76 



KNOWLEDGE 



[Apeil 1, 1898. 



layer after layer was laid down. Between the parallel 

 ranges of the "Caledonian" chain, long valleys of 

 subsidence may have existed like that in which the East 

 African lakes have arisen at the present day. By an 

 opposite movement, along the planes of gradual faulting, 

 the intervening ridges may have prolonged their existence, 

 and may have maintained the level of the continent. By 



i^j^^^~ 



Fig. 1. — Sketch-map of Ireland, showing the direction of the 

 principal axes of folding. The lines represent the trend of both 

 anticlinal and synclinal axes. Lines with dots represent the 

 " Caledonian " folding ; thick lines, the " llercynian " folding. 



our own times, a succession of later earth-movements has 

 complicated the relations between the Devonian sandstones 

 and the land-surface that gave them birth ; but we may 

 still see in the great chain of Leinster one of the real 

 highlands of "Caledonian" times. 



The marine Carboniferous beds abut directly on a great 

 part of this chain, with no exposure of Old Eed Sandstone 

 round their margins ; hence the ridge stood out as a long 

 island even in the Carboniferous sea. To this day it 

 forms the most continuous portion of the mountain-rim of 

 Ireland, though shorn of its former schistose peaks by 

 whole eras of denudation, and though the round back of 

 the granite-core is now laid bare to view. 



The "Caledonian" uplift was characterised by a feature 

 common in true mountain - chains — the intrusion of 

 granite along the more important lines of elevation. As 

 the long arch formed, the igneous mass rose with it, 

 melting off its lower layers, sending ofif veins into higher 

 ones, and inducing crystallisation and foliation in the 

 argillaceous beds along the contact-zone. Hence the back- 

 bone of Leinster became strengthened from below ; and its 

 double structure is seen clearly in any traverse of the 

 range. 



Bound Newry, again, granite forms a hard ridge inti- 

 mately connected with the "Caledonian" folds; and at 

 Castlewellan, a little further north, the igneous invader 

 has been caught, as it were, in the act, and is seen to be 

 stuck full of fragments of Ordovician or Silurian strata, 

 which present every stage of alteration, from mere baking 

 to almost complete absorption. It is very reasonable to 



suppose that the characters that distinguish the Newry 

 granite from that of Leinster are induced by the amount of 

 foreign material absorbed by it in the portion now exposed. 

 Further evidence of the support given to the "Cale- 

 donian " folds by the intrusion of granite is seen in the 

 exposures in the county of Cavan. At and near Crossdoney, 

 a granite of very various grain and character comes to the 

 surface among the Ordovician shales. It is a miniature 

 picture of the structure of the Leinster chain, and suggests 

 the vast extent of similar features hidden throughout 

 Ireland beneath the blanket of Carboniferous rocks. 



When we go north or west, we are confronted with the 

 schistose ranges, which may be of any age between the 

 date of the " Huronian " uplift and the Devonian period. 

 Unconformities show that there were movements, unclassi- 

 fied in the broad scheme of Bertrand, before Ordovician 

 times; but the great folding of the country, like that of 

 the Scotch Highlands, clearly occurred about the close 

 of the Silurian period. To this we owe the green and 

 romantic range of the Sperrins, a highland scarcely 

 visited, even by the dwellers on its flanks ; also the 

 whole present structure of wilder Donegal, with its 

 ridges and valleys running north-east and south-west, still 

 preserving the general trend of the Caledonian folds ; 

 and, again, the superb coast-scenery of Slieve Liaga 

 and Achil Island, where cliffs of two thousand feet 

 remind us of the mass of " Caledonian " land that has 

 become lost in the Atlantic. The uplift of Mweelrea, 

 with its fossiliferous Wenlock zones, and of the Wenlock 

 and Ludlow beds of the Dingle promontory, dates from 

 the same period of unrest. In the latter spot one of the 

 fractures reached the surface, and our unique volcano of 

 Wenlock age threw its bombs briskly in the air, as a sign 

 that the Silurian gulfs were about to pass into dry land, 



A great part, then, of the mountain-rim of Ireland is of 

 extreme antiquity ; and in other places the pre-Devonian 

 surface has been, as it were, restored to us after many 

 strange vicissitudes. The Carboniferous subsidence con- 

 verted the region of the British Isles into an archipelago ; 

 and in Ireland the separate islands can sometimes be 

 traced out by the conglomerates formed in the Carbon- 

 iferous beds upon their flanks. This invasion of the sea 

 left its mark upon the whole centre of the present Ireland, 

 through the uniform deposition of the blue-grey Carbon- 

 iferous Limestone. The denudation, and the actual solu- 

 tion, of this rock have given us the landscapes of the 

 great plain ; these become often impressive in their very 

 breadth, and are never monotonous to those who love to 

 watch the cloud-shadows move across the bogland or the 

 lake, in a picture that takes half its life and colour from 

 the changing temper of the sky. 



The great limestone-sea was thrust out, very gradually 

 at first, by what is known in Europe as the Hercynian 

 uplift, named after the forest-ranges of Central Germany. 

 The sandy beaches that were formed as the sea shaDowed 

 give us ledges of hard rock at the present day, such as that 

 on the crest of C'uilcagh, where the Shannon first forms 

 into a stream. The trend of the Hercynian folds was no 

 doubt diverted locally by the surviving knots of the 

 Caledonian chains ; but in many places the pre-Devonian 

 land gave way. It was thus worked up again, and was 

 brought into new prominence, and into a new scheme of 

 arrangement, in the cores of the Hercynian folds." 



From the west of Kerry to Waterford, away on across 

 Pembrokeshire and the South Welsh coalfield, under 

 Oxfordshire and London, and through Belgium and Central 



* Compare W. J. Sollas, " Gteology of Dublin and its Neighbour- 

 hood," Proc Geo!. Assoc, Tol. XIl'l. (1893), p. 113. 



