110 THE ROYAL SOCIETY OF CANADA 



relief (34), (24, c). This linear outcrop cutting across an irregular 

 terrane is proof that they are breaks along a very even surface of 

 parting, and that this surface now stands at a high angle of dip. The 

 fact that these faults are steeply inclined at the surface is, of course, 

 no guarantee that they maintain this attitude indefinitely, and they 

 probably dip at flatter angles at depth. But when many faults, at 

 different topographic elevations, and separating widely different 

 geologic horizons, are all steep, then they must be supposed to remain 

 steep for hundreds, if not thousands of feet, and a common cause for 

 this attitude must be sought. 



A measurement of 22 faults in the sections of the western part of 

 the Blairmore sheet gave an average dip of the reverse faults of slightly 

 over 70 degrees, which is nearly twice the average dip of reverse 

 faults as stated by Leith (20, a). The average angle of inclination 

 of the strata which they cut to these same fault planes is just under 

 31 degrees, which is not far from the average dip of reverse faults. 

 This angle of 31 degrees between the dips of the strata and the fault 

 is believed to be significant, and furnishes one of the clues to the ex- 

 planation of the anomalous attitude of the reverse faults, and of the 

 general structure of the region. 



A prominent feature in the northern part of the eastern structural 

 area, so well shown on Rose's map of the Blairmore area, is the Turtle 

 Mountain anticline. The southern half dozen sections of this map 

 illustrate another general structural feature of the region connected 

 with this anticline. These sections show that the western limb of the 

 anticline is formed of steeply dipping fault blocks of repeated strips 

 of strata, whereas the eastern limb is more folded than faulted, and 

 the strata are in general much flatter, though the folding is irregular 

 in detail. The southern continuation of this anticline exhibits a 

 similar though a less pronounced relation of a steeper western and a 

 flatter eastern limb (24, c). This steep western and flat eastern limb 

 is an effect that would be caused, leaving the faulting out of con- 

 sideration, by the westward rotation of a previously formed sym- 

 metric anticline. 



The Lewis Thrust 



The salient features of this great overthrust are of such import- 

 ance in a consideration of the structural features of the region that they 

 will be given here. 



Extent. — The LcAvis overthrust (see Fig. 1) was first recognized 

 and named by Bailey Willis in northwestern Montana in 1901 

 (45, f). In 1906 Daly traced the thrust into Alberta (12, h) and in 



