ORIGIN OF THE GREAT LAKES.-The question of the origin of our lakes is one that requires more observation and study than have yet been given to it before we can be said to have solved all the problems it involves. There are, however, certain facts connected with the structure of the lake basins, and some deductions from these facts, which may be regarded as steps already taken toward the full understanding of the subject. These facts and deductions are briefly as follows:

1st. Lake Superior lies in a synclinal trough, and its mode of formation therefore hardly admits of question, though its sides are deeply scored with ice-marks, and its form and area may have been somewhat modified by this agent.

2d. Lake Huron, Lake Michigan, Lake Erie, and Lake Ontario are excavated basins, wrought out of once continuous sheets of sedimentary strata by a mechanical agent, and that ice or water, or both.

That they have been filled with ice, and that this ice formed great moving glaciers we may consider proved. The west end of Lake Erie may be said to be carved out of the Corniferous limestone by ice action; as its bottom and sides and islands-horizontal, vertical, and even overhanging surfaces are all furrowed by glacial grooves, which are parallel with the major axis of the lake.

All our great lakes are probably very ancient, as since the close of the Devonian period the area they occupy has never been submerged beneath the ocean, and their formation may have begun during the Coal Measure epoch.

The Laurentian belt, which stretches from Labrador to the Lake of the Woods, and thence northward to the Arctic sea, forms the oldest known portion of the earth's surface. The shores of this ancient continent, then high and mountainous, were washed by the Silurian sea, where the débris of the land was deposited in strata that subsequently rose to the surface, and formed a broad low margin to the central mountain belt, just as the Cretaceous and Tertiary strata flank the Alleghanies in the Southern States.

In the lapse of countless ages, all the mountain peaks and chains of the Laurentian continents have been removed and carried into the sea, and this has been done by rivers of water and rivers of ice. That these mountains once existed there can be no reasonable doubt, for their truncated bases remain as witnesses, and it is scarcely less certain that glaciers have flowed down their slopes of sufficient magnitude and reach to deeply score the plain which encircled them.

It will be noticed that all the great lakes of the continent hold certain relations to the curving belt of Laurentian highlands.

Some of them are embraced in the foldings of the Eozoic rocks, and fill synclinal troughs; but most of the series, from Great Bear Lake to Lake Ontario, exhibit the same geological and physical structure, are basins of excavation in the paleozoic plain that flanks in a parallel belt the Laurentian area. Few of us have any conception of the enormous general and local erosion which that plain has suffered. Those who will take the trouble to examine the section across Lake Ontario, from the Alleghanies to the Laurentian hills of Canada, and compare it with the other sections in the Lake Winnepeg district, radial to the Laurentian arch, given by Mr. Hind in his report on the Assiniboin country, will be sure to find the comparison interesting and suggestive; suggestive especially of a community of structure and history, and of an inseparable connection between the lake phenomena and the topographical features of the Laurentian highlands flanked by the paleozoic plain.

In estimating the influences that might have affected the number and magnitude of glaciers on the sides of the Laurentian mountains, it should not be forgotten that the Cretaceous sea swept the western shore of the Paleozoic and Laurentian continent from the Gulf of Mexico to the Arctic Ocean; and whether we consider this sea as a broad expanse of water simply dotted with islands, or a strait traversed by a tropical current, we have in either case conditions peculi

arly favorable to the formation of great glacial masses of ice, i. e. a broad evaporating surface of warm water swept by westerly winds that carried all suspended moisture immediately on to a mountain belt, which served as a sufficient condenser.

This, at least, may be positively asserted in regard to the agency of ice in the excavation of the lake basins, that their bottoms and sides wherever exposed to observation, if composed of resistant materials, bear indisputable evidence of ice action, proving that these basins were filled by moving glaciers in the last ice period if never before, and that part, at least, of the erosion by which they were formed is due to these glaciers.

No other agent than glacial ice, as it seems to me is capable of excavating broad, deep, boat-shaped basins, like those which hold our lakes.

