94 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



were strongly folded with a general synclinal arrangement along an axis 

 running roughly parallel to the Cobequid belt, the dips at South Jog- 

 gins averaging 19° and in other parts from 0° to 45° or more (Plate 9). 

 Strata characterized by such thickness and so diverse attitudes must 

 have been covered by a constructional topography of relatively great 

 strength. That topography is now lost, and a new one has taken its 

 place. From the railroad station at Wentworth, one looks down over 

 a vast expanse of this Carboniferous district and is struck by its even- 

 ness (Plate 9). A vast plain lies before the observer. It is highest at 

 the three hundred-foot contour where it abuts against the Cobequids. 

 Farther away, it sinks to an average elevation of two hundred feet, which 

 persists throughout most of the basin, except where narrow valleys are 

 incised below the plain. The plain is not absolutely level, but is gently 

 rolling, as is typically displayed on the railroad from Maccan to South 

 Joggins and along the Joggins shore. 



The rocks of the Colchester district differ from those of Cumberland 

 in including only the Lower Carboniferous series and therewith a larger 

 proportion of limestones than occur in the northern trough. The struc- 

 tures are here more complicated. The beds stand at all angles to the 

 horizontal plane and are frequently interrupted by faults of large throw. 

 The great gorge at Truro affords an especially fine view of the steeply 

 inclined sediments. The present topography is again, however, quite 

 independent of structure. The lowland stands at an average height 

 above the sea of about two hundred feet. Gentle ridge-like swells on 

 the harder beds break the monotony of an otherwise nearly perfect 

 plain, in which youngish valleys are sunk, comparable to those of 

 Cumberland County. 



Now, in accounting for this great amount of denudation in both dis- 

 tricts, we have not only to deal with a possible Jura-Cretaceous cycle and 

 a possible Tertiary cycle ; some place must be made for the enormous 

 Permian denudation implied by the unconformity between the Carbo- 

 niferous and the Triassic, and for Triassic denudation on unsubmerged 

 Carboniferous. Then, too, a mid-Carboniferous period of subaerial con- 

 ditions must have elapsed, since we find an unconformable relationship 

 between the Lower and Middle Carboniferous strata. However, not- 

 withstanding this latitude offered us in placing the dates of the various 

 erosion-periods which, from time to time, have worn down the construc- 

 tional Carboniferous relief, I have come to the belief that the existing 

 lowlands owe their existence essentially to the same Tertiary cycle of 

 wasting that explains for us the Annapolis Valley. 



