ORIGIN OF LAKES. The causes of lakes are as various as their forms. In general they may be considered as consequence of natural inter ference with drainage. They may exist on a new land surface, when they may be called orig inal consequent lakes, since they are formed in consequence of original irregularities in the land; they may result from the normal develop ment of rivers, and may then be called lakes of normal derclopment and they may be due to some accidental interference with preexisting drainage, when they may be called lakes of acci dental origin. All lakes fall into one of these three great classes.
Original consequent lakes are illustrated by the shallow lakes of Florida, which exist in de pressions on a raised sea 'bottom; the same condi tion exists in the Siberian plains, and in the Ar gentine plains. Consequent lakes are also found in shallow basins on the beds of extinct lakes. Thus the Great Salt Lake is in a depression in the deposits of a much larger lake that once existed there. Any other new land surface, as a lava flow. or a thick sheet of glacial drift which obscures the old land, may have depres sions in which ponds or lakes develop. There are many small lakes of this origin in the glaciated belt of America and Europe.
Of lakes of normal development there are also numerous illustrations. Abandoned meanders of rivers, forming oxbow lakes, and abandoned river channels on deltas, shut off from the river by the deposit of river silt. are instances. The growth of alluvial fans by streams coining from a mountain into a more level valley sometimes dams the river in the main valley. Thns Tulare Lake in California is made by a broad, low, alluvial fan made by King Inver, which comes down from the Sierra Nevada. Still another kind is found where stream development is tak ing place in a region of limestone or other solu ble rock. Under these conditions some of the drainage is underground, the surface settles here and there by undermining, and little basins or sink-holes are formed toward wide!' the water drains. If the hole in the centre becomes filled, ponds are caused.
Far the most common cause for lakes is some accident to a stream, so interfering with its normal development as locally to transform its valley to a basin. An avalanche across a river dams back a lake, and the growth of a moun tain barrier makes a still greater dam. The warping of valleys during mountain growth also makes basins. Lake Geneva in the Alps has been ascribed to this origin. There are basins where the rocks across a valley have been faulted —for example, in southern Oregon, in Ireland, and in the ease of the Dead Sea. \Viten the
land has subsided and the sea entered the mouths of river valleys, the building of bars across the drowned valleys often shuts in the water, forming lakes. These may be made salt by the occasional overflows of the sea, or they may be completely disconnected from the sea. Such lakes are illustrated by the shut-in bays on the south shore of the Great Lakes. Sinking of parts of the land during earthquake shocks forms basins, as in the 'sunk' country of Arkan sas, in the :\lississippi Valley, that was shaken by the earthquake of Lava dams hold back river water, as is illustrated in the Auvergne region of Central France; by Snag Lake, near Mount Shasta ; by the Lake of Tiberias, in the .Jordan Valley; and by many other lakes in volcanic regions. After the vol canic' energy has subsided, volcanic craters are occupied by lakes, as in the Eifel region of Germany; Lake Nemi, near Rome; Averno, near Naples; and many other places. Such lakes are especially large and deep when the cra ter bottom has subsided, as in Crater Lake, Ore gon. But perhaps the most important single cause for lakes is the glacial accident. Ily moraine dams and by dams of other classes of glacial deposits a vast number of lakes in Northeastern America and Northwestern Europe have been formed. Without doubt the number of glacial lakes and ponds ill Northeastern America and Northwest ern Europe is several hundred thousand. There are estimated to be 10.000 lakes in Minnesota alone, due in one way or another to the glacier. In consequence of the interference of drainage by the ;glacial accident, glaciated regions are characterized by an abundance of lakes, while unglariated regions have relatively few. In addi tion to the deposit of materials forming a dam across stream valleys, glaciers have scoured out many. basins, known as rock basins. Seneca and Cayuga Lake valleys in central New York have been thus formed. the Great Lakes owe at least a part of their depth to this ea use, and the same is true of 50111e of the Alpine lakes. notably the Italian lakes Como, Lugano, and Maggiore. Many lakes are the result of a eomhination of causes. For example, the Great Lakes are evi dently in old river valleys, deepened to some ex tent by glacial erosion, further deepened by a warping of the earth's crust, and with their depth still further increased by dams of drift in the preexisting valleys. The Alpine lakes also seem to combine valley warping, glacier erosion. and glacial-drift dams among their causes.