THE EEPDBLIC TROUGH. 539 



nearly 45°, the several fonuations swing aronnd snccessively in horseshoe 

 form through an angle of 180° from the northeastern to the southwestern 

 side, and the fold, so far as it affects the Algonkian rocks, abruptly termi- 

 nates. Through the greater part of the length of the trough tlie rocks on 

 the two sides of the axial plane have been squeezed nearly into parallelism. 

 None of the many surface observations show in tlio Islipeming quartzite a 

 dip less than 80°. The formations in the underlying Lower Marquette 

 series dip at a uniformly higher angle on the eastern side, being either 

 vertical or slightly overturned toward the west, while on the western side, 

 owing to the absence of the lower series for nnich of the wav, observations 

 are rare, but a similar divergence in dip is found in two or three places. 



If the base of the Goodrich quartzite be developed into a horizontal 

 straight line along any cross-section (thus approximately restoring the 

 conditions to what they Avere before the last folding), it will be seen that 

 the rocks of the underlying lower series on the two sides of the trough 

 dip toward each other (Atlas Sheet XI, sees. A'A' and B'B'). This conver- 

 gence in dip along a developed section points clearly to the existence of a 

 gentle syncline in the Lower Marquette series before Upper Marquette time, 

 within the limits of the rocks included in the present fold. The very slight 

 discordance between the strikes of the members of the two series, which, 

 broadly regarded, is measurable in feet per mile rather than in degrees, 

 would indicate that the axis of the later fold is sensibly parallel to that of the 

 older, while the greater thickness of the lower series remaining on the east 

 side of the present trough, as compared with that remaining on the west? 

 gives good ground for the inference that the axis of the old syncline lay 

 somewhat east of the present axis. This previously existing synclinal axis 

 doubtless determined in the later folding the position of the present trough. 



It has been said that the trough as a whole pitches, at its southeast end, 

 toward the northwest at an angle of about 45°. This is not far from the 

 average pitch at the surface. With depth this angle slowly diminishes, 

 and at about 900 feet below the surface it is less than 40°. The distance 

 in which the turn is made at the southeast end of the trough is relatively 

 very short. The average radius of the generalized curve into which the 

 base of the Goodrich quartzite has been thrown can be very little greater 



