Ornithichnology. 335 



considerable distance, the slighter indentations first disappearing, 

 and finally those that were deepest ; so that, after the mud had been 

 consolidated into stone, several successive layers might be split off, 

 each one containing an Ornithichnite. In the highest layer the track 

 would be smallest, and its more delicate extremities would be want- 

 ing. Each successive *ayer beneath, would exhibit it more and 

 more perfect, until the precise layer was reached, on which the bird 

 originally trod. A few layers beneath this, might exhibit the track 

 imperfectly, but it would soon be lost. Now, by looking back to my 

 description of the actual manner in which the Ornithichnites occur, 

 it will be seen that the facts correspond to these deductions of theory. 



The results above stated, however, would be very much modified 

 by circumstances. The more quietly the deposition took place, after 

 the track had been impressed upon the mud, the longer time would 

 it require, and the greater the number of superimposed layers, be- 

 fore it would be effaced. But if a sudden and more tumultuous rise 

 of waters, either from a land flood, or a violent storm acting on the 

 ocean, should bring a coarser coat of materials over the track, some- 

 what violently, it might be filled up and effaced at once, as the spe- 

 cimens show was sometimes the case. Or should the matter de- 

 posited in the track, assume a concretionary form, so as in fact to 

 become a real petrified foot, the depression in the superimposed lay- 

 ers would almost immediately disappear, as I find to have been the 

 case frequently with O. gigonteus and O. tuberosus. 



There is one fact respecting these foot marks, which deserves to 

 be mentioned, and which is not so easy to explain. Where success- 

 ive layers of the rock are bent downwards by the impression, the 

 curves are sometimes not placed perpendicularly above one another, 

 but they are considerably oblique ; so that when the track is visible 

 on both sides of the specimen, on one side it appears thrown forward, 

 or backward, or laterally, an inch or two. I have noticed as great 

 a difference as this, where the rock is not more than an inch thick. 



I can conceive of only two modes in which such an effect could 

 be produced. It could result, as it seems to me, in no way, from a 

 slide of the animal's foot in the mud. But suppose the impression 

 made in mud, which was so very yielding that a slight action upon it 

 would cause the upper portion of it, almost suspended by the water, 

 to be carried somewhat forward, in the direction in which the disturb- 

 ing force impelled it. Suppose now, either winds or floods should 

 produce a gentle current, where a track had been made in such mud ; 

 might not the impression be gradually slid a little from its original 



