insect, or a few grains of sand, will sink, while blocks of granite will be thrust up in the air.

GLACIER TABLES.

by the rills, and at length chokes up the holes in the glacier through which the water flows. As the ice subsides, a pyramid or cone of sand, sometimes 20 or 30 feet high, and 80 or 100 in circumference, is left standing.

UNION OF GLACIERS-FORMATION OF MEDIAL MORAINE.

The medial moraine appears to be formed by the union, in a common valley, of two glaciers from separate sources. The lateral moraines, common to each, unite upon the surface and mark, by a band of stones in the centre, often for miles, the line of separation between them. The accompanying diagram will illustrate the formation of the medial moraines, the arrows pointing in the direction of movement of the glacier.

Sometimes large portions of rock fall into holes and openings between the ice of the glacier and the side of the mountain, which forms its bed. In this situation they grind and chafe the mountain itself, producing grooves and scratches in the direc

tion of the moving ice. These polished rocks are found in places where glaciers have long ceased to exist, and thus give traces of the icy stream in an unexpected manner, and help to confirm a theory which we must now briefly state.

In 1816, the interest which had been excited in glaciers was still further enhanced, by the announcement made by Dr. Playfair, that they might

be reckoned among important geological agents, in their power of transporting rocks from place to place. In many countries of Europe, a number of single pieces of rock are scattered over certain districts, with which they have no geological

connexion, the mountain range with which they assimilate, being, perhaps, hundreds of miles off. Such isolated pieces of rock are called erratic rocks or boulders. Many unsuccessful attempts had been made to account for the fact that these boulders had been carried to such great distances from what appeared to be their native bed. It was a startling idea that the slow and silent, but mighty agency of the glacier, might have produced this effect. The theory was stoutly opposed in many quarters, but gradually gained ground. Several Swiss savans began diligently to study the glaciers, with a view to the new doctrine, and among them a distinguished naturalist, M. Agassiz, at first an opponent, became ultimately one of the most zealous advocates of the theory. Large boulders of granite are dispersed over the declivities of the Jura mountains, which are of limestone, and in such a manner as to favour the idea that they were brought to those places, and deposited, by glaciers, in the form of glacier walls. These boulders, on the Jura, were found at a much lower level than any to which a glacier now descends. From these, and similar facts, Agassiz was led to the conclusion, that by far the greater part of Switzerland was at some remote period entirely covered with accumulations of snow, which, when the temperature of our globe increased, were gradually melted, and, in retreating to the more elevated portions of the country, deposited these boulders at different levels on the slopes of the Jura range. A great belt of these stones ex

tends for miles, at a height of about 800 feet above the level of the lake of Neufchatel. One of these blocks, situated about two miles to the west of the town of Neufchatel, is called the Pierre à Bot, or the Toad-stone, from its rude resemblance to the form of a crouching toad. The stone is 50 feet long, 20 wide, and 40 high, and contains about

Professor

40,000 cubic feet of solid material. Playfair, speaking of the Toad-stone, says, "A current of water, however powerful, could never have carried it up an acclivity, but would have deposited it in the first valley it came to, and would, in a much less distance, have rounded its