DOWNWARPING AT CLOSE OF NIAGARAN AND ACADIAN 147 



conditions in rocks of Cambrian age where the evidence as to their 

 early origin is conclusive. 



In all of the Lockport localities that have been described the 

 warped surface directly underlies till or marine clays. During the 

 field season of 191 5 a similar warped surface was discovered to be 

 characteristic of the uppermost beds of the Middle Cambrian in 

 British Columbia. These are dolomites and form the top of the 

 Eldon formation. The basal beds of the Upper Cambrian rest 

 directly upon this warped surface and, as if to yield further con- 

 firmation of the fact that the beds forming the top of the Eldon 

 suffered prolonged exposure to the air and that such a condition 

 continued during the period of deposition of the basal beds of the 

 overlying Bosworth formation, the latter is full of mud-cracks, 

 ripple-marks, and casts of salt crystals 2 inches or more in diameter. 



The phenomenon was studied in the amphitheater north of 

 Castle Mountain, and there is here no question that the warping 

 was essentially contemporaneous with the deposition of the strata. 

 It is interesting to note that the only known occurrences of this 

 peculiar type of warped structure in both cases occur at the top of 

 a dolomite overlain by shales with salt crystals and evidences of 

 salinity. In the Lockport, Grabau 1 is of the opinion that the over- 

 lying Vernon suggests the accumulation of fine loess-like material, 

 chiefly as wind-blown dust. This is of interest in connection with 

 the theory of the eolian origin of the salt deposits of India discussed 

 by Holland and Christie. 2 The bed immediately overlying the 

 warped structure at the top of the Eldon is almost a pure dolomite, 

 and the warped layers whose depressions it fills contain almost as 

 little calcium carbonate. Grabau 3 describes the section between 

 the Lockport and the Salina as composed of a "stratum of thin- 

 layered bituminous accretionary limestone, forming flat, imbri- 

 cating, shell-like domes" overlain by 2 feet of yellow impure 

 limestone, which is in turn succeeded by the green shale forming 

 the base of the Salina. In this paper on the early Paleozoic Delta 

 deposits of North America, Grabau goes into the physical conditions 



1 Bull. Geol. Soc. Amer., XXIV (1913), 490. 



2 Rec. Geol. Survey India, XXXVIII, Part 2 (1909), 154-86. 



3 Bull. Geol. Soc. Amer., XXIV (1913), 491. 



