GEOLOGY OF THE PHILIPSBURG REGION OF QUEBEC 319 



as well as several partly overturned synclines and anticlines — all 

 in limestone of the lower part of the grayer fades of Logan's Ai. 



It might be well to mention at once that one important charac- 

 teristic of the entire series is that the average outcrop gives little 

 or no evidence of any folding whatsoever, so that it is particularly 

 difficult and often impossible to tell when the same stratum has 

 been repeated in a close fold. Most of the beds — if they may be 

 called beds — are thick, and the trend of the rocks, although it can 

 usually be ascertained in general, is not easily measured with exact- 

 ness because the bedding is so obscure in many places. For this 

 reason and for reasons to appear later in connection with a discussion 

 of minor folding, an accurate measurement of thickness is well-nigh 

 impossible. 



The lowest beds of Ai rest upon crumpled beds of black slates 

 interstratified with thin bedded, brownish weathering calcareous 

 strata. These slates occupy a narrow strip of lake shore from 

 Philipsburg south. They show marked signs of deformation, but 

 in no outcrop do they appear to have suffered folding contemporane- 

 ously with the overlying limestones. Although no fossils appear 

 to substantiate the general belief that the slates are of Trenton 

 age, neither is there paleontological proof that they belong to the 

 older age of the overlying rocks. The structural evidence, on the 

 other hand, though scanty, would argue for Logan's interpretation, 

 that the limestones were thrust over the slates. The folding and 

 faulting just mentioned in the overlying Ai beds can be explained 

 best by a thrusting force from the southeast. The same force if 

 strong enough, could have acted on the entire mass of Hmestone as 

 a competent block, thrusting it to west and north. 



Although in the large, the lake shore exposure of the strata 

 comprising Ai bears out the general evidence of neighboring regions 

 regarding the direction of the thrusting force, there is clear evidence 

 locally of overthrusts from the northwest. These thrusts directly 

 opposed to the direction of the main thrusting force can be explained 

 on the basis of initial dip or a vertical shifting of the horizon of 

 major thrust.^ 



'Bailey Willis, "Mechanics of Appalachian Structure," U.S. Geol. Surv., Ann. 

 lUpt. 13, Part 2 (1892). 



