476 
GEOLOGY OF THE FOURTH DISTRICT. 
the lower part of a mass of siliceous shale, and above alternating with argillaceous 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. 
The Tully limestone and the shale below disappear on Cayuga lake, four miles from Ithaca, 
the black shale extending about two miles farther 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 portion rising 
above the lake, which results from the undulation farther south. The succeeding group of 
shales and sandstones approaches the level of Seneca lake north of Hector falls, and Cayuga 
lake near its head. 
The Chemung group, like the preceding, 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 lime¬ 
stone, the calcareous matter arising principally from the contained shells. This group is well 
characterized at Ithaca, at the inclined plane of the railroad; it extends also, far above tho 
rocks here visible, attaining a much greater thickness, as can be seen in the valley of Che¬ 
mung, 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. 
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. 
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, except that the casts of 
mud furrows are more abundant and large, being often an inch in diameter and several feet in 
length. The surface of most of the layers is smooth, or even glazed with a thin coaling 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. 
In some localities the sandstone is replaced by a kind of sandy shale, being a mixture of 
sand and clay; and the whole is rippled, the markings affecting each thin layer, and showing 
that it was deposited from water in motion which might transport from different directions the 
two materials of the rock. This group appears to have been deposited from an ocean alter¬ 
nately at rest, and disturbed. Thick masses of sandy shale occur, bearing ripple-marks 
through their whole depth ; these are succeeded by others of variable thickness, without ripple- 
