ANTICLINAL MOUNTAIN RIDGES I 77 



The search for a force to supply this supposed upward pres- 

 sure involves further difficulty (p. 29). 



Just what the action was which produced these arches it is difficult to 

 determine. It is possible that volcanic rocks, escaping in a molten state 

 through fissures in lower beds, raised the Columbia lava and superimposed 

 beds into arches. In the continuation of the ridge cut through at Union gap, 

 which forms the south wall of Moxee valley, molten rock forced up from 

 below escaped through fissures in the Columbia lava, but raised the lighter 

 beds above into a long, narrow ridge. In this instance the intruded lava has 

 been clearly exposed by the erosion of a longitudinal valley along portions of 

 the crest of the uplift. 



In another place (p. 54) Selah ridge is cited as another possi- 

 ble occurrence of uplift by intrusion. "There are also reasons 

 for suggesting that the scoriaceous basalt at the base of the sec- 

 tion may have been a subsequent intrusion." In a careful study 

 of the basalt section at the latter locality with a view to sub- 

 stantiate the earlier observation, the present writer failed utterly 

 to find any reason for supposing any of the basalt intrusive. 

 Both here and in the other locality cited the extremely scoria- 

 ceous character of the basalt was observed, but was taken by him 

 to indicate the basalt to be extrusive rather than intrusive. 

 Moreover, in both cases the basalt in question is superficial in 

 position as compared with that shown in the deeper cuts of 

 Union and Umptanum gaps. 



Relative to the structural features of central Washington, 

 then, the later and more detailed observations conflict with the 

 results of earlier reconnoissance in these respects : 



The mountain ridges described as monoclinal fault-blocks, 

 tilted along lines of fracture, are found to be gentle anticlinal 

 folds, with no evidence of faulting at any one of the several 

 localities cited. 



The assumption that lateral pressure had no part in this 

 deformation is opposed by the structure section which shows 

 compression, as well as by the occurrence of overturned strata 

 on the sides of one of the synclines. 



The hypothesis that igneous intrusions produced the defor- 

 mation rests upon field evidence, which impresses the later 



observer as wholly inadequate. 



George Otis Smith. 



