142 



SCIENCE. 



[Vol. X. No. 241 



the Manchester Literary and Philosophical Society, Oct. 7, 1884, p. 

 5), and later independently advanced by H. Carvill Lewis, be true, a 

 number of diamonds may have been formed in the Kentucky perido- 

 tite ; but the general paucity of carbon in the rock adjacent to the 

 peridotite is certainly discouraging to the prospector. 



The best time to search for gems in that locality is immediately 

 after a heavy rain, when they are most likely to be well exposed 

 upon the surface. It is proposed to keep up the search economi- 

 cally by those most interested, by furnishing to responsible indi- 

 viduals in the vicinity a number of rough diamonds mounted in 

 rings, for comparison, that they may know what to look for under 

 the most favorable circumstances. J. S. Diller. 



Geo. F. Kunz. 



New York, Sept. 12. 



The Classification of Lakes. 



Several years ago I presented to the Boston Society of Natural 

 History a paper on the classification of lake-basins, in which the 

 many varieties of lakes were grouped under three heads, according 

 as they were made by constructive, destructive, or obstructive pro- 

 cesses. The first heading included lakes made by mountain- 

 folding and other displacements ; the second consisted chiefly of 

 basins of glacial erosion ; the third contained the greatest number 

 of varieties, such as lakes held by lava, ice, and drift barriers, delta 

 and ox-bow lakes, and some others. The classification proved 

 satisfactory, in so far as it suggested a systematic arrangement of 

 all kinds of lakes that have been described ; but it now appears 

 unsatisfactory, inasmuch as its arrangement is artificial, without 

 reference to the natural relations of lakes to the development of the 

 •drainage systems of which they are a part. A more natural classi- 

 fication is here presented in outline. 



When a new land rises from below the sea, or when an old land 

 is seized by active mountain-growth, new rivers establish them- 

 selves upon the surface in accordance with the slopes presented, 

 and at once set to work at their long task of carrying away all of 

 the mass that stands above sea-level. At first, before the 

 water-ways are well cut, the drainage is commonly imperfect : 

 lakes stand in the undrained depressions. Such lakes are the mani- 

 fest signs of immaturity in the life of their drainage system. We 

 see examples of them on new land in southern Florida ; and on a 

 region lately and actively disturbed in southern Idaho, among the 

 blocks of faulted country described by Russell. But as time passes, 

 the streams fill up the basins and cut down the barriers, and the 

 lakes disappear. A mature river of uninterrupted development has 

 mo such immature features remaining. The life of most rivers is, 

 however, so long, that few, if any, complete their original tasks un- 

 disturbed. Later mountain-growth may repeatedly obstruct their 

 flow ; lakes appear again, and the river is rejuvenated. Lake 

 Lucerne is thus, as Heim has shown, a sign of local rejuvenation in 

 the generally mature Reuss. The head waters of the Missouri 

 have lately advanced from such rejuvenation ; visitoft to the Na- 

 tional Park may see that the Yellowstone has just regained its former 

 steady flow by cutting down a gate through the mountains above 

 Livingston, and so draining the lake that not long ago stood for a 

 time in Paradise Valley. The absence of lakes in the Alleghany 

 Mountains, that was a matter of surprise to Lyell, does not indicate 

 any peculiarity in the growth of the mountains, but only that they 

 .and their drainage system are very old. 



The disappearance of original and mountain-made lakes is there- 

 fore a sign of advancing development in a river. Conversely, the for- 

 mation of small shallow lakes of quite another character marks ado- 

 lescence and middle life. During adolescence, when the head-water 

 streams are increasing in number and size, and making rapid con- 

 quest of land-waste, the lower trunk-stream may be overloaded 

 ■with silt, and build up its flood-plain so fast that its smaller tribu- 

 taries cannot keep pace with it : so the lakes are formed on either 

 side of the Red River of Louisiana, arranged like leaves on a stem ; 

 the lower Danube seems to present a similar case. The f^ood- 

 plains of well-matured streams have so gentle a slope that their 

 channels meander through great curves. When a meander is aban- 

 <ioned for a cut-off, it remains for a time as a crescentic lake. 

