176 GEORGE OTIS SMITH 



cessive lava sheets, as well as between the vertical columns, must 

 reasonably be expected to have been important in whatever flexing 

 took place. Such displacements, however, were at no place 

 observed, and are doubtless on a minute scale, although of con- 

 siderable importance in the aggregate. 



A consideration of the cause of deformation of the nature 

 described here would lead to the discussion of a much broader 

 question. It would involve the origin of the Cascade range, the 

 uplift of which is believed to belong to the same period as the 

 production of these east-west ridges. Such a discussion will be 

 in order after the results of detailed mapping over larger areas 

 of the Cascade mountains are in hand. It may, however, be in 

 accord with the purpose of this paper to take exception to cer- 

 tain assumptions as to origin with which the geologist appar- 

 ently sometimes enters such a field. An example follows '} 



The arches were raised by a force acting from below upward, and not by 

 lateral pressure which forced the strata into ridges and troughs, as is com- 

 mon especially in the Appalachian mountains. 



This distinction between the supposed monoclinal structure 

 with faulting and the more common type of deformation appears 

 to be based in large part upon certain a priori hypotheses 

 developed in the course of work farther south, viz. : tilted 

 orographic blocks are expressive of lateral extension rather than 

 lateral compression, and the region is one characterized by 

 depression as well as by fracturing. Therefore, central Wash- 

 ington, like southern Oregon, was at once considered as charac- 

 terized by deformation which involves extension rather than 

 compression. It is now suggested that such assumptions are 

 not the only ones that can be made to account for the facts. A 

 glance at the structure sections will show this assumption of 

 extension to be unsupported by the field evidence. The occur- 

 rence of overturned strata on the limbs of folds is, moreover, 

 forcibly suggestive of some degree of lateral compression, while 

 the fact that the Yakima cuts below the floors of several of the 

 transverse synclinal valleys may be considered good evidence 

 that these valleys are not sunken areas, but have simply been 

 uplifted less than the bordering ridges. 



^ Bull. No. 108, p. 29. 