If the elevation of temperature and retreat northward of the glaciers of the lake basins were not uniform and continuous, but alternated with periods of repose, we should find these periods marked by excavated basins, each of which would serve to measure the reach of the glacier at the time of its formation, the lowest basin being the oldest, the others formed in succession afterwards. Such a cause would be sufficient to account for any local expansions of the troughs of the old ice rivers.

Where glaciers flow down from highlands on to a plain or into the sea, the excavating action of the ice mass must terminate somewhat abruptly in the formation of a basin-like cavity, beyond which would be a rim of rock, with whatever of débris the glacier has brought down to form a terminal moraine.

When glaciers reach the sea, the great weight of the ice mass must plough up the sea bottom out to the point where the greater gravity of water lifts the ice from its bed, and bears it away as an iceberg.

If it is true, as the facts I have cited indicate, that our

lakes are but portions of great excavated channels locally filled with drift material, the fiords of the northern Atlantic and Pacific coast present remarkable parallels to them; and I would suggest Puget's Sound, Hood's Canal, and other portions of that wonderful system of navigable channels about Vancouver's Island, as affording interesting and instructive subjects for comparison. Like our lakes their channels are for the most part excavated from sedimentary strata which form a low and comparatively level margin to the bases of mountain chains and peaks.. They too have their depths and shallows, their basins and bars, and probably all who have seen them will assent to Professor Dana's view, that they are the "result of subaerial excavation," in which glaciers performed an important part.

The "Loess" of the Mississippi Valley. The "Bluff formation" of the West, sometimes called "Loess," from its resemblance to the Loess of the Rhine, I have on a preceding page designated as a lacustrine non-glacial Drift deposit. It seems to be the sediment precipitated from the waters of our great inland sea in its shallow and more quiet portions, to which icebergs, with their gravel and boulders, had no access, and where the glacial mud was represented only by an impalpable powder, which mingled with the wash of the adjacent land, land shells, etc.

It is evidently one of the most recent of the deposits which come into the series of Drift phenomena, and was apparently thrown down while the broad water surface which once stretched over the region where it is found was narrowing by drainage and evaporation, till, by its total disappearthis sheet of calcareous mud was left.

ance,

It underlies much of the prairie region, and once filled, often to the brim, the troughs of the Mississippi and Missouri, so deeply excavated during the glacial epoch. When the system of drainage was re-established the new rivers began the excavation of their ancient valleys in the Loess. When they had cut into or through this stratum, so that it

stood up in escarpments on either side, man came and called it the Bluff formation, because it composed or capped the bold bluffs of the river-banks. It is often, however, only a facing to the rocky cliffs, which are the true walls of these valleys, and which are monuments of an age long anterior to the date of its deposition. - Annals of the Lyceum of Natural History of New York, 1869.

OUR NATIVE TREES AND SHRUBS.

BY REV. J. W. CHICKERING, JR.

Ir has long been a favorite aspiration of the writer, at some time in life, to have an arboretum collected from our woods and waysides. But despairing of that, I would in this article give a list of those native shrubs and trees, which seem to promise to repay transplanting, and which would in beauty, and many of them in novelty, to any but the botanist, vie with those imported.

Of the trees of early spring, it is a pity that the Silver Maple (Acer dasycarpum), and the Sugar Maple (A. saccharinum), were not more generally known and valued, as flowering trees. The former is the earliest tree I know in this latitude, and the beauty of the long, yellow tassels of the latter, commends itself to every observer. Then for grounds of any extent the different Birches, the White (Betula alba), the Paper (B. papyracea), the Yellow (B. excelsa), and the Black (B. lenta), are in early spring most attractive ornaments, for the grace and variety of the spray of their delicate catkins. Then the Tulip Tree (Lirioden dron tulipifera), and the Cucumber Tree (Magnolia acuminata), both perfectly hardy in New York and New England, should be seen much more frequently in cultivated grounds. The Barberry (Berberis vulgaris) forms a pleasing clump