78 PROCEEDINGS OF THE WASHINGTON MEETING 



curves we see that between the marked -depression of the middle of the (Upper) 

 Silurian and the Devonian there was a long period of relatively low sealevel or 

 high stand of the continent, not due to any disturbance in Michigan, but per- 

 haps due to disturbances in the other hemisphere. The question is whether 

 the base of the Devonian shall be drawn at the beginning or the end of this 

 time of uplift. If we assume, as seems likely, that the beginning is the best 

 line, the most sharply marked, and the one most nearly coeval throughout the 

 world, then the Detroit River series would be Devonian. 



We have, however, an alternation of a fauna so Devonian that it was at first 

 taken for Hamilton (Traverse) and then a recurrence of Silurian fauna. But 

 this Silurian fauna is of a local peculiar type — that associated with brines — 

 that may easily be assumed to have lingered on in some Dead Sea or Aral Sea. 

 The real difficulty is that in New York we have a Lower Devonian fauna of 

 a quite different type. We must either assume these to have formed during 

 the unconformity, so well described by Stauffer, or assume that New York and 

 Michigan were in different provinces for a length of time that seems unlikely. 

 But the division line between Devonian and Silurian must depend on dias- 

 trophic studies outside of Michigan. 



RECESSION OF NIAGARA FALLS REMEASURED IN 1311, 

 BY J. W. SPENCER 



{Abstract) 



In October, 1914, just ten years after my previous survey, which had been 

 the fifth (the fourth was that of Kibbe, in 1890), I remeasured the crest-line 

 of the main cataract of Niagara. The principal changes were not at. but adja- 

 cent to, the apex. Here had been a narrow Y-shaped incision in the upper 

 strata only, which has widened to one of U-shape 50 feet broad. To the west 

 blocks of rock 400 feet long and from zero to 55 feet wide had fallen, thus 

 straightening the line. The fallen area here and elsewhere aggregated about 

 two-thirds of an acre, corresponding to a mean recession for the full width of 

 the falls of scarcely 2.5 feet a year. The diversion from the river had been 

 more or less compensated by the late prevailing high water. This reduces the 

 mean annual rate Of recession during 72 years to 4 feet a year, compared with 

 1.2 feet during the previous 02 years, but adds 5 per cent to the computed age 

 of the falls. 



The eastern end of the great cataract at the time of Hennepin (1678) has 

 been investigated, the location of the western end having been previously 

 found. Thus it appears that the mean rate of recession during 236 years was 

 approximately 3.75 feet a year. This factor would increase the calculated age 

 of the fall by 10 per cent, which figure still comes within the limit originally 

 provided for (39.000 ± 4,000 years), but on the side of increased age. 



Additional soundings in the upper part of the gorge were also made. In no 

 case did I find depth of 100 feet, in contrast with those of 186 to 192 feet a 

 short distance below. These confirm the evidence of a late reduction in the 

 height of the falls, as previously described, and also mentioned by Kalm in 

 1750. The reduction in height was due to the falling of the gorge walls at the 

 Whirlpool Rapids, thereby damming and raising the river level above them. 



I made another sounding (of 210 feet) near that of 183 feet, previously de- 



