Page:Popular Science Monthly Volume 31.djvu/348

334 other acting as principal. In all the larger lakes there has evidently been a local sinking of the surface, the glacier having been only auxiliary. The numerous small lakes in Middle and Western New York lying in the direction of the glacier-flow, and having frequent groovings on their adjacent walls, have been credited wholly to the glacier. But, as nearly the whole of this lake-region lies within the Niagara limestone formation, it is not improbable that the falling of cave-roofs may have greatly aided the work of the ice-plow. Many small lakes and ponds, as in Kentucky, Tennessee, and Southern Indiana, are due wholly to the falling down of cave-roofs. In South-Eastern Missouri and Eastern Arkansas are lakes and lakelets where these roofs were shaken down by the earthquake of 1811. Lakes Borgne and Pontchartrain have been captured from the Gulf by the delta of the Mississippi; while numerous small lakes, called bayous, have been formed by changes in the river-bed, the deposit of sediment at both their inlets and outlets having kept them filled with water. Crater-lakes are not infrequent. These basins, but containing no water, abound in New Mexico, Arizona, and Southern California, while many of the beautiful lakes of the Italian Peninsula are but the filled craters of extinct volcanoes.

"Why is the North the land of lakes? In order intelligently to answer this question, let us see what has been going on at the South. Between the Alleghanies and the Blue Ridge is a long, narrow valley extending nearly the whole length of these parallel ranges; and, but for the breaks in whose walls, the whole extent must have been a basin of water. At Harper's Ferry, the Potomac, and near the Natural Bridge, the James River, have broken through the Blue Ridge carrying the waters of the upper half of the valley to the Atlantic. Farther to the south the Kanawha and the Tennessee drain the lower half into the tributaries of the Gulf of Mexico. These last-named rivers have also broken through the Cumberland Mountains, draining another considerable valley between these and the Alleghanies. Could these several outlets be closed, as they probably once were, large lakes would again rapidly form, and the work of abrasion and drainage begin anew. Could the Knobs and the Muldro Hills unite again at the falls of the Ohio, a large shallow lake would form, covering parts of Ohio, Indiana, and some of the fairest portions of Kentucky. Should the bluffs of the Wabash come together at the mouth of the Salimony, another shallow lake would result, whose outlet would probably be the Maumee. Commencing at Richmond, Indiana, itself situated in a small lake-basin, and thence northeastwardly half-way across Ohio, is a succession of shallow depressions once filled with water, and through which still flow the streams whose unceasing work has cut through their margins and emptied them of their contents. In many of these ancient lake-basins the draining is not yet completed, the lowest parts being still marshes or ponds of water. The Mohawk and the