312 
j AsSEMBLr 
The course of the lime^ones just described, can be easily traced by 
the elevated terrace formed by the line of their northern outcrop. Their 
superior hardness withstood the erosive action of water, while the sof- 
ter gypseous rocks, and water limestones on the north, were excavated 
to a much lower level, producing in one case a valley, in the other a 
corresponding elevation. Where this terrace is unbroken, the alluvial 
hills for the most part terminate in the valley north, or on the gypseous 
formation; and the deposition of transported matter in such abundance 
along this line, is doubtless owing to the partial check which the cur- 
rent received from the limestone elevation south. 
The terrace here described, must not be, as it often is, confounded 
with that extending east from Lewiston, the mountain ridge; the latter 
is entirely a different rock, though the outline is produced by similar means 
and from similar causes. The mountain ridge is not perceptible east of 
Rochester. 
In this county, as in Seneca, the black shale succeeds the Seneca 
limestone; and possesses the same essential characters. It appears in 
the banks of several small streams in the eastern part of the county, 
though its immediate connexion with the limestone has not been ob- 
served. On Flint creek, three miles from Vienna, this shale is exten- 
sively developed. The lower part is compact and highly bituminous; 
it contains much pyrites, and the decomposing shale effloresces large 
quantities of sulphate of alumine. On the Canandaigua outlet it is 
not so well developed; nor at any intermediate point; for the force of the 
original water having widened the channel, leaves the shale now covered 
with alluvium. On Mud creek, it is seen above the locality of the Se- 
neca limestone, near HilPs mill. 
The different members of the shale series succeed the last mentioned in 
the same order and with the same characters as in Seneca county. These 
I shall speak of together, deferring the minute distinctions to a more de- 
tailed report. Several members of the group are well exhibited in the 
ravines and on the shore of Seneca lake, above Geneva. Along Flint 
creek, from two miles below Orleans to the village of Bethel, they are 
seen in great perfection. The upper mass is particularly well developed 
at Monteith's point, on Canandaigua lake, and the Encrinal limestone 
appears about half a mile below. The shale is seen in many places 
along the eastern shore, and in all the ravines entering the lake, for se- 
ven or eight miles south of Canandaigua, 
