170 GEORGE OTIS SMITH 



made by Mr, Calkins and the present writer along this east-west 

 ridge convinced us that the structure is that of a broad arch, 

 somewhat unsymmetrical, in that on the northern limb the dips 

 are steeper than on the opposite side, and for short distances 

 the strata are even overturned. This anticlinal structure in the 

 basalt flows is well exhibited at Union Gap, where the Yakima 

 river has made a steep-sided cut, a mile in length and 800 feet 

 in depth, across the ridge. 



The next ridge to the north is Selah ridge, also mentioned 

 above. In the earlier description (p. 29) it is stated that here 

 "the arch is broken longitudinally and the ridge assumes the 

 faulted character described in the preceding paragraph," i. e., it 

 is a "monoclinal ridge." A more detailed description is given 

 on a following page (p. 54): 



The structure of Selah ridge is too complex to be described intelligently 

 without the aid of map and detailed section, but these are not to be had on 

 account of the lack of an accurate survey. The ridge is an exception to 

 other similar uplifts in the same region, for the reason that it changes from 

 a monoclinal ridge at the west, where the dip is toward the north, to a mono- 

 clinal ridge at the east having a gentle slope to the south. The general 

 form of the ridge is that of a long, narrow arch, broken at the west end 

 by a fault on the south side, and by another fault on the north side, for 

 the greater portion of its length. 



On the preceding page the structural relations at Selah gap 

 are considered somewhat more fully: 



In the west end of the ridge, separated from the main portion by the deep 

 transverse canyon cut by Yakima river, the east side of the fold is wanting. 

 It may have been cut away by the Naches, which flows along its base, but 

 more probably was carried down by a fault which may have been continuous 

 with a break to be seen on the south side of the ridge and east of the Yakima. 

 There is also a cross-break running north and south which determined the 

 course of the Yakima river. This is shown by the lack of correspondence in 

 the dip of the light-colored John Day beds (EUensburg formation) occurring 

 on the sides of the canyon. 



The detailed study given to this locality when it was mapped 

 afforded no reason for considering that Selah ridge possesses a 

 structure more complex than other ridges of the vicinity. There 

 appears to be no necessity for dividing it "into sections having 

 various structures." It is true that farther west where it merges 



