1 68 GEORGE OTIS SMITH 



discussion of the structural features. Only two important forma- 

 tions are involved, and both extend eastward to the Columbia. 

 The older is the great series of Miocene basalts, measuring 

 several thousand feet in thickness, and this is overlain by 

 Miocene sediments 1,500 feet thick in the best-observed sec- 

 tion. To these two series the names "Yakima basalt" and 

 "Ellensburg formation" have been given. Reference will also 

 be made to the Wenas basalt, which consists of one or more 

 thin lava flows interbedded with the lower portion of the 

 Ellensburg formation, unimportant except as it often affords 

 more exact datum planes for the determination of structure. 

 Topographically the region is one of moderate relief, and the 

 general absence of erosion subsequent to the latest deforma- 

 tion of the region is a result expressive of the aridity of the 

 climate. 



The first recognition of the Basin range structure in central 

 Washington is given by Professor Russell in these words : ^ 



All of the formations mentioned in the preceding section were originally 

 spread out in essentially horizontal sheets, but since the youngest member in 

 the series was laid down they have been broken into blocks, and the blocks 

 tilted and upturned so as to form prominent mountain ridges with horizon- 

 tally floored valleys between. A structure has thus been given to the beds 

 on which depends nearly all of the present topographic diversity of the 

 region 



Briefly stated, the main structural features in our field are (i) narrow, 

 sharp-crested ridges having a prevailing east-and-west trend, due to the 

 upheaval of the borders .of orographic blocks ; (2) broad and comparatively 

 gentle north-and-south elevation, produced apparently by an arching of the 

 strata, parallel to and probably of the same date as the much greater uplift 

 forming the Cascade mountains ; (3) regions where the rocks have been but 

 little disturbed and now form plains and valleys. 



The author distinguishes monoclinal ridges and monoclinal 

 folds. Of the former he states : 



The prevailing and most typical structure has been produced by the 

 breaking of the strata and the upturning of the beds on one side of the 

 lines of fracture. The fragments, more or less regular, into which the rocks 

 have been broken are termed orographic blocks. (P. 28.) 



The monoclinal folds are thus described : 



^ Op. cit., pp. 28-31. 



