MEDINA SANDSTONE. 41 
The above woodcut will give an idea of this appearance; in other instances the lines are 
more curved, though generally when seen on a large scale, they are nearly straight. From 
the interlamination of shaly matter, the strata to which this structure is confined often appear 
in wedge-form layers of greater or less extent, enclosed between parallel lines. In most in¬ 
stances this structure is clearly produced by deposition alone ; but where the lines of lamina¬ 
tion, as indicated by color, are strongly curved as in the wood- 
cut No. 4, it seems impossible that it could have been produced 
by deposition. As this rock is everywhere free from distur¬ 
bances or upliftings, there could have been no lateral move¬ 
ment, except on a small scale, to have produced this effect. 
The infiltration of finer particles, with a greater proportion of 
coloring matter, may perhaps explain the appearance without 
having recourse to any extraneous agency. This structure is 
seen in the more siliceous strata on the Genesee below the 
lower falls, and at Medina. 
In many places where this diagonal lamination is exhibited, there is a tendency to the pro¬ 
duction of conglomerate in the associated strata. Thin layers of pebbles and angular frag¬ 
ments are of frequent occurrence at Rochester, Medina and other places, and associated with 
them and the more sandy portions of the rock, are frequently found fragments of what appear 
to be shells of Lingula. The latter may be seen in loose specimens somewhat weathered, 
below the lower falls of the Genesee, at Rochester. 
In specimens which I have more recently collected, the fragments of shells appear as if 
drifted by waves and spread evenly over the surface, precisely in the same manner as com¬ 
minuted shells upon the shore of the ocean. In instances of this kind, after walking for a 
mile upon a sandy beach, without seeing anything other than a stranded Fucus, I have sud¬ 
denly come upon a space several feet in extent, which, apparently from a slight eddy, or 
inequality in the surface, was covered with comminuted shells and fragments of coral. The 
analogy to some of our sandstones where organic remains are rare, is thus most strikingly 
exhibited. 
Accretions .—In the Second division of this rock, at Lewiston, and other places, we notice 
upon the surface small points and blotches of fine black sand, scarcely cohering, and which 
crumbles on the touch. These in many places are larger; and at Lockport, in the thin-bedded 
sandstone, they are frequently several inches in diameter. All have the same incoherent 
structure, appearing like a mass of black or dark colored sand, around which the strata have 
been deposited. From the manner of their occurrence, and from the stratum frequently being 
a little elevated just around them, it is evident that they were formed during the deposition of 
the mass. Their limits are well defined, and they separate from the stratum, frequently leav¬ 
ing a circular hole in the quarried slab. A specimen of this kind may be seen in the State 
Collection. In a few instances I have observed these masses of black sand surrounded by a 
firm coating or shell of sandstone an inch thick, like the ordinary rock, but separating from 
the stratum, and presenting a smooth spheroid. This peculiar form of accretion I have seen 
Geol. 4th Dist. 6 
