THE DEVONIAN SECTION OF ITHACA, N. Y. 581 



upward into the high hills of Ashland, near the top of which the 

 member is terminated by a thin conglomerate lentil called the Fall 

 Creek conglomerate lentil. Immediately below this conglomerate 

 the horizon is indicated by a band of thin-bedded sandstone, often 

 calcareous, containing a great number of shells of Leptoslrophia 

 nervosa and Orthothetes chemungensis , to which the name Ashland 

 Leptoslrophia zone is applied. 



It was noted that the formations of this section, extending from 

 the top of the Onondaga limestone formation to the top of the sec- 

 tion, belong to the Erie division of Vanuxem and Hall. This con- 

 stitutes a natural series according to the rules of the Survey; the 

 upper boundary of which, according to the original definition, should 

 be the Catskill division (or series). Although the Wellsburg sand- 

 stone member appears to be the upper member of the Chemung 

 formation of this particular section, the author expressed the con- 

 viction that the stratigraphic horizon of the Fall Creek conglom- 

 erate does not mark the termination, chronologically, of the Chemung 

 fauna. 



DISCUSSION 



The original definition of the section. — This geological section 

 of the Devonian rocks running through Ithaca was constructed 

 along the boundary line between the Third and Fourth Districts 

 of the state. Lardner Vanuxem wrote the Final Report on the 

 Third District (1842), and James Hall the Final Report on the Fourth 

 District (1843). The adopted classification and nomenclature of 

 the Paleozoic rocks, up to the base of the Carboniferous, for North 

 America has been constructed on the general lines which were 

 announced chiefly by Vanuxem and Hall, and published in these 

 two reports. 



The significance of this particular section was expressed by 

 Vanuxem in his report (1842), in the following passage: 



The Erie division embraces the rocks above those of the Helderberg division^ 

 extending to the Catskill group. It presents through several counties on both 

 sides of the boundary of the Third and Fourth Districts two well-deiined parts, 

 separated by the Tully limestone and the Genesee slate; these latter are boundary 

 masses of the two parts, being comparatively very thin, and of no great extent of 

 range. The lower part of the division consists of the Marcellus shales and the 



