THE use of Niagara as a geological chronometer dates from the visit there of Lyell in 1835. He recognised that the Falls must date from the close of the Glacial period, and that the Niagara gorge must have been excavated since the retreat of the glaciers from the Great Lakes. The necessary assumptions as to uniformity of rate and condition being granted, he held that the length of the gorge divided by the amount which the Falls recede up-stream annually would give the length of post-Glacial times for the Niagara district in years. He realised the uncertainty of some of the data, but estimated that the age of the Falls is about 35,000 years. The problem, however, is not to be solved by simple rule of three, for the data are complex, and there are many variable factors. Lyell himself used one of the unknown elements to explain the formation of the Niagara Whirlpool. He rightly attributed it to the existence of a channel filled with drifts, which are now worn away more quickly by the river than the rocks of the old river banks; and if part of the existing gorge had been formed by the re-excavation of a channel filled with drift, the process would have been much quicker than if the Falls had to cut their way for the whole distance through the hard Niagara limestone. Lyell's estimate has therefore been greatly reduced by some later geologists, and Dr. G. K. Gilbert has allowed the Niagara Falls a life of only some 7000 years, with a possibility of even considerably less.