TOPOGRAPHY AND GEOLOGY 



The more ordinary muddy sediments are now found in the foothills of the 

 Leinster chain, and also in a broad area stretching from Drogheda and 

 Cavan to Belfast Lough. 



At the close of Silurian times, the subterranean forces began their work 



ao-ain in Europe. Volcanic eruptions had already indicated a considerable 



amount of unrest. Off Portrane, a cone had been 



Portrane and reared, spouting out its lavas and ashes into the sea 



Lambay Island, m which the corals grew — an interesting precursor of 

 the conditions that prevail in the Pacific of to-day. The 

 neck of this volcano, cold and crystalline, now forms Lambay Island ; and 

 the famous green " Lambay porphyry " is the mass that last consolidated in 

 the vent. In Kerry again, we have a unique little volcano, of Gotlandian 

 age, which has left its lavas and banks of agglomerate in the cliffs of the 

 Dingle promontory. Then the wrinkling of the crust set in. A series of 

 huge folds were formed, with axes running north-east and south-west. 

 Sometimes these were pressed over obhquely, and became broken through, 

 while one part moved over another along surfaces of shding known as 

 thrust-planes. Old rocks, that ought to have been comfortably buried 

 down below, were thus brought to the surface, and became piled on others 

 of far later date. The Huronian chains were in part remoulded, and frag- 

 ments of them were worked up into these new Caledonian chains. The 

 latter take their name from the Grampian region, which was conspicuously 

 involved in these disturbances at the close of Silurian times. Thus some 

 of the leading hnes of Ireland became early impressed upon our area. The 

 north-east and south-west " Caledonian " trend, the trend of the axes in 

 Scandinavia and in Scotland, is clearly seen in the structure of Donegal and 

 the Ox Mountains, in the axis from Cavan to Belfast, and notably in the 

 Leinster chain. The folding was accompanied by the uprise of molten 

 granite from below. This hot igneous rock, squeezed upward by the earth 

 pressures, filled the arches of the anticlinals, inch by inch, as they were 

 formed. It attacked its surroundings, melting mass after mass from 

 the walls, absorbing them into its substance, and sending insidious offshoots 

 into the adjacent shales and sandstones. The sedimentary rocks forming 

 the arches thus became baked and crystalline, and in places are bound to 

 the invading granite by a network of interlacing veins. As the weather 

 worked down against the uprising chains, the coating of sediments was 

 often worn away, and the granite, now cold and hard, was exposed as a 

 moorland in the midst. 



The backbone of Leinster, running south-west from Dalkey to the junc- 

 tion of the Barrow and the Nore, a distance of seventy 

 The Leinster miles, was thus formed by the Caledonian movements. 

 Chain. On its flanks, Ordovician, and, perhaps, Gotlandian, 



strata, rise in contorted masses, consisting of dark 

 shales for the most part, and easily cut into by the rivers that flow from the 

 central axis. Picturesque ravines and valleys, like those of the upper Liffey, 

 with woods and old demesnes along them, mark this region on the east or 

 west. In Wicklow, similar features, including the Glen of the Downs and 

 the Devil's Glen, have been carved out of the older strata of the Bray 

 series, which have also become involved in the flanks of the chain. As a 

 contrast to this varied country, the high moors of Dublin, Wicklow, and 

 Carlow, stretch in a uniform series of great domes, heather-clad and impres- 

 sive in their vastness, where the granite core comes to light along the axis 

 of the chain. 



