284 



KNOWLEDGE. 



[Decbmbbb, 1902. 



Ciirboiiiforoiis trees fjatherin^ on its tlanks. Tlie seenery, 

 Init for the greater proportion of schist tliat still ovt-rlay 

 the j^ranile core, was essentially that of the ])res('nt day. 

 The sea-lloor then sank, until the witters spi'eail across the 

 Irish niiillauils, and tiually covered the stuliborn heights 

 of Donegal. By the opening of the Upi)er Carboniferous 

 ejioeli, the Backbone of Leinster was probably entirely 

 submerged. 



The striking Iandsca])e of early Carboniferous days was 

 thus buried in soft liniestone-inud, in shell-banks and 

 coral reefs, and seemed for ever lost to view. Coal- 

 Measure forests stretched across the country, which, in 

 ])liu;e of mountain ridges, now exhibited a monotony of 

 sandy and swampy tlats. The Hercynian uplift, however, 

 altered these conditions ; new folds set in, accommodating 

 themselves to the unseen Leinster Chain below ; the arch 

 of Coal-Measures and limestone was broken away piece- 

 meal from the buried mass of mountains ; and at last the 

 granite itself again began to tie revealed. 



Denudation at tirst was slow enough, through the arid 

 Triassic days ; but, little by little, the ancient features were 

 restored. Even the great glacial incident at the close of the 

 Cainozoic era only moulded the general outlines, and left 

 undulating mounds of gravel against the severer mountain- 

 slopes. These glacial deposits, which have been largely 

 re-arranged by rivers, are nowadays mostly cultivated, and 

 can easily be picked out, as smooth green areas, under the 

 heather-clad or rocky hills. They often fill the old notches 

 and stream-cuts of pre-glacial days. When we speculate 

 as to the age of this or that valley in the Leinster Chain, 

 we are tempted to ask if any of them can belong to the 

 remote and primary Caledonian drainage system. There 

 must have been original transverse valleys, which became 

 flooded and infilled by the deposits of the Carboniferous 

 sea. Can any of these have survived, to be re-excavated 

 by the streams of later days ? 



Let it suifice that modern denudation is paring away 

 the Carboniferous Limestone from the antique surface of 

 the hills. The glacial gravels show how vast a quantity 

 of this limestone has been broken up in recent 

 times. And the loug-lost Caledonian highland, perhaps 

 reduced to half its former height, emerges along 

 the present lines of foothills, and forms the central 

 granite moors. 



When Mr. Weaver,* in 1818, first gave a detailed 

 account of the structure of the Leinster Chain, he traced 

 the junction of the granite and the schists from Killiney 

 southward. He noted the interlamination of granite and 

 mica-slate in Glenmalure, and the cap of the latter rock 

 left behind three thousand feet up on Lugnaquillia. 

 Weaver says that the passes across the range were in his 

 day scarcely practicable, even in summer ; and Glenmalure, 

 which is now abandoned, seems then to have been the best 

 of them. The military road, made after 1798, to command 

 the heads of the glens, gave Weaver access to the 

 interior of the range. 



In our time, there remain large central tracts which are 

 scarcely ever traversed. They lie remote from any cart- 

 track, and are difficult to visit in a day. But the chief 

 beauty of the mountains lies in the contrasts betAveen the 

 granite and the stratified foothills. The streams that form 

 broad basins on the central moorland give rise to caiions 

 in the jointed rocks of the lower ground. The grey 

 quarlzite of the Bray series forms a series of striking 

 summits between the sea and the main watershed ; while 

 the splintery Silurian shales, converted along the junction 

 into gleaming mica-schists, weather into rocky slopes, on 



* "Memoir on the Geological Relations of the East of Ireland," 

 Trans. Oeol. Soc, Ser. I., Vol. V. (1821), p. 117. 



which the lines of stratification are apparent. Where 

 the granite has eaten into them, as at Luggela, near 

 Jloundwood, or on the crest of Douce, or down against 

 the sea in the typical section of Killiney, andalusite ami 

 garnet have developed ; and a banded structure is often 

 found in the adjacent granite. This was noted by Weaver,* 

 who speaks of it as an approach to gneiss ; and Sollasf 

 regards it as due in some places to original flow, and in 

 others to pressure acting on the solid mass, in continuation 

 of the forces that uphejived the Caledonian chain. The 

 granite has been stated to l»e but little modified by 

 absorjition of matter from the walls of its caMron ; but it 

 often becomes richer in dark mica near its margins, and 

 assumes here and there the aspect of a composite gneiss. 

 On the whole, however, as Haughton long ago pointed out, 

 it jjreserves a remarkable uniformity throughout the length 

 of the great mass. 



To those who would appreciate the structure of the 

 chain, and the contrasts of scenery on its flanks, the famous 

 Military Road offers a ready introduction. There are 

 some five miles of climliing on it out of Dublin, until we 

 reach the watershed, where we are far up on the old Slieve 

 Roe, a region of heather and high-lying peat, with the 

 crag above Lough Bray prominent before us in the south. 

 Most of the rock-exposures and romantic river-clefts lie in 

 the older Palaeozoic foothills ; the granite furnishes a 

 gathering- ground for the streams, and its long slopes are 

 only here aud there broken by a cirque or some uuburied 

 talus. The lower Lough Bray lies in a semicircular 

 hollow, which was once occupied by a residual glacier, aud 

 so remained protected while other craggy features 

 disappeared. It is divided from the upper lake by a huge 

 glacial dam, from which we look into a second cirque, 

 carved out originally when the denuding forces were more 

 severe. Already the grass is climbing across the old rock- 

 walls, and the deltas of sand, formed of fragments of 

 felspar aud quartz and mica, are spreading out into the lakes. 

 Lough Tay, farther south, has similarly been invaded by 

 the green delta of Luggela ; and the fine crag above it 

 belongs to the older order of mountain-landscapes. Only 

 the young granite ranges, like the Mont Blanc maasif. the 

 Pelvoux, or, in their own small way, the Mournes, retain 

 their array of peaks and pinnacles. Even among the 

 Mournes, or the equally Cainozoic Red Hills of Skye, 

 the decay and rounding-oft' of the older features 

 are already obvious. The characteristic smooth dome 

 is already prevalent in these wind-swept granite 

 uplands. 



If we i^limb again from the last cottage at Lough Bray, 

 we find, in a waste moorland, the infant Liffey rising as a 

 mountain-stream. It pours down a fine and wooded 

 valley to the east, aud then cuts deeply into the Silurian 

 shales. At Pollaphuca, " Puck's Pool," the cascade and 

 chasm are the last highland feature ; the river then goes 

 off wandering into the limestone lowlands of Kildare, 

 bends northward at KilcuUen, and finally cuts through 

 the recent and glacial gravels on its way to the sea at 

 Dublin. M\id-brown and commercial though it may be 

 within the city, two miles out it speaks of the unbroken 

 country. At Lei.xlip (the " salmon-leap "), the name 

 reminds us of the Norsemen, the strong sea-power that 

 founded Dublin ; but the water, pouring across the rocky 

 ledges, is still fresh from that larger Ireland which waits 

 for us ou the Leinster hiUs. 



* Op. «7.,p. 132. 



t "Contributions to a knowledge of the G-ranites of Leinster," 

 Trans. R. Iris/i Aead., Vol. XXIX. (1891), pj). 11)2 and 4i)f)-502i 

 also "Geology of Dublin," Fror. Oeol. A.uoc., Xol. XIII. (18'.>.5) 

 pp. 108-111. ■ 



