20 
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 exami- 
nation. 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. 
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 impres- 
sions of rain-drops and ripple-marks. Another speci- 
men 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 superficial and almost imperceptible. They are 
generally circular, but some are ovoid. Some have the 
edge equally raised around, as if struck by a perpen- 
dicular drop; and others have the edge on one part 
faintly developed, while another part is very sharp and 
well defined, as if the drop had struck obliquely. It 
has been suggested, that these fossil rain-drops may 
