[)-J, BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



on the amygdaloid. The trench has, on the other hand, all the appear- 

 ance of a subsequent valley worn out in the second cycle by a river 

 already well adjusted in an earlier cycle. Finally, we shall see that the 

 structure and topography of the Acadian Carboniferous areas will sug- 

 gest a subaerial origin for the neighboring Triassic lowland. The broad, 

 low, gently rolling plains of the Colchester and Cumberland districts 

 flank the Cobequids, and are continuous with the great Carboniferous 

 lowland of New Brunswick on the northwest. Their discussion will be 

 postponed until the date of the upland facet has been fixed, for this 

 date will be found to have an important bearing on the problem of their 

 interpretation. 



Granting a subaerial origin for the Triassic lowland, it is natural 

 to attempt a scheme for the pre-glacial drainage that conditioned the 

 erosion. It is evident that the picture must be incomplete. Much 

 of the drainage must have been longitudinal. Some of it was trans- 

 verse through courses still well preserved in the notches at Digby 

 Gut, Sandy Cove, Petit Passage, and Grand Passage (Plate 7). At 

 least two of these are located on faults causing dislocations in the trap 

 visible in the field. 1 The offsetting of the trap ridge at all four notches 

 is most simply explained by as many upthrows on the west. It looks as 

 if these faults were older than the upland peneplain, that either a con- 

 sequent stream of the former cycle or a later subsequent stream occupied 

 each fault-zone, and that the notch in each case was deepened about as 

 fast as the adjoining lowlands were worn out. In like manner, the 

 Connecticut traversing the Holyoke range in Massachusetts has kept 

 open its notch, the product of two cycles. Since the recent drowning of 

 southwest Nova Scotia, tidal scour has somewhat widened the passes. 

 In passing, it may be noted that the fault at Digby Gut lies close to the 

 hinge-line about which I have posited the westward warping of the 

 peneplain. 



The Geological Dates of the Peneplains. — In the absence of 

 the sediments which must have been deposited on the sea-floor during 

 the formation of the peneplain, it is not possible to deduce directly the 

 date of the facet in geological time. If I am right in correlating the 

 upland facets of North Mountain and the Southern Plateau, it follows 

 that the peneplanation must have occurred in post Triassic times. Beyond 

 this general fact, the Acadian record will not permit us to go, except as 

 that record is interpreted in terms of better known regions. Using the 



1 Dawson, Acadian Geology, ed. 2, p. 9(3. Cf. Bailey, Geol. Surv. Canada, Ann. 

 Rep. M., 1896, Vol. 9, p. 131, and Dawson, op. cit., p. 95. 



