Erratics.-These are of all shapes and sizes, occasionally reaching colossal proportions, and containing many hundred feet. Some are rounded, others are angular, and not a few exhibit marks of scarification. They may rest on base-rock, and, if carefully poised, may be made to oscillate by the form of the land, or these large blocks may appear on the till, angular débris, and hills of gravel. As a general rule, they prove to have been carried from higher to lower levels in Scotland, though many exceptions are recorded. There is one at the height of 1,020 feet on the Pentland Hills, which may have traveled westerly as much as eighty miles. It probably passed from one mountain across a wide valley before attaining its final resting-place. This is not so striking as the blocks lying nearly over the recently-completed Hoosic Tunnel, in Western Massachusetts, one of which weighs 510 tons, and has been transported from Oak Hill across a valley 1,300 feet deep. It has hundreds of lusty comrades, scattered in a southeasterly course for thirty miles.

Sometimes a large block is revealed by the washing away of the till around it. Those on the surface of gravel may have been carried by floating ice. To such blocks it is not easy to assign limits of the distance traveled, since icebergs may float for thousands of miles without melting.

ORIGIN OF THE COLD CLIMATE.-The question of the cause of the glacial cold has been discussed warmly for a long time. The opinion seems to be gaining ground that purely geological causes are not sufficient to account for the magnitude of the glacial distribution. The precession of the equinoxes, changing the times of the seasons, and the eccentricity of the earth's path around the sun, lengthening the winters and increasing precipitation of moisture, when combined with certain changes in the courses of ocean-currents, and some elevation of land in the north, may have together been instrumental in bringing around a period of intense cold. If it be possible to use the orbital changes as a guide to a chronological date for this term of cold, we can say it began about 240,000 years since, and continued for 160,000 years, terminating 80,000 years before A. D. 1260. The cold would have culminated about 30,000 years after its beginning.

Granting such figures, we can understand that the glacial must have been the dark age in the earth's history-a terrible blight upon the flourishing faunas and floras existing in tertiary times in northern latitudes. The presence of warm temperate plants in Greenland has always excited interest, even to the proposal of very wild theories to account for the genial climate there of preglacial days. It may be that the American Sequoia traveled across the bridge anciently connecting Greenland with Iceland and Scotland, and that the renowned cedars of Lebanon are the cousins of their famed relations in California; but the connection has been severed by the ruthless ice-flow, and is not likely to be reëstablished, unless our sun shall carry his sys

tem of planets through a much warmer region than the space now encircling us.

Centres of Dispersion.-All existing glaciers flow from higher to lower levels as a rule-the only exception being that already stated, when the ice may be forced up-hill for a short distance. This may be well exemplified in the Alpine glaciers of the present day. These streams of ice all flow from the summits and axes of particular mountains along the valleys, and spread over the neighboring plains. The action is radial-proceeding from a central point or line outward.

The geologists have concluded that most of the ancient ice-movements in Northern Europe have been from centres of dispersion, like those in Switzerland. Examples are numerous. One of the most interesting is exhibited in Switzerland. The traveler finds there two prominent centres of glacial radiation-the Bernese and the Mont Blanc regions. Glaciers now flow westerly into the great valley of Switzerland and toward the Rhone from the former, and in the latter group the streams discharge upon the Italian plains on the south, and toward the vale of Chamouni on the north. A careful study of the vale of Chamouni shows that ice once filled it to the brim, for the embossed rocks carry striæ even to the height of 5,000 feet.

Search for the sources of bowlders proves that large blocks on the southern flanks of the Jura Mountains must have been derived from Mont Blanc, sixty or eighty miles distant. Instead of passing down the Arne at Chamouni, the blocks proceeded northerly toward the Rhone, and thus across the great valley of Switzerland to the Jura. The magnitude of this ancient action equals much of the wonderful glacial phenomena of other districts in Europe, though hardly equal to what may be seen on this continent. But, being satisfied of the former enormous extension of the Alpine glaciers from examination of the striations and the dispersion of blocks, it is easy to generalize and refer similar phenomena in other countries, whose glaciers are extinct, to the same mighty cause.

In Scotland there may have been a centre of dispersion for glaciers from Ben Nevis, another in the south part of the province. In England, one in the Cumberland region; in Wales, one from Mount Snowdon. It is easy to discover the evidence of radial dispersion.

A combination of the glacial and iceberg agencies may be discerned in a map in Mr. Geikie's work, showing the courses of the striæ marked upon the rocks of Scandinavia. They diverge from the central water-shed between Norway and Sweden-part pushing toward Iceland and Scotland, and part directed toward Lapland and the Baltic Sea. The distribution of the bowlders corresponds with these marks. Furthermore, these ice-masses seem to have come in contact with the water of the Baltic, and part have floated over Germany till high land obstructed farther movement, and a part may have been caught by the outflowing Baltic current, carried over the North Sea to the south part of England, and perhaps Iceland. At least, bowlders of Scandinavian origin are common in these regions, and have probably migrated in the way described. On the east shore of Scotland they are plenty; but, between these and those south of the Thames, none have been found, which fact has given rise to the theory of dispersion by means of icebergs through the Baltic.

In years past the prominent topic of discussion in scientific associations has been the character of the ice-movements in the Glacier period. One school has stoutly defended icebergs as the active agent, the other has vigorously insisted upon land glaciation. The example before us seems to require both these agents to account for all the phenomena of this period. Both classes seem to be right, though neither can explain all the facts. Nature's domain is so vast that human intellects do not seem to be capable of grasping the whole truth at We are like the mariners who seek to penetrate to the northpole. They have penetrated a little way beyond Spitzbergen-they have gone nearer the goal through the straits west of Greenland, and have made great exertions in some other quarters. Each party has its theory of the character of the unknown region, as derived from a partial survey. By-and-by the whole of this area will be known, and it

once.