THE DEVONIAN SECTION OF ITHACA, N. Y. 587 



Rule 3 of the United States Geological Survey Rules 0} Nomen- 

 clature and Classification contains the following provision for dis- 

 criminating a "formation": 



Lines of separation are drawn at points in the stratigraphic column where 

 lithologic characters change, or where there are breaks in the continuity of 

 sedimentation or other evidence of important geologic events. 



In the original definition the line at the base was drawn where the 

 "soft argillaceous rock of a green color" of the Cashaqua shale 

 succeeds the "fissile black shale" of the "Genesee black shale." 

 The upper boundary, although marked by the "thick-bedded sand- 

 stones" (Portage sandstones) was distinctly drawn on paleonto- 

 logical evidence. Hall stated: 



Still it must be acknowledged that in lithological characters there is no 

 abrupt change, or evidence of very different conditions in the ocean from which 

 they were deposited, from the termination of the Tully limestone, to the final 

 deposition of the Chemung group (p. 229). 



After stating that there are some general differences noted on pass- 

 ing upward by which the Portage and Chemung rocks can be dis- 

 tinguished as masses, he adds: 



When we apply the test of organic remains, we find an equally or even more 

 strongly marked difference in the two groups, and upon this alone a distinction 

 between the two should be made. 



Later studies have confirmed both of these points, and upon purely 

 lithologic grounds it would be necessary to go down to the Tully 

 limestone to find a sharply defined hthologic lower limit for the 

 rocks in question. There is no satisfactory upper hmit of a purely 

 lithologic nature — not till we reach a definite pebbly conglomerate 

 or a red sandstone such as has been supposed to mark the upper 

 boundary of the Chemung formation. Nevertheless, the rule defines 

 more clearly what is intended by "unity of constitution" by stating 

 that in determining the unity of constitution, which is to character- 

 ize a formation, "all available lines of evidence, including paleon- 

 tology, shall be considered" and "when two formations of closely 

 similar hthologic character are in contact, it will sometimes be neces- 

 sary to depend almost entirely on the contained fossils in separating 

 them." 



These statements distinctly apply to the case in hand. The 



