202 DYNAMICAL GEOLOGY. 



show it. Occasionally a lake bursts its bounds, and produces in a few hours 

 the devastating effects of the most violent of torrents. But such effects are 

 rare, except where man has interfered. 



The large lakes have many of the characteristics of the ocean. The 

 wind, waves, and currents are effective agents of wear and deposition along 

 the shores, and about bays and the mouths of rivers. 



The waves work landward on shelving shores, as along the sea border, 

 while a littoral current usually runs parallel, or nearly so, with the coast ; 

 and between the two the depositions of sand and making of beaches and 

 sand-bars take place. 



The nearly total absence of tides makes marked differences in the 

 effects. The change of level in seashore action with the tidal movements 

 fails. Abrasion sets back the cliffs, but makes a sloping surface at their 

 base. 



The tide on Lake Michigan has a range of three inches at spring tides 

 and li at neap tides. Large oscillations of the surface are produced by storm 

 winds, and lighter ones by floods in the region. On Lake Erie, at Buffalo, 

 the difference between the levels produced by two gales, one from the S. W., 

 and the other off shore, from the K. E., was 15|- feet (Whittlesey). Small 

 but short tide-like changes of level, called seiches, a few inches in height, 

 observed on Lake Geneva and other Swiss lakes, are attributed by Forel to 

 local variations of atmospheric pressure — an impulse so given producing 

 a long-continued series of oscillations. Larger seiches are supposed to be 

 due to earthquake shocks. 



For a thorough discussion of lacustrine methods of work xmder varying conditions of 

 levels, see the Memoir of G. K. Gilbert on Lake Bonneville, U. S. G. S., 4to, 1890. 



Past geological ages had their fresh-water lakes as well as rivers. But 

 the great lakes and rivers of the world belong to later history, the era 

 of full-grown continents. Yet the lakes of greatest geological interest 

 are not those of the present era, but of that next preceding. Those of 

 North America formed over the emerging land of the Rocky Mountain 

 region had great area, and received abundant debris for lacustrine deposits 

 from a newly made mountain range. 



But another condition existed; for the great lake-basins were subsiding 

 areas, so that the deposits continued thickening, as the subsidence made 

 progress, until 5000 to 10,000 feet of beds were laid down, — as the region 

 of modern coral reefs is described, on page 149, as subsiding while the reefs 

 thickened. 



These Tertiary lacustrine formations prove their fresh-water origin by 

 containing remains of abundant fresh-water and terrestrial life, from Quad- 

 rupeds or Mammals, of many more kinds than now exist in North America, 

 to Snakes and Turtles, Eishes, and Insects and even Butterflies, besides leaves 

 and other relics of the forest. 



