1877.] Concerning Foot-Prints. 407 
The same principles of comparative anatomy that enabled Cuvier 
to reconstruct the skeletons of Tertiary mammals, a few bones of 
which were discovered near Paris, also give the ichnologist the 
power of calling again into being the forms of the animals which 
in times long passed impressed their foot-prints on the sand. 
Let us see, first of all, how the records of these ancient foot- 
_ steps have been preserved for indefinite ages, so as to appear as 
fresh and well defined as if made but yesterday. It is evident 
that if a track is left in the loose dry sand, it is poorly defined 
and soon becomes obliterated ; but if impressed on the wet sand 
at low tide, or on mud of the proper consistence, it may retain 
its form for a considerable time. The first and most common 
means by which such foot-prints are indefinitely preserved is by 
the rising tide filling and covering the impressions with the mud 
and sand borne on by the advancing waves. Each tide by de- 
positing a sheet of sediment over the trodden surface would not 
only tend to bury the foot-prints deeper and deeper and thus se- 
cure their preservation, but the new deposit thus spread out by 
the waters might receive a series of records in its turn,. made by 
the feet of the birds and reptiles that walked over it, and by the 
drops of rain that pattered down on the plastic surface, or by the 
retreating wavelets that rippled over the soft mud. Such in- 
scriptions when once entered on the day-book of nature are im- 
perishable until the rocky tablets that they form are again 
ground down to sand and dust in the great cycle of changes to 
which they are subjected. 
Such preservation of foot-prints can nowhere be better seen 
than on the shores of the Bay of Fundy, where, owing to the 
great difference between high and low tide, —in some places 
amounting to seventy feet, — a broad extent of smooth, shining 
mud is left exposed at low water. Some portion of this soft sur- 
face is sure to be trodden by the numerous birds that feed along 
the shore, or to have its surface pitted by a passing shower ; often, 
too, the mud is left in regular ripples by the retreating tide, and 
sometimes a leaf is borne out by the wind and dropped on the 
plastic surface, to record the character of the vegetation that 
fringes the shore. The red mud with all these inscriptions upon 
it is somewhat hardened by the warmth of the sun, so that it 
retains its place when the advancing tide rushes in. As the 
waters then sift down the fine mud which they hold in suspen- 
sion, it fills each foot-print and rain-drop impression, and impris- 
ons the leaves that are fast on the bottom; and thus is finished 
