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[Assembly 
two hundred feet of the next series, before described, consisting in the 
lower part of a mass of silicious shale, and above alternating with ar- 
gillaceous sandstone. The surfaces of this rock are often rippled and 
covered with minute fragments of vegetables, which seem to follow the 
course of the marking, and accumulate or diminish with the ripple 
wave. The same appearance is presented by a beach of sand, where 
the ebbing tide leaves fine fragments of vegetable matter arranged in 
quantity and direction proportionate to the wave. 
These layers of sandstone furnish the finest flag stones in the county, 
being of any required thickness, and often twenty feet in length, and 
from five to ten in width. 
The Tully limestone and the shale below disappear on Cayuga lake, 
four miles from Ithaca, the black shale extending about two miles far- 
ther south; and on the western side of the county, in consequence of 
the greater elevation of Seneca lake, the black shale disappears near the 
southern boundary of Seneca county, with the exception of a small por- 
tion rising above the lake, which results from the undulation farther 
south. The succeeding group of shales and sandstones approach the 
level of Seneca lake north of Hector falls, and Cayuga lake near its 
head. 
The Ithaca group follows the rocks last described. Like the preced- 
ing, it consists of alternations of shale, both slaty and compact, and ar- 
gillaceous 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 rail-road; it extends also much above the 
rocks here visible, attaining a much greater thickness, as can be seen in 
the valley of Chemung, south of Seneca lake. In the rocks of this se- 
ries, 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, Leptsena, Orlhis, Pterinea, &c. 
These diminish farther west; a few only of the more characteristic, oc- 
curring 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 Ludlow- 
ville shales. These fragments being the lighter part of the animal, were 
