Page:Popular Science Monthly Volume 49.djvu/186

172 to the Mississippi was completed, the upper lakes were rapidly lowered, and this re-established the life of the Niagara for some time longer. Upon the basis of calculations made it would appear that the change of outlet for the upper lakes from the Niagara to the Mississippi will not be more than another five thousand years hence—to us living a matter of indifference, but only showing how the present days are simply passing events in the history of the lakes. In the meanwhile the waters of Lake Ontario will more and more flood the head of its basin. However, the end of the lakes is so far removed in geological time that, until such great changes in the configuration of the land shall have obtained of which we have no prophetic vision, the lakes will continue to exist.

.—When the rate of movement of the earth's crust, as determined in the history of Niagara Falls, is applied to the deserted strands of Warren Water and its successsorssuccessors [sic], it is estimated that since the commencement of the Warren epoch fifty thousand or sixty thousand years have elapsed. This estimate, although based upon the most analytical knowledge obtainable, can, after all, be regarded as only approximate. The time ratio tells us that the lakes are still youthful; although in terms of solar years very old, yet perhaps not older than the human race.

The vicissitudes between the end of the ice age proper and the birth of Warren Water are too little known to enter into any proportional division of time, except that the ice age culminated prior to or at the date of the closing of the old valleys with drift.

The Great Lakes are the most striking feature of the eastern part of the continent, yet what we know of their history has been mostly discovered within the last few years. In this sketch only some of the more important and generalized results have been given. Many of the observations are beyond doubt, but there is plenty of room for students to add to our knowledge and correct our imperfect work. While the history of the lakes can be told with considerable certainty, the attempt at computing their age in terms of solar years has the same fascination, although not so extravagant, as the speculations concerning the antiquity of the earth itself, as the former question probably comes within the human period.

College Observatory publishes a list of fourteen new variable stars of long period, in addition to those previously announced, which have been discovered by Mrs. Fleming from the examination of Henry Draper memorial photographs. The spectrum of one of these stars is of the fourth type, while all the other stars have spectra of the third type, with the hydrogen lines also bright.