TOPOGRAPHY AND GEOLOGY. 5 



This rock, with its broad even joints and powdery products of weathering, 

 gives rise in the course of ages to round-backed hills, with few conspicuous 

 peaks, and with valleys smoothed by crumbling debris. The contrast be- 

 tween its characters and those of the stratified masses round it is well seen 

 at the Upper Lake of Glendalough (Fig. i), where the sheer walls of shale 

 and schist abut on the spurs of Lugnaquilla. 



In the corresponding axis of Newry, granite has similarly welled up, and 

 at Castlewellan it is seen to be stuck full of frag- 

 The Axis of ments derived from its stratified neighbours. The 



Newry. whole Newry granite probably owes its darkened 



character to the material absorbed by it ; and the in- 

 clusions in it are often completely altered and crystalline, and are pene- 

 trated on a microscopic scale by the granite that attacked them. The 

 Ordovician and Gotlandian rocks of Louth, Monaghan, and Down, form a 

 broken country of small and frequent hills, with one of the most irregular 

 surfaces to be found in Ireland. 



As alread}- liinted, the west and north-west highlands were certainly 

 refolded in Caledonian times. Old knots of gneiss, 

 like that of East Tyrone, had the younger masses 

 lyroiie. pressed against them, and formed " eyes " round which 



the Caledonian earth-waves flowed. Granite veins 

 traversed them, becoming especially conspicuous in the counties of Mayo 

 and Donegal. It is often difficult to distinguish between the older Huro- 

 nian granites and the new, throughout this mingled region of the West. But 

 the trend of the Ox Mountains, with their granite 

 The core, invading the schists and amphibolites, and run- 



Ox Mountains. ning from Castlebar to Sligo, and the lines of fold and 

 fracture in Donegal, such as the great glen from 

 Gweebarra Bay to Sheep Haven, are clearly due to 

 Highlands of the Caledonian system of movements. At the same 

 Donegal and Mayo, time, the Gotlandian beds were uplifted high and dry 

 in Mayo, and have since been carved out into the 

 noble masses of Muilrea and Ben Gorm, which look down on Killary 

 Harbour. The quartzite cone of Croagh Patrick is now known to belong to 

 the same series of strata, which have thus contributed largely to the rugged 

 scenery of the west. 



This uplift at the close of Gotlandian times formed a continental area 

 on which detritus began to gather, Vi^hile the great lakes spread across the 

 hollows. The sea still lay to the south-east across Devonshire and Belgium ; 

 but the Irish and Scotch areas were included in the land. The weather 

 soon laid hold of the Caledonian masses, and rolled down sand and pebbles 

 fiom them into the lakes. Under the burden of debris thus poured into 

 them, the lake floors sank, as those of Eastern Africa have done since the 

 time of their formation, and thousands of feet of freshwater strata were 

 thus enabled to accumulate. This was the origin of the Old Red Sand- 

 stone, laid down in the Devonian period. 



The boundaries of the old lakes are nowadays difficult to determine. The 

 sandstone and conglomerate that form a hilly land between Lough Erne 

 and Pomeroy may have been at one time continuous with corresponding 

 beds in Southern Scotland. The great masses of the south of Ireland may 

 have been connected on the east with the Devonian estuary of Hereford 

 and Wales. In any case, the lake deposits extended far and wide across 



