to an inch, I reduce glens 12 miles long to the size of glacial striae a foot long, and they are alike in shape. If, on the other hand, I begin with hair-lines engraved by ice with fine sand upon glassy quartz, and magnify them with a microscope, they take the proportions of larger striae upon the same stone. I can get to grooves like Gweebarra and Kenmare river by easy steps along Irish rocks. But hair-lines, Irish glens, and Norwegian fjords are all grooves of one pattern, though engraved upon different scales. If ice made one set of grooves, bigger ice might make the biggest. A finished ordnance- map and a rubbing taken off a glaciated rock show that glens and striae are very like when the large scale is reduced from a mile to an inch. A very little chipping and shaping would convert a few square yards of glaciated Irish rock into a tolerable model of the island of which it is part. I have not grown to be 70 miles high ; but, in growing to be 50 years old, I have seen as much of the world as if I had looked down upon it, and I remember, on the reduced scale, as if I looked upon a model. Looking thus back upon all the countries wbich I have seen, the hills and dales appear to record that the very same ice-engines which are shaping the earth's crust in high latitudes and in high lands, also shaped the surface of the British isles when those engines were larger, longer, broader, deeper, and heavier.
XXIII. Sea-marks. — Having seen and copied sea-marks at many places, I see that shelves and floors carved all round Ireland by the sea will unite in time. Unless Ireland is raised, it will be polished off the face of the earth by waves. But the new surface will only be like older buried surfaces, and like the surface of Bute or Anglesea, or any other low country, which is like a geological map without mountain-shading.
Looking at the work done by the sea round the Irish coast, and at ice-work and drift and sedimentary rocks, no measure for " Denudation " is left, except the full sum of sedimentary rocks from Irish mud to Laurentian gneiss and the granite, which was sedimentary before it was last fused.
Wide hollows and narrow grooves were dug out of the solid in Ireland since the formation of Antrim Chalk and Basalt. Most of that work still bears the marks of ice. Enough of glacial debris is strewn over the low lands to fill up many of the grooves in the hills ; and these records are carved upon Irish hills in plain lines, which a child may soon learn to read.
XXIV. Conclusion. — Denudation is part of geology. Ireland has been largely denuded. Glacial and marine action are the most powerful known to me. Glaciers and the sea shaped Ireland, as I believe. Rivers and weathering have done little to obliterate the tool-marks of ice and the sea, since the end of the last of a series of glacial periods *.
- April 3, 1873. It has been pointed out to me, that as early as 1840-42,
P. Merian, of Basle, showed that ice-fractures are completely closed. The first maker of a snowball proved the " regelation " of ice-crystals under pressure ; and the fact is now generally understood.
