72 
DEANE ON FOSSIL FOOTPRINTS. 
ere it became obliterated or injured, it in turn was overspread and preserved by 
materials deposited by the succeeding overflow of waters. In this way, stratum upon 
stratum was superimposed in an inclined position, each one a vivid register of the 
movements of the animal that trod upon it. The vast thickness of these accumulated 
deposits is amazing. Footprints, however, only occur at occasional intervals in the 
order of deposition. It is usual to find several thin fossil-bearing layers in contact, 
and this species of rock is separated by wide intervals of non-fossiliferous strata, a 
fact that suggests the idea that the visits of the birds were periodical. It is usual to 
see upon the same surface considerable variety in the character of the footprints, but 
being simultaneously impressed it is certain that the birds, although specifically 
distinct, were gregarious, and that their instincts were harmonious. A stratum may 
contain several species of ornithic footprints, one or two species of quadrupedal 
impressions, while its entire surface is completely spotted by the indentations of 
falling rain. 
When a fossil stratum is raised from its bed, its inferior surface is seen to be an 
exact cast of the impressions upon the stratum beneath, which are consequently in 
relief and disclose the precise form of the plantar surface of the foot. It sometimes 
happens that the impress upon the upper stratum is transmitted to several inferior 
layers, often as many as five or six, a phenomenon that has been explained upon the 
supposition that each stratum yielded to the weight of the bird. But the 
disproportion between the size of the footprint and the thickness of the strata forbids 
this solution. Such impressions are always imperfect; an unblemished imprint never 
transmits its form to the layer beneath, unless it be so extremely thin as to bend by 
the weight of the bird. An explanation which is undoubtedly correct has been 
suggested by Mr. Marsh. He thinks it due to the plastic state of the strata at the 
time of the animal’s transit; the foot in sinking perforated the strata, which closed 
again upon the withdrawal of the foot, and thus left rude representations of its 
outline. In such cases there is no evidence of the articulations of the foot, a 
depression representing each toe is all. It is not unusual in this class of impressions 
to see upon the upper layers a depression projecting backwards from the heel, much 
longer than the foot itself, made by the leg in sinking through the unresisting 
medium, which has led to the belief of peculiar species. 
The existence of the ancient birds in respect to size embraced a vast scale. Many 
families were small, and others of stupendous proportions, far exceeding those of 
modern times, and only approximated by those gigantic remains recently disclosed in the 
island of New Zealand. From the character of the fossil footprints, it is inferred that 
the birds belonged to those orders that resort to the shores of accumulated waters 
for subsistence and reproduction. Their feet were massive and were terminated by 
blunt nails; their stride was extensive, and as a whole they were evidently waders, 
