DOWNWARPING AT CLOSE OF NIAGARAN AND ACADIAN 149 



observed, not only in the Castle Mountain section along the 

 Canadian Pacific Railway, but in the Mount Robson section of the 

 Grand Trunk Pacific 200 miles to the northwest. In each case 

 it is followed by reddish-purple, green, and yellow shales, with 

 ripple-marks, mud-cracks, and casts of salt crystals. 



Salt crystals, previously known only from pre-Cambrian and 

 post-Ordovician rocks, have been assumed, and rightly, to indicate 

 arid conditions and more or less emergence. The peculiar type of 

 downwarping described appears to be the natural result of the 

 quiet subaerial exposure of recently consolidated dolomitic lime- 

 stones under conditions of aridity. Whether or not the down- 

 warping described is necessarily contemporaneous with aridity, it 

 is certainly a feature due to subaerial exposure, and the probabili- 

 ties are in favor of its formation at the time of the deposition of 

 the strata rather than subsequently. This necessarily involves the 

 assumption that the joints along whose channels the solution was 

 localized came into existence very soon after the deposition of the 

 strata and were relatively contemporaneous with the deposition 

 of the beds. The evidence as to the early origin of the warped 

 structures is so conclusive that we are justified in disregarding 

 such coincidences as the immediate superposition, where so far 

 discovered, of tills and clays upon the warped Lockport and such 

 inferences as that, since the solution took place along joint planes, 

 it must be comparatively recent. Have we not here rather a slight 

 measure of the duration of the time-break in the deposition between 

 the Middle and Upper Cambrian, and between the Niagaran and 

 the Cayugan, and are we not justified in bearing in mind the 

 principle of relatively contemporaneous consolidation and joint- 

 ing in rocks ? 



