Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/917

 the alluvial plain, bounded by secondary banks, from 10 to 25 feet in height, depositing alluvium on the one bank and horizontally denuding the other. This secondary denuding, though important from being exercised along the whole course of the river, is of much less importance than that exercised at the elbows of the curves of the stream.

I have gone thus at length into the origin of these cliffs, owing to Mr. Mackintosh having described them as having been formed by the sea, and having, through misapprehension, represented me as holding that opinion.

General Conclusions.

1. After the deposition of the Esker drift, the country appears to have gradually risen, probably to an elevation of from 200 to 300 feet higher than at present ; but before this elevation was reached, a pause appears to have taken place, during which great denudation took place — the sea having eroded the cliffs of glacial drift in Western Lancashire and Cheshire, back and back, until the great low-lying plains, now covered with peat-moss, came into existence.

2. At the present time, nearly two-thirds of the Irish Sea is within the 30-fathom line, which runs on the east side, nearly in a straight line from the Mull of Galloway to St. David's Head, passing to the west of the Isle of Man. The whole of this tract would therefore become land if there were an elevation of 200 feet. Between this line and the coast of Ireland is a deep channel, generally about a mile broad, of an average depth of from 60 to 70 fathoms ; the deepest portions, or rather points, are off Magee Island, 84 fathoms (504 feet), and off Larne, 112 fathoms (672 feet) : near this point is the Highland Rock, in the shallow called " the Maidens," in only 6 fathoms, being a fall of 648 feet in half a mile. Off the coast of Galloway is a narrow channel, about 24 miles long ; its deepest point is 149 fathoms, or 894 feet, opposite Belfast Lough. The deepest water in a straight line between Lancaster and Dundalk is 57 fathoms (342 feet) ; between Dublin and Holyhead, 93 fathoms. This latter channel shallows to the south, towards the " line of least depth " between England and Wales and Ireland, which runs in a curve pointing south, from the peninsula of Caernarvon to Arklow, in Ireland, marking probably the watershed from which the rivers, which probably formed the long narrow channels occurring in the otherwise flat surface of the Irish Sea-bed, described above. Southward of this line, the deepest portion of which is only 44 fathoms deep (264 feet), the channels again deepen, increasing in depth towards the open sea, the average being about 60 fathoms.

3. If in postglacial times the land rose as much as 280 feet above its present level, the coast-line ran from the Mull of Galloway to St. David's Head, and Ireland was connected with Wales by a narrow isthmus over this " line of least depth," over which the postglacial mammals, the Germanic flora, and man himself may have migrated. This connexion would appear to have taken place