18117.] DEAKE — THE GEOLOGY OF IXDIAX TERKITOKY. 335 



northern part of the Indian Territory. Thus the strike swings 

 around so as to become parallel to the western end of the Ozark 

 mountain uplift. This secondary system of folds is more charac- 

 teristic of the northern part of the region, thus further showing its 

 intimate connection with the Ozark mountains. So it appears that 

 the force that threw the Ouachita mountain system into east and 

 west folds at the same time uplifted the Ozark mountains to the 

 north and started secondary north-south folds. 



The sketch map of the Choctaw coal field shows that there are 

 three groups of the south 80° west folds that are especially promi- 

 nent. The group on the north is anticlinal and runs westward 

 from immediately south of Pocola, passing near Farmer and Milton, 

 and running through Sans Bois to the west end of Sans Bois prairie. 

 This group includes three separate anticlinal folds, or the Backbone, 

 the Bokoshe-Miiton and the Sans Bois anticlines. 



The group lying immediately south of this is composed of three 

 separate synclines, the Sugar Loaf, the Cavaniol and the Sans Bois 

 synclines, giving axial direction to the mountains of those names. 

 The third group, farther to the south, is anticlinal and extends from 

 immediately north of Heavener westward, with some minor deflec- 

 tions, passing near Krebs and through McAlester. This is a con- 

 tinuous anticlinal fold. To the north of these three groups the 

 folds are more gentle and irregular, while to the south of them the 

 folding is also more irregular and decidedly more violent and 

 accompanied by faulting. So in the southern part of the field the 

 structure can only be worked out by detailed study. 



Folds of the second system, or those running nearly northeast- 

 southwest, are all gentle and not so extensive. The most promi- 

 nent folds of this system are those separating the Sugar Loaf, Cav- 

 aniol and Sans Bois mountains, and those lying between Ward and 

 Whitefield. Of all these the one separating Cavaniol and Sans 

 Bois mountains is the most prominent, and it is only a gentle roll. 



Characteristics and Origin of the Folds a/id -Faults. — The inten- 

 sity of folding and faulting increases quite regularly southward, as 

 is shown in Sees, i, 2, 3 and 4, PI. 11, and this intensity of folding 

 extends further westward in the southern part of the area. Along 

 any given east and west line the folding is usually more intense to 

 the east ; this, however, is due chiefly to the southward deflec- 

 tion of the west end of the principal folds. As a rule, the strata 

 on the north side of anticlinal folds dip more steeply than on the 



