I am speaking, experienced Greenlandic circumstances, rather than those of any other part of the world-then this sheet of snow or ice possibly grew to be hundreds, if not thousands, of feet in thickness. Such is the case in Greenland at the present time. The fine snow there accumulates on the mountaintops, and is only got rid of by its freezing into a sheet, which is always moving down to the lowest level. In temperate and tropical climates, rivers carry off the excess of moisture-in Arctic countries this can only be done by the moving ice-sheets, termed "glaciers." The Greenland glaciers debouch into. the sea itself. The ice-sheet forms grand sea-cliffs, hundreds of feet high, into whose bases the angry sea eats caverns, until the toppling mass falls over, and floats away as an iceberg. Or the great icesheet thrusts itself right into the sea, creeping along its bottom until it comes to water deep enough to buoy up, break off, and float away the extreme end.

You will have no difficulty in perceiving that the immense mechanical force exercised by such glaciers on the solid hard rocks over which they creep must be immense. You can easily conceive how the latter must be ground down and pounded into mud; and also, how fragments would be broken off, frozen into the great icy mass, and slowly carried away. When that portion of the glacier into which some huge fragment has thus been frozen, reaches the sea, it would be broken off, and floated away as an iceberg,

carrying the enclosed fragment of rock with it. Away drifts the iceberg, carried by oceanic currents in a southerly direction, until the warmer waters gradually melt it, and then down drops the rock to the bottom of the sea, to rest perhaps thousands of miles away from its parent source.

Fig. 157.

Tellina Balthica -a

The friction of a moving glacier elicits just enough heat to melt a portion of the ice, which flows away as water, carrying with it the finer mud or sand set free by attrition. Hence all the water flowing into the sea is turbid with mud, and this mud, as it gradually settles to the sea-bottom, is there forming what will some day be a geological deposit. In this mud Arctic mollusca live and die, and will also some day be found fossilized. It was in a common marine similar bed to this that I was shell in the Boulder dropped. Down I sank amid the Clays. oozy mud, displacing the strata, and more or less causing them to assume a contorted appearance. Well do I remember the effect produced by the largest boulders, dropped in a similar way into the same strata. They sank so deeply as to cause thin beds of shells, which had previously been horizontal, to wrap over and become almost vertical. In the Norfolk cliffs near Cromer, where what is known as the "Coast Boulder Clay" attains a great thickness, you may see masses of chalk imbedded, which cannot be less

than two hundred feet in length. The soft sand and clay beds near are so contorted that you would imagine an earthquake had produced the disturbance; but it was caused simply by the melting icebergs dropping their stony burdens. For ages this process went on-the land glaciers grinding down the solid rocks, and the sea currents strewing the débris over the ocean floor. The icebergs, also, added no little to the accumulating mass.

I am told that along the North Atlantic sea-floor there is going on a similar deposit. The thousands of icebergs which set out from the north every year gradually melt as they near the more southerly latitudes. There is a great stream of warm water called the "Gulf Stream," which sets out from the Gulf of Mexico, crosses the Atlantic, and impinges on the southern and south-western coasts of Great Britain. When the northern icebergs come into contact with it, they rapidly melt, so that, of course, the sea-bottom in that place might be expected to be heaped up with the débris they had dropped. Actual soundings prove this to be the case; so that if the North Atlantic sea-floor could be upheaved, you would have a series of loose deposits of sand, mud, boulders, &c., not unlike those. which were formed during my own lifetime.

I am not left without a natural barometer to fix the depth to which the dry land went down. In North Wales is a hill called Moel Tryfaen, and near its summit, at seventeen hundred feet above the

sea-level, is an old sea-beach, formed when the submergence had reached its maximum. After this there came as gradual an upheaval, and this is marked in various places in Great Britain by a graduated series of raised beaches, ranging in height from that above given to those only a few feet above high-water mark. Gradually the land appeared more extensively above the water. The climate was still intensely cold and Arctic. The icebergs coming from Scandinavia frequently brought with them Arctic plants growing on the frozen mass of gravel or sand. Whenever these icebergs stranded on the coast, these plants were able to migrate inland, and very soon they covered the new land with an Arctic and sub-Arctic flora. Those soft beds of sand or mud lying along the sea-bottom which first came within the influence of the surface-currents, were very much worn away or denuded. This was especially the case with an extensive sheet known as the "Chalky Boulder Clay," from its containing so many small rounded pebbles of chalk, as well as large boulders of other rocks.

Among farmers, this goes by the name of "Heavy lands," and the bed is usually found occupying the highest grounds, having been denuded by marine currents into what are now valleys. A good deal of the material thus worn away was carried by the waves to form beds of later date, which sometimes go by the name of "Post-glacial," although they were really deposited during the Glacial epoch.