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. 



