THE YAMPA PLATEAU. 203 



displacement ; where this displacement is by faulting the cliffs are escarp- 

 ments, but where it is by flexure the flexed beds remain, and the slope of 

 the cliffs conforms in a general way with the dip of the strata. Hence these 

 lines of cliffs have not receded from the locus of displacement, although they 

 are so precipitous as so furnish conditions favorable to rapid degradation by 

 direct sapping, corrasion, and general erosion. From these facts we inevi- 

 tably conclude that the displacement is of late date. But the Triassic beds 

 on the upheaved side are gone, and in many places a great thickness of 

 Carboniferous beds also, while on the thrown side a notable thickness of 

 Trias yet remains. The non-recession of the cliffs proves that the displace- 

 ment was not ended long ago, if indeed the movement has yet ceased, while 

 the occurrence of the beds on the thrown side, which have been degraded 

 from the upheaved side, lead us to refer the inception of the upheaval to an 

 epoch of much earlier date, and to infer that the displacement was slow, yet 

 not so slow that degradation was able to keep apace. 



When we come hereafter to discuss the relation of this displacement to 

 Other facts of displacement, degradation, and sedimentation, it will appear 

 that this displacement was not upheaval in relation to the general region 

 under discussion in this chapter, but was in fact downthrow, whatever it 

 might have been in relation to the level of the sea or in that other relation, 

 which yet may be a very different thing, i.e., to the center of the earth. 



SCHOLIUM. 



Early in this volume I distinctly defined my use of the terms "upheaval" 

 "subsidence," "uplift," and "downthrow," and restricted the meaning of these 

 words in such a manner that they should relate only to adjacent and compared 

 beds of rock, but I recognize three categories of relations that may be 

 expressed by these terms, viz, the relation of parts of a geological horizon 

 to each other, the relation of parts of a geological ho'rizon to the level of 

 the sea, and the relation of parts of a geological horizon to the center of 

 the earth. For these several categories it would be a great advantage to 

 geological science, by leading to greater simplicity and precision of de- 

 scription and to clearer conceptions, if different verbal representatives could 

 be used for the different ideas. 



