PALEOZOIC LAND AREAS. 261 



land areas there during Palaeozoic time. As will be seen later, the eleva- 

 tion which accompanied an orographic movement did not affect the whole 

 area uniformly, but some regions were raised more than others, and indeed 

 there is some evidence to prove that some portions of the area Avere actu- 

 ally depressed while others were being raised. In a general way, therefore, 

 it may be said of the southern area that the distribution of land areas was 

 probably somewhat more widely spaced than in later times, and that inte- 

 rior depressions existed that were afterwards raised above ocean level, and 

 even became parts of prominent mountain masses as the outlying land- 

 masses were depressed. 



The Late Palaeozoic Movement. 



The existence of land areas toward the close of Palaeozoic time has been 

 frequently suspected by western geologists from the evidence of shallow 

 water and shore-line conditions in the beds which have been considered upon 

 somewhat meager and often conflicting palreontological evidence to belong 

 in different localities to the upper Carboniferous, Permian, or Trias ; but, so 

 far as I know, no actual unconformity has hitherto been observed. In the 

 summer of 1882 I first noticed what seemed conclusive evidence of the exist- 

 ence of such an unconformity in the Elk mountains, but it was not until 

 two years later that actual field work with my assistants, Messrs. Cross and 

 Eldridge, enabled me to fix its horizon as in the middle or upper part of the 

 Carboniferous.* Since that time I have found such corroborative evidence 

 of its existence in various parts of the Rocky Mountains as justifies the con- 

 clusion that a general orographic movement took place throughout this re- 

 gion, whose effects may probably be found to have been felt beyond it. It 

 is a movement that is in many ways difficult to define. Firstly, on account 

 of the wide range of most of the abundant molluscan species which are 

 found in Carboniferous beds, owing to which palreontological evidence by 

 itself is thus far of but little value in determining the relative position of 

 any beds except those at the two extremities of the series. Further, because 

 the dynamic disturbances that accompanied the movement were very un- 

 equally distributed, and their effects are to be observed, as a rule, only in 

 regions which were again violently disturbed during the succeeding move- 

 ment, where they were consequently much obscured. Its determination as 

 occurring in middle or late Carboniferous time has, therefore, necessarily 

 been founded mainly on the stratigraphical relations and lithological 

 character of the beds. 



That it was not earlier than middle Carboniferous is proved by the finding 



* A notice of this, and of the Jurassic unconformity observed in the same region, was published 

 in the Sixth Annual Report of the I >ireetor of the U. S. fteol. Survey, 1885, p. 64. 



