. The Niagara book : a complete souvenir of Niagara Falls : containing sketches, stories and essays--descriptive, humorous, historical and scientific. described must have converted the cataract intoa vertical fall. In a few score years the process ofretreat of the steep over which the water fell musthave begun the excavation of the great may help th^ reader to conceive the advance ofthe process to imagine a great auger boring awayupon some soft material, the tool while turningbeing drawn slowly across the surface. In thesimilitude, the whirling waters at the base of thecascade with th
. The Niagara book : a complete souvenir of Niagara Falls : containing sketches, stories and essays--descriptive, humorous, historical and scientific. described must have converted the cataract intoa vertical fall. In a few score years the process ofretreat of the steep over which the water fell musthave begun the excavation of the great may help th^ reader to conceive the advance ofthe process to imagine a great auger boring awayupon some soft material, the tool while turningbeing drawn slowly across the surface. In thesimilitude, the whirling waters at the base of thecascade with their armament of stones, representsthe auger, and the wide field of strata which have beencarved the material which is bored by the moving many years geologists, who are ever trying tomeasure the duration of the past, have endeavored tocompute the time which has elapsed since the excava-tion of the gorge below Niagara Falls began. Itseemed at first likely that the time occupied in thisgreat work might be reckoned in a somewhat definiteway. Long ago it became evident that the Falls wereslowly advancing up the river through the undermin-. THE GEOLOGY OF NIAGARA FALLS. 89 ing of their base and the consequent crumbling of theoverhanging limestone at the foot of the precipice. In1842, Dr. James Hall made a careful map showing theposition of the different parts of the Falls^ which werereferred to monuments from which subsequent surveyscould do work that would afford a basis for compari-sons. A third of a century later another survey wasmade by officers of the U. S. Engineers. In 1886 S. Woodward made yet another careful map of theregion. It now appears, however, according to K. Gilbert, that one or more of these delineationsis somewhat in error, for at certain places the outlineof the front projects beyond the position indicated byHalls survey. After a careful consideration of thesediscrepancies, Mr. Gilbert says : *Nevertheless a crit-ical study, not merely of th
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