The Historical and Actual Significance of the Terms, 

 "Ithaca Group" and "Ithaca Fauna." 



It is quite essential to acquire a clear conception of the precise footing 

 upon which the terms Ithaca group and Ithaca fauna now rest in the New 

 York nomenclature, in order to fully apprehend the difficulties of applying 

 either in the region eastward of the Ithaca meridian. 



The name Ithaca group was first used by Professor James Hall (1839). 

 In discussing the geologic formations of Tompkins and Chemung counties, in 

 the Third Annual Report of the Fourth Geological District (pp. 318-322), 

 the account given of this formation and its distribution is so full that I 

 reproduce it here in its entirety. 



" The Ithaca group follows'the rocks last described [the shales and flags immediately overlying 

 the Genesee slate]. Like the preceding, it consists of alternations of shale, both slaty and compact, 

 and argillaceous sandstone, but differs from it in the contained fossils, and in some particulars of its 

 lithological character. It sometimes contains thin layers of impure limestone, the calcareous matter 

 arising principally from the contained shells. This group is well characterized in Ithaca at the inclined 

 plane of the railroad; it extends also much above the rocks here visible, attaining a much greater 

 thickness, as can be seen in the valley of the Chemung, south of Seneca lake. In the rocks of this 

 series individuals of two species of ferns have been found, precursors of the great abundance of that 

 tribe in the coal formation; and among the many testaceous fossils are Producta, Leptcena., Orthis, 

 Pterinea, etc. These diminish farther west, a few only of the more characteristic occurring on Seneca 

 lake. 



"With the deposition of the Tully limestone the family of trilobites ceased to exist, yet we find 

 with the characteristic fossils of this group the buckler of Dipleura Dekayi and Calymene bufo, with 

 other fossils which lived at the period of the deposition of the Moscow and Ludlowville shales. 

 These fragments, being the lighter part of the animal, were floated upwards with the detritus of the 

 lower rocks and deposited at this era. Similar instances occur in some of the lower rocks, but the 

 occurrence of such fossils is not to be considered as characteristic of a rock, or as evidence of their 

 existence at the time of its deposition. 



"At Hector Falls, and above, we find about four hundred feet of this group exposed; the lower 

 part contains the ferns of Ithaca, and above some of the other fossils. At this place we find a few 

 thick layers of sandstone, very compact and firm, which have been quarried by Mr. Lawrence. Few 

 durable building stones are found in this county, if we except this sandstone, which, however, is little 

 used. It furnishes the fine flagstones used in Ithaca and elsewhere. In general characters it differs but 

 little from that of the group below, but Fucoides are found in greater quantities on the surface, many 

 stems of which attain a diameter of two or three incbe=. One species, the most abundant, occurs on 

 the under side of the layers, as if growing on the bottom of mud and clay, when overwhelmed with the 

 inundation of sand. This species is always straight or anastomosing at various angles, sometimes pre- 

 senting imperfect reticulations, and at other times a fancied resemblance to a bird's foot. It occurs on 

 many layers through a considerable extent, though separated by thick masses of shale. The surface of 

 most of the layers is smooth, or even glazed with a thin coating of shale, which appears to have flowed 

 over it, leaving marks of unequal deposition and little ridges or prominences where the paste was less 

 fluid. The deposition of these shales and sandstones progressed slowly, considerable time having 

 elapsed between the deposition of the different layers, and in some instances a lower stratum became 

 partially indurated before the succeeding deposit was made. There are numerous proofs of the general 

 operation, and in particular of the latter, where we find, near Jefferson, the surface of a layer worn 

 smooth and grooved, as if by a current, transporting some hard body over it. The scratches do not 



46 



