S04 [Assembly 
the best characterized and most extensive formations in the State. The 
lower portions of this series are indurated marls, sometimes slightly sili- 
ceous in the lowest strata, but becoming almost entirely argillaceous, 
containing seams of fibrous gypsum and small masses or nodules of se- 
lenite and anhydrous gypsum. Above this we find a considerable thick- 
ness of a tough indurated marl, of a green or bluish green colour, with 
seams of flesh coloured gypsum. This rock is exceedingly difficult to 
remove, even by blasting, but after a few days, or even a few hours ex- 
posure, it crumbles into fine particles, and finally dissolves into a clayey 
mass. 
The succeeding strata are more calcareous, and in some places become 
a tolerably pure limestone, of a slaty structure. The green marl, like 
that below, often alternates with these strata. Towards the upper part 
of the series, we find frequent alternations of the green or bluish green 
marl, and a compact earthy limestone. Thin strata of vesicular lime- 
stone are observed toward the top of the series. 
These rocks embrace all the gypsum of importance in the 4th Dis- 
trict, and may with propriety be called the gypseous rocks, as they 
are not likely to be confounded with any other formation. These 
rocks, west of the Genesee river, are all bituminous, as is the gyp- 
sum in many places. The gypsum in the lower part of the forma- 
tion, is limited to thin seams or nodules, but after the commencement 
of the slaty limestones, it is found in large masses, or beds, of a few 
feet thickness, and limited extent. These masses are of a flattened 
spherical form, and are from a few pounds to fifty tons weight, com- 
monly from five to twenty tons. The existence of these beds of gyp- 
sum is indicated by appearances of the surface, which above the mass 
is raised into a knoll or hillock. Sometimes the surface, to a conside- 
rable extent, is covered with these hillocks, which resemble small mounds 
of earth. When the soil is removed from one of these elevations, a 
corresponding convex surface is presented by the limestone beneath, 
which is cracked and broken in every direction, as if by some elevating 
force. On removing the stone, further evidence of such force is per- 
ceived. The layers of limestone dip upon every side of the mass of 
gypsum, and as this mass thins off at its edges, the strata of limestone 
meet those below and assume their original, nearly horizontal position. 
From such appearances we cannot doubt but the gypsum has been form- 
ed since the surrounding rock become indurated; and that the produc- 
tion of these masses has disturbed the overlying strata. The decompo- 
sition of large quantities of iron pyrites might, perhaps, explain the for- 