 When rivers get on so far as to form large deltas, lakes often col- 

 lect in the areas of less sedimentation between the divaricating 



channels. Deltas that are built on land, where the descent of a 

 stream is suddenly lessened and its enclosing valley-slopes disap- 

 pear, do not often hold lakes on their own surface ; for their slope 

 is, although gentle, rather too steep for that : but they commonly 

 enough form a lake by obstructing the stream in whose valley they 

 are built. Tulare Lake in southern California has been explained by 

 Whitney in this way. 



The contest for drainage area that goes on between streams 

 heading on the opposite slopes of a divide sometimes produces little 

 lakes. The victorious stream forces the divide to migrate slowly 

 away from its steeper slope, and the stream that is thus robbed of 

 its head waters may have its diminished volume clogged by the fan- 

 deltas of side-branches farther down its valley. Heim has ex- 

 plained the lakes of the Engadine in this way. The Maira has, like 

 an Italian brigand, plundered the Inn of two or more of its upper 

 streams, and the Inn is consequently ponded back at San Moritz 

 and Silvaplana. On the other hand, the victorious stream may by 

 this sort of conquest so greatly enlarge its volume, and thereby so 

 quickly cut down its upper valley, that its lower course will be 

 flooded with gravel and sand, and its weaker side-streams ponded 

 back. No cases of this kind are described, to my knowledge, but 

 they will very likely be found ; or we may at least expect them to 

 appear when the northern branches of the Indus cut their way 

 backwards through the innermost range of the Himalaya, and gain 

 possession of the drainage of the plateaus beyond ; for then, as the 

 high-level waters find a steep outlet to a low-level discharge, they 

 will carve out canons the like of which even Dutton has not seen, 

 and the heavy wash of waste will shut in lakes in lateral ravines at 

 many points along the lower valleys. 



In its old age, a river settles down to a quiet, easy, steady-going 

 existence. It has overcome the difficulties of its youth, it has cor- 

 rected the defects that arose from a period of too rapid growth, it 

 has adjusted the contentions along the boundary-lines of its several 

 members, and has established peaceful relations with its neighbors : 

 its lakes disappear, and ft flows along channels that meet no as- 

 cending slope on their way to the sea. 



Certain accidents to which rivers are subject are responsible 

 for many lakes. Accidents of the hot kind, as they may be called 

 for elementaiy distinction, are seen in lava-flows, which build great 

 dams across valleys : the marshes around the edge of the Snake 

 River lava-sheets seem to be lakes of this sort, verging on extinc- 

 tion : crater lakes are associated with other forms of eruption. 

 Accidents of the cold kind are the glacial invasions : we are per- 

 haps disposed to overrate the general importance of these in the 

 long history of the world, because the last one was so recent, and has 

 left its numerous traces so near the centres of our civilization ; but 

 the temporary importance of the last glacial accident in explaining 

 our home geography and our human history can hardly be exagger- 

 ated. During the presence of the ice, especially during its retreat, 

 short-lived lakes were common about its margin. Claypole has 

 just described the extinct ' Lake Cuyahoga ' in Ohio as of this kind. 

 We owe many prairies to such lakes. The rivers running from the 

 ice-front, overloaded with sand and silt, filled up their valleys and 

 ponded back their non-glacial side-streams ; their shore-lines have 

 been briefly described in Ohio and Wisconsin, but the lakes them- 

 selves were drained when their flood-plain barriers were terraced ; 

 they form an extinct species, closely allied to the existing Danube 

 and Red River type. As the ice-sheet melts away, it discloses a 

 surface on which the drift has been so irregularly accumulated that 

 the new drainage is everywhere embarrassed, and lakes are for a 

 time very numerous. Moreover, the erosion accomplished by the 

 ice, especially near the centres of glaciation, must be held responsi- 

 ble for many, though by no means for most, of these lakes. Canada 

 is the American type, and Finland the European, of land-surface 

 in this condition. The drainage is seen to be very immature, but 

 the immaturity is not at all of the kind that characterized the first 

 settlement of rivers on these old lands : it is a case, not of rejuve- 

 nation, but of regeneration ; the icy baptism of the lands has con- 

 verted their streams to a new spirit of lacustrine hesitation unknown 

 before. We cannot, however, expect the conversion to last very long : 

 there is already apparent a backsliding to the earlier faith of steady 

 flow, to which undisturbed rivers adhere closely throughout their 

 life. 



