574 HENRY SHALER WILLIAMS 



{Te7ith Annual Report, United States Geological Survey) is based 

 upon its lithologic uniformity, it is certain that in all cases in 

 which the lithologic changes affect the upper or lower limits of 

 a formation (which is expressed by thinning or thickening of 

 the formation) there must be discordance between the forma- 

 tional continuity and the time represented by it. It requires but 

 a moment's reflection, further, to show that two sections in dif- 

 ferent regions may, on other evidence, be known to represent 

 the same interval of time but present no similarity lithologically; 

 this can receive only the one interpretation that formational dis- 

 continuity does represent time uniformity, which is the converse 

 of the original assumption. 



In other words, while it is practicable in some cases to 

 assume that formations which are clearly continuous may be 

 deposited during the same period of time, it is clear that litho- 

 logic uniformity (by which the continuity of the formation is 

 recognized) is not a safe guide in making chronologic correla- 

 tions, however much value may be placed upon the lithologic 

 divisions of a standard geologic section, as natural divisions of 

 a geologic column made on a time basis. The tracing of time 

 equivalences by formational continuity is unsatisfactory, not 

 because of any failure on the part of a formation, as a litho- 

 logic unit, to represent a definite period of geologic time, but 

 because the time relations of the formation are not expressed 

 by any of the lithologic characters by which one formation is 

 distinguished from another. The confusion the geologist is apt 

 to fall into in discussing this point may be illustrated by the 

 measurement of the altitude of a rock outcropping on a moun- 

 tain side. The base of the Olean conglomerate, for instance, as 

 it appears at Olean Rock City may represent exactly the alti- 

 tude of 2,340 feet above the level of the sea {McKean County 

 Report, Secofid Pe?insylvania Geological Survey, R. 59), but its 

 altitude above the sea has no relationship whatever to any of 

 its lithological peculiarities. In forming an altitude scale, it 

 is in the region a conspicuous mark for the altitude at whicn 

 it lies, and if its dip be considered, the continuity of the 



