November 5, 1908J 



NA TURE 



Before the astronomical study of them was commenced 

 a very few years ago, if we accepted the available records 

 the cromlechs were all directed helter-skelter, their 

 sight-lines were without any meaning, and no astro- 

 nomical or practical use was served by them, except, 

 perhaps, as tombs. A comparatively few observations 

 have sufficed to show the absurd inaccuracy of these 

 views ; for full light we may be content to wait for the 

 authoritative inquiries now happily commenced. That 

 our knowledge will be largely increased in many 

 directions there is no room to doubt. 



Norman Lockyer. 



^lAGAUA AS A GEOLOGICAL CHRONOMETER. 

 T^HE use of Niagara as a geological chronometer 



■^ dates from the visit there of Lyell in 1835. He 

 recognised that the Falls must date from the close of 

 the Glacial period, and that the Niagara gorge must 

 have been excavated since the retreat of the glaciers 

 from the Great Lakes. The necessary assumptions 

 as to uniformity of rate and condition being granted, 

 he held that the length of the gorge divided by the 

 amount which the Falls recede up-stream annually 

 would give the length of post-Glacial times for the 

 Niagara district in years. He realised the uncertainty 

 of some of the data, but estimated that the age of the 

 Falls is about 35,000 years. The problem, however, is 

 not to be solved by simple rule of three, for the data 

 are complex, and there are many variable factors. 

 Lyell himself used one of the unknown elements to ex- 

 plain the formation of the Niagara Whirlpool. He 

 rightly attributed it to the existence of a channel filled 

 with drifts, which are now worn away more quicklv bv 

 the river than the rocks of the old river banks; and if 

 part of the existing gorge had been formed by the 

 re-excavation of a channel filled with drift, the process 

 would have been much quicker than if the Falls had 

 to cut their way for the whole distance through the 

 hard Niagara limestone. Lyell's estimate has there- 

 fore been greatly reduced by some later geologists, 

 and Dr. G. K. Gilbert has allowed the Niagara Falls 

 a life of only some 7000 years, with a possibility of 

 even considerably less. 



The last contribution to the Niagara question is a 

 monograph by Dr. J. W. W. Spencer, published by the 

 Canadian Geological Survey.' It makes two impor- 

 tant additions to the known facts. A series of borings 

 ha-i been made to determine the course of the former 

 river channel which is exposed at the Niagara Whirl- 

 pool, and the Niagara River below the Falls has been 

 carefully sounded. The soundings have proved the 

 existence of a basin 192 feet deep immediately below 

 the Falls ; the river then shallows, until at the Canti- 

 lever Bridge the depth is only 86 feet. The basin is 

 due to the filling up of the channel by material that 

 has fallen in from the sides of the gorge after the 

 Falls have passed up-stream, a fact proved by work 

 undertaken in connection with the foundations of the 

 bridge. 



Dr. .Spencer, in addition to these important con- 

 tributions to the facts, has carefully re-discussed the 

 evidence and shown how complicated the problem is, 

 owing to the numerous post-GIacial changes in the 

 physical geography of the Niagara area during the 

 lifetime of the Falls. .According to his calculations, 

 the Falls have receded up-stream at a mean rate of 4.2 

 feet per annum, during the sixty-three years between 

 1842 and 1905. The rate of retreat is not uniform, for 

 the process consists in the cutting of a V-shaped 

 groove, which is gradually widened during a period 



I " The Falls of Niag.ira : their Evolution and Varying Relations to the 

 r.reat Lakes; Characteristics of the Power and the Effertsof its Diversion." 

 (Ottawa : Geo!. Survey of Canada, 1907.) Pp. xxxi + 490 ", plates and maps. 



NO. 2036, VOL. 7q] 



when there is no recession of the notch; the edge of 

 the Falls thus becomes straighter, and then the forma- 

 tion of the horse-shoe curve begins again. By the 

 double process 7I acres of the river bed above the 

 Falls have been removed since 1842. 



The^ precise measurements of the recession of the 

 Falls in recent years have been accompanied by in- 

 creasing recognition of the extreme complexity of the 

 problem. The existing river system connected with 

 the Great Lakes necessarily dates from the close of 

 the Glacial period in that area ; for it was not until 

 the ice had disappeared that rivers could be formed, 

 and many of them had their sources in the extensive 

 glacial lakes along the receding ice-front. The course 

 of these rivers altered as the lake levels were lowered, 

 and also in consequence of earth-movements, possibly 

 due to the removal of the ice-load. 



When the waters of Niagaia first fell from the 

 plateau into the basin of Lake Ontario they had a 

 fall of only 35 feet, for the lake then stood at the level 

 of some of its uppermost beaches, and the river dis- 

 charged directly into the lake. The power of the 

 Falls was then comparatively small, for they had only 

 20 per cent, of their present height, and only 15 per 

 cent, of the present volume. For the Niagara River 

 was then fed only by the overflow from a compara- 

 tively diminutive lake in the lowest depression on the 

 plains now covered by Lake Erie. The drainage from 

 the Great Lakes, instead of passing through Lake 

 Erie into the Niagara River, was collected into Lake 

 Huron, and was discharged through the gap contain- 

 ing Lake Nipissing to the valley of the Ottawa River. 

 This arrangement was disturbed by the subsidence 

 of the country to the north-east of Lake Ontario, 

 whereby the level of that lake was lowered, and the 

 outlet from Lake Huron to the Ottawa River closed. 

 A fresh channel was opened from the southern end 

 of Lake Huron through a valley now filled up with 

 drift into Lake Ontario. Further movements led to 

 the closing of this outlet, and the waters of Lake 

 Huron flooded the valleys of the southern tributaries 

 and the area that is now Lake St. Clair. The level 

 of the lake rose until it found an outlet at the head of 

 the Detroit River into Lake Erie, and thus at length 

 Niagara received the overflow from the Great Lakes. 



A further complication is introduced by the fact that 

 for one period the Great Lakes had an escape south- 

 ward from Lake Michigan, near Chicago, into the 

 Mississippi ; but this outlet appears to have existed for 

 a comparatively short time. 



The value of Niagara as a geological clock is there- 

 fore open to grave suspicion, for the erosive power 

 of the Falls must have varied enormously, both with 

 the varying resistance of the rocks and with the vary- 

 ing volume of the Niagara River and height of its 

 Fails. Nevertheless, Dr. Spencer is delightfully con- 

 fident of the exact accuracy of his conclusions. " The 

 work of the Falls of Niagara along its whole course 

 has now been made known," and the author claims 

 that his work has brought the dates of the various 

 geographical episodes at Niagara out of the realm of 

 speculation. He rejects the shorter estimates of the 

 length of the Niagara Falls, and somewhat exceeds 

 the result adopted by Lyell, for he assigns them an 

 age of 39,000 years. He also claims to have deter- 

 mined in years the date of the chief episodes in the 

 life-history of the Falls. According to Dr. Spencer, 

 the overflow from Lake Michigan to the Mississippi 

 occurred from 2000 to 2500 years ago ; the Falls 

 were at the Whirlpool 3000 years ago, and the capture 

 of the discharge from the Great Lakes bv the Niagara 

 River happened 3500 years ago. 



The future of the Great Lakes and of Niagara is 

 threatened by two dangers. The diversion of water 



