Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/270

 Clew Bay, to glaciation at 2000 feet upon a hill-top near Galway Bay. But this quartz hill stands there like the stone pillar at Penryn, to mark, 1st, the depth of the ice, and, 2nd, the depth of the rock which has been removed from that region. On lower hills are marks of local Connemara glaciation ; but at the head of Galway Bay is a bed of Boulder-clay full of limestone from the central regions, resting upon rocks striated in the direction of the sea. A great body of ice at some time or other passed off Ireland from the N.E. to S.W. by way of Galway, according to the high and low marks which I more fully described in ' Frost and Fire,' and which are now mapped by the Geological Survey *.

The high marks upon Shan Folagh pointed the way to seek for more knowledge nine years ago. The lines ruled upon the hill-top were produced upon a map, and touched Cushendal, in Antrim. In 1872, after nine years, I went there to see what I could find. I found first unmistakable marks of a local Antrim system proportionate to the size of the hills.

Next I found out a tall trap-hill called Slieve Mish, and went to the top of it. I found it a great glaciated " Tor," beside a rock- groove which crosses the range. In the groove, at about 500 feet above the sea-level, I found and copied stria? pointing N.E. and S.W. ; therefore this groove was filled with ice of some kind. The long axis of the hill is nearly north and south ; the rock is weathered ; but at the northern end it is deeply grooved. If these are old weathered ice-marks, as I believe them to be, then all the marks aim right over Ireland at the Twelve Pins of Connemara, and over the sea at the Firth of Clyde. In mapping the glaciation of Ireland a line may fairly be drawn from Shan Folagh to Slieve Mish, and the ancient ice may be reckoned to have been more than 2000 feet deep from one side of Ireland to the other, within the bounds of Ulster and Connaught. That makes the northern ice-system in Ireland.

Theoretical General Glaciation.

Of late years a school of geologists have taken up a glacial theory which their adversaries condemn. The advanced glacial theory, so far as I understand it, is that during a late geological period the whole of the Northern Hemisphere, from the Pole down to regions near the Equator, was covered by a continuous thick crust of ice. It grew then, as ice grows now, by evaporation about equatorial regions and by condensation about the Poles and about high grounds. From the Polar regions, where the ice was many thousands of feet deep, as from the chief condensing-point and gathering-ground, this general ice-system spread southwards, because any pile of plastic materials so spreads. No one imagines that all the water evaporated condensed at any one spot, at the Pole or elsewhere ; but the greatest condensation was about the coldest region near the Pole.

Accordingly the ice moved thence with a general southerly movement, along meridians, during the greatest development of the last


 * A specimen from the hill-top was shown.