THE TOPOGRAPHY AND GEOLOGY OF 



IRELAND. 



Ireland, lying on the western rim of the great Eurasian continent, occupies 

 a position of extreme geological interest. The line along which a large 

 body of water meets with continental land is now recognised as one of in- 

 stability and unrest. It has long been obvious that the breakers are wearing 

 away the rocks at one point, while at another they are depositing beds of 

 shingle and fine sand. But, at the same time, the very floor of the ocean is 

 rising or falling with the slow movements of the crust ; the ocean is thus 

 forced to recede before the elevation of a new coast-line, or is allowed, by 

 subsidence, to creep in upon the land. Each movement, this way or that, 

 leaves its record m the rocks. The masses that are already solid become 

 crumpled together like a cloth ; old marine deposits are forced up to form 

 the outposts of a continent, until we find the shells entombed in them lying 

 thousands of feet above the sea ; or the land surface, carved by rain and 

 rivers, sinks beyond the reach of the destroying agents, and is gently buried 

 beneath sheets of sediment in the ocean. The resistance or yielding of the 

 border-lands that protect a continent often determines the fate of the con- 

 tinent itself. The rocks of Ireland thus record the main features of the 

 history of Western Europe. 



The present outline of the country is, geologically speaking, of modern 

 date. The island rises, in fact, from the continental plateau, and is essen- 

 tially a part of Europe. The line marking a depth of lOO fathoms upon the 

 Admiralty charts runs from Norway, outside the Shetlands and the Outer 

 Hebrides, keeps west of the Irish coast by 25 to 100 miles, and then passes 

 down southward until it almost touches Spain. Beyond this line, the depths 

 increase rapidl)', as we reach true oceanic waters. Only 100 miles west of 

 Co. Mayo, we find a depth of 1,000 fathoms, and 300 miles west of Co. 

 Kerry we have the abyssal depth of 2,700 fathoms, or more than 16,000 

 feet. On the east, the channel between Ireland and Scotland is, at one 

 point, only thirteen miles wide ; and at Wexford it is only some fifty miles 

 across to Wales. Between Stranraer and Larne, there is a singular depres- 

 sion, reaching down to 140 fathoms (840 feet) ; but this is quite local, and 

 the sea between Ireland and Great Britain is rarely deeper than seventy 

 fathoms. The small granite hills of Killiney, in- Co. Dublin, could be cut 

 off at the sea level and pushed across from Kingstown to Holyhead, without 

 their summits ever becoming covered by the waves. On the other side of 

 England, the broad North Sea, except for one channel that reaches down to 

 300 fathoms close against the Norwegian coast, is similarly a mere film of 

 water on the submerged plateau, and is rarely fifty fathoms deep. The 

 connexion of Ireland with the continental mass is further emphasised when 

 we note the outline of its coast. On the east, it is fairly smooth, with few 

 conspicuous inlets ; on the west the sea runs up by a number of long valleys 

 into the land. This is the essential feature of the indented western coast 

 of Scotland, and of the corresponding coast of Norway ; in fact, the edge 



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