C. H. Hitchcock on Impressions on Clay. 395 
upon the clay, illustrate the Herpystezoum Marshii and H. mini- 
mum of the-fossil foot-marks. No difference in character be- 
tween the Triassic and alluvial specimens has been noticed, 
The trails of Aunelids have been found upon stone. Refer- 
erence is made to those upon the Clinton gronp of the New York 
Silurian rocks, described in the 2d volume of the Paleontology 
of New York, page 30, 31, figures 13 and 14. Some differences 
exist between the impressions occurring in the different forma- 
tions. But much of the diversity may be referred to the disparity 
of size in the animals, and the partial filling up of the fossil trails 
with sand. 
These tracks afford a presumption respecting the time of man’s 
appearance upon the globe. Among these impressions in Hadley 
as already mentioned, are those of man. Now, if he had lived 
) when ichnolites were formed, it is probable that he would have 
left similar traces of his existence on stone: but in the whole 
/ Series of rocks below the alluvial, no tracks made by human feet 
| ve been discovered. 
The phenomenon of raindrops on stone receives a beautiful illus- 
tration at the Hadley locality. These are preserved most perfectly 
when it barely sprinkles. In a heavy or long continued shower, 
‘ So many impressions are made, that they coalesce, and leave no 
' distinet trace of their separate existence. The surface in this case 
tesembles a chopped sea. Specimens of raindrop impressions de- 
noting every variety of shower, have been found apon stone and 
Upon this alluvial clay. In no particular are they dissimilar. On 
page 502 of the Final Report on Geol, Mass., It is stated that 
some specimens of raindrop impressions dehoted the direction of 
| 
nat of ripple marks. 7 are formed by the action of waves 
Upon the bottom, arranging small quantities of sand and clay in 
ontinuou ridges, Specimens of ripple marks upon stone and 
