148 LANCASTER D. BURLING 



of Niagara time in great detail, but does not mention the warped 

 surfaces. Likewise Clarke and Ruedemann, in their memoir on 

 the Guelph, 1 do not mention these structures in their discussion 

 of the conditions of life and sedimentation during the prevalence of 

 this fauna. Writers agree, however, that there was a shallowing 

 of the sea near the close of Lockport sedimentation and a gradual 

 increase in its salinity and the magnesian content of its waters. 

 Arguing from the extraordinary thickness of the Guelph mollus- 

 can shells, Kindle and Taylor 2 postulate the subjection of the 

 bottom of the Silurian sea at this time to intense wave-action. 

 Calvin 3 accepts the theory which had already been suggested by 

 Hall that "at the close of the Niagara huge mounds and ridges were 

 built on the bottom of the shallow Silurian sea, in part by the 

 accumulation in situ of corals, crinoids, and molluscous shells, and 

 in part by the drift of calcareous sediments under strong currents." 

 I am inclined to the opinion, and this appears to be corroborated 

 by the position and physical character of the sediments involved, 

 that the sagging of these beds, both those in the Eldon of British 

 Columbia and those in the Lockport of New York, was largely 

 caused by the gentle scour of water at a time soon after deposition. 

 In each case the horizon of the warped structures is the locus of 

 pronounced changes in the paleontologic record. In the Lockport 

 the time was one of a prolonged emergence and marked the close 

 of the Niagaran; in the Eldon it marks the close of the Middle 

 Cambrian or Acadian and doubtless indicates a similar period of 

 emergence at that time. Walcott, who did not, however, have the 

 advantage of having seen the salt crystals of the Bosworth forma- 

 tion, says: "It is difficult to resist the conclusion that the 268 feet 

 of shales forming the base of the Bosworth Upper Cambrian section 

 were deposited in fresh or brackish water or on a river flood plain 

 or delta such as Barrell describes so graphically in his studies of 

 the Geological Importance of Sedimentation. ," 4 The warped structure 

 at the top of the Middle Cambrian in British Columbia has been 



1 New York State Museum, Mem. No. 5 (1903), 1 14-21. 



2 Geol. Atlas of the U.S., U.S. Geol. Survey, Niagara Folio (No. 190), 1914, p. 116. 



3 Geol. Survey Iowa, Rept., 1896, p. 129. 



4 Problems of American Geology (Yale, 1915), p. 185. 



