418 
[Assembly 
The same causes which here produced the small nodules of gypsum, 
were in operation over a large extent, to form the immense quantity 
which occupies a place in the rocks beneath the drab limesone. Ow- 
ing to this circumstance only, we find no fossils in the gypseous rocks, 
for none could exist in a sea where sulphuric acid was a free ingredient. 
The continuation of the drab limestone on the east side of the river 
is found in Rush and Mendon, and has already been noticed in the sur- 
vey of Monroe county. 
Onondaga limestone. — The Onondaga limestone succeeds the last 
described rock ; it scarcely extends into the town of Lima, but it forms 
the substratum of the northern portion of Avon, and in the river valley 
extends as far south as the centre of the town. This rock is quarried 
in the Conesus Lake outlet, and on a small stream a short distance fur- 
ther east ; at these places only a few feet of the upper part of the mass s 
is seen. It is easily quarried in blocks of large dimensions, and is near- 
ly free from hornstone. The stone is wrought for use on the Genesee 
Valley canal. The fossils at this place consist chiefly of Strophomena 
rugosa, Atrypa affinis, Delthyris, and some fragments of Trilobites. 
The greater portion of this rock on the east side of the river is covered 
by a deep alluvium which renders it difficult to trace its bearing and out- 
crop with extreme accuracy. 
On the west side of the river, this rock first makes its appearance in 
the southeast Conner of Caledonia, near the town line. At this place 
a very extensive quarry has been opened, on the west bank of a small 
stream, on land of Mr. Christie. From this place large quantities of 
stone have been taken for the construction of locks, aqueducts &c. for 
which purpose no better stone can be found. It is entirely free from 
seams, and is easily quarried and dressed. The whole thickness ex- 
posed does not exceed ten feet ; the courses varying from one to two 
and a half feet. None of the layers are continuous of the same thick- 
ness, sometimes a thick one thins out entirely and its place is taken by 
two thin ones, or a thin layer in one place becomes a thick one at a 
few rods distance. Sometimes the courses are separated by a thin ir- 
regular course of hornstone, at others this hornstone is in the centre or 
near the surface of a layer of limestone. The presence of hornstone 
in the limestone injures the mass for working, and no. blocks are dress- 
ed which contain large nodules of hornstone. In some parts of the quar- 
ry, fossils are abundant, while in other parts very few are lound ; indi- 
viduals of the Cyathophylli are often found of very large size. 
