granite appears at the other 

 side of the river (south), and graduates into a grey variety of fel- 

 spathic gneiss, with black mica." 



" August 22, 1856. — In Dunlewy quarry the crystalline limestone, 

 forming a coarse statuary-marble, associated with thin bands of 

 quartz-rock, is greatly contorted, and rests on contorted hornblendic 

 micaceous slate, penetrated by thin vertical dykes of felspathic 

 granite, containing garnets. From this quarry, by the Poison Glen 

 to Ballygihen, the transition from gneiss to granite is finely exhi- 

 bited ; the granite ultimately retaining planes or joints parallel to 



