layed, and even the articulations. Moreover,
one of the tracks on the inferior surface interferes with the outer
track in the superior, and tends in an opposite direction, so that
this last-described footstep must have been made before the other. It
is also observable, that, while all the other tracks are superficial,
this last penetrates the whole thickness of the slab; thus showing
that the different deposits continued some time in a soft state.

On the surfaces of this slab, particularly on the upper, there are
various marks besides those of the feet, some of which seem to have
been made by straws, or portions of grass, or sticks; and there is a
curved line some inches in length, which seems to have arisen from
shrinkage.

In the collection of Mr. Marsh,[B] there were two slabs of great size,
each measuring ten by six feet, having a great number of impressions
of feet, and about the same thickness as the slab under examination.
One of these presented depressions; and the other, corresponding
reliefs. These very interesting relations were necessarily parted in
the sale of Mr. Marsh's collection; one of them being obtained for the
Boston Society of Natural History, and the other for the collection of
Amherst College.

[Footnote B: Mr. Marsh was a mechanic of the town of Greenfield,
and procured his subsistence by his daily labor. Being employed by
Dr. Deane in obtaining the sandstone slabs of Ornithichnites, he
acquired a taste for the pursuit, entered into it with
extraordinary ardor, and accumulated by his own labors a great
collection of fine specimens. He unfortunately fell into a
consumption, and died in 1852. The collection was sold at public
auction for a sum between two and three thousand dollars. The
specimens were purchased by the Boston Society of Natural History,
by Amherst College, and by varioud colleges and scientific
associations in this country.]

The _Physical Impressions_, according to Professor D'Orbigny, are
of three kinds, viz.: 1st, Rain-drops; 2d, Ripple-marks; and 3d,
Coprolites. I have a slab which exhibits two leptodactylous
tracks very distinct, about an inch and a half long, surrounded
by impressions of rain-drops and ripple-marks. Another specimen
exhibits the impressions of rain in a more distinct and remarkable
manner. The imprints are of various sizes, from those which might
be made by a common pea to others four times its diameter; some
are deep, others supe