October, 1903.] 



KNOWLEDGE. 



235 



TIRERRILL AND DRUMAHAIR. 



By GrBENVIl.LE A. J. CoLE, M.E.I. A., F.O.S. 



The names of these two baronies are delightful, however 

 corrupt they may be in their present form and spelling. 

 Meeting on the borders of Sligo and Leitrim, Tirerrill and 

 Drumahair occupy a country that is purely Irish, purely 

 western, and yet is among the neatest and most attrac- 

 tive. You may leave Dublin in the morning and be in the 

 midst of it long before uoon ; and then you will certainly 

 desire to stay there and explore the ridges day by day. 

 Lough Gill bounds the area on the north ; and beyond rise 

 the great scarps of limestone that shut it off from the sea 

 at Donegal. 



There is something fantastic in the landsca])e as we 

 alight at the little station of Collooney, or of Ballysadare, 

 on the main line to Sligo. A band of singular hills, knob- 

 like and excrescent, runs into the south-west and is 

 continued north-eastward as far as Manorhamilton. A 

 fairly level lowland divides them from the sea, but from 

 this rises the huge outlier of Knocknarea, crowned by its 

 cairn, its sides almost vertical beneath a grassy dome. 

 Not far east of it is Sligo, set more beautifully, perhaps, 

 than any city in our islands, but showing little of its 

 individuality from the level of the railway. There are 

 some places that should bo approached vnth deliberation ; 

 and Sligo certainly cannot be realised, unless one comes 

 over the last hill from Ballysadare and looks across the roof- 

 tops to Benbulbeu and the broken cliff-wall of Glencar. 

 Away on the left a blue sea stretches, and, after a little 

 time, we grasp the fact that those cloud-like masses beyond 

 it, twenty-five miles away, are the highlands of Donegal. 



Cities, however, are not to the puipose in the present 

 article. We may go east at once into Tirerrill. One of 

 the odd and rocky bosses faces us as we climb from 

 Ballysadare ; it is clothed below with woods of larch and 

 fir, and shows on its higher slopes bare rounded surfaces 

 of white or warm pink-brown. We touch the same rock 

 beside the roadway in Gleunagoolagh, above Ballydawley 

 Lough, where we look south from the ridge into a more 

 fertile lowland, liarked by high Carboniferous masses in 

 the region of Lough Arrow. Our ridge, in fact, is formed 

 of granitoid rock, running across a Carboniferous country. 

 North-west of it, the limestone type prevails, giving us 

 scarps and precipices worthy of the boldest parts of 

 Yorkshire ; south-east of it the beds dip under the higher 

 series of shales and sandstones, which even bear coal, on 

 heights of thirteen hundred feet and upwards, in the 

 cloud-swept country round Lough Allen. 



The granite nowhere cuts the Carboniferous strata. On 

 the contrary, these abut against it without a sign of 

 alteration, or along a faulted junction that masks their 

 true relations. We have here to deal with a portion of 

 that older Ireland which formed a floor for the Car- 

 boniferous sea ; our ridge was at one time a long 

 promontory or island, until it became entirely submerged. 

 By its powers of resistance, it has again asserted itself 

 during the prolonged denudation of the country ; while in 

 places the stratified rocks have slipped down fault-planes 

 from its flanks, and have given it thus a greater prominence 

 in the landscape. 



We may c;ill the rock of this old axis a granit«, but it 

 varies considerably from point to point. On the charming 

 wooded slope above Ballydawley Lough, it shows strong 

 bands fonned of lighter and darker grou]is of minerals ; 

 it is, in lact, a banded gneiss. Black mica gleams in 

 aliuiidaiic(! along out! layer, accompauied by green j)yro.\ene 

 anil beautiful clear brown garnets ; (juartz and various 

 felspars form another layer ; while here and there a great 

 vein of quartz runs up along the genei'al banding. In 



some places a patchy appearance presents itself : and soon 

 we find whole lumps of foreign rock embedded in the 

 granitoid gneiss. These are dark green and rich in horn- 

 blende ; they weather away more rapidly than their sur- 

 roundings, and thus come to lie in hollows of the gneiss, 

 while the banding of the gneiss itseK runs round them, and 

 is emphasised where they happen to be most abundant. 



These dark inclusions were first noticed by Mr. Hardman, 

 and are well described, though not explained, in the 

 memoir of the Irish Geological Survey on the district. 

 Their interest lies in the fact that they must be older than 

 the rock in which they are embedded. From our modern 

 standpoint, we may agree that the gneiss is an old igneous 

 mass, that penetrated some arch in the earth's crust in 

 pre-Carbouiferous times. In the lumps of hornblendic 

 rock, technically called compact diorites and amphibolites, 

 we have a relic of the rocks forming the arch, I'ocks much 

 older than the granite which was forced among them. 



This slope in Gleunagoolagh gives us, then, one of 

 those visions into the inner workings of the crust which 

 charm us by their very incongruity. Back and l).ick our 

 thoughts may lead us, till we see the molten mass, hea^dng 

 with its imprisoned water, and its components that would 

 be gases if they could, oozing against the rocks that bound 

 it, and constantly enlarging its borders by melting off 

 fragments from the walls. Study of other districts shows 

 that amphibolites and pyroxenites, rocks rich in minerals 

 of the hornblende-augite group, can develop from a 

 number of materials, when these are attacked by a granite 

 magma ; the altered and crystallising masses are, more- 

 over, likely, under the same influences, to give rise to a 

 pi'of usion of garnet. Hence our inclusions of amphibolite, 

 which we shall meet in greater variety as we go eastward, 

 may represent a whole series of pre-existing rocks, manv 

 of them originally igneous, some of them argillaceous, and 

 some calcareous, like the blocks of intensely metamorphosed 

 limestone found in the ancient crater of Vesuvius. 



What, then, is the banding of the gneiss but a record 

 of the flow of the viscid granite magma ? The pressure 

 under which it flowed helped to arrange each constituent 

 as it was formed, and perhaps even distorted and broke 

 some crystals that had consolidated in a resting stage, and 

 then were moved again. The structure in the ridge is 

 essentially that of a mass forced onward even while it 

 cooled ; and the included blocks are often dragged out, 

 until their longer axes lie along the planes of flow. 



More than this, we soon become aware that the stronglv 

 banded and darkened gneiss of Gleunagoolagh is by no 

 means typical of the ridge. The true rock appears to be 

 a pale fine-grained granite, consisting of quartz and 

 felspar, with a trace of mica here and there. It is this 

 that provides the beautiful white slabs which shine high 

 up on Slieve Daeane amid the heather. But, wherever 

 dark inclusions of foreign matter occur, the rock also 

 becomes full of black mica and other dusky minerals, 

 while even its lighter layers are found to contain brilliant 

 little garnets. The microscope helps us to trace these 

 garnets to the included blocks, and we soon see how the 

 invading rock has l)ecome enriched at the expense of 

 earlier masses. 



Specimens can be found, measuring some two feet in 

 each direction, which would serve as museum records of 

 these subtle processes of admixture ; but it is the broad 

 survey, and the actual tramp across the hills, that force the 

 conclusions on us, when we come to compaiv observations 

 made many miles apart. And the story of the towulands 

 along Lough Gill is found to be that of the whole of 

 Donegal, and doubtless of many other parts of western 

 Ireland — the story of the invasion of an ancient and 

 already altered sedimentary series, containing sheets of 



