146 AN ATTEMPT TO DESCRIBE THE ANIMALS THAT MADE 



of copper that was first deposited can be from those superimposed 

 afterwards. 



The oblique direction in which the impressions often pass 

 through successive layers, while their distinctness is not impaired, 

 is a matter of surprise, and not so easily explained. Sometimes 

 the track seems to advance, and sometimes to recede, and some- 

 times to move laterally on the successive layers, taking the lowest 

 one as the fixed basis. This might proceed in part from the ob- 

 lique direction in which the foot of the animal was exerted ; as 

 when running, for instance, the impression would be made so as 

 to reach the successive layers farther and farther backward, be- 

 cause the legs incline forward; or suppose the surface to be in- 

 clined, and the animal going directly or obliquely up or down upon 

 it. It is clear that the impression, in such case, would be com- 

 municated to the successive layers obliquely to the surface, so as 

 to produce the phenomena which we actually observe. Again, if 

 the tracks be made beneath the water, on light, loamy mud, it is 

 easy to see that waves or currents might produce slight movements 

 in the successive deposits, without destroying the impressions. Or 

 if the surface were slightly inclined, gravity would produce the 

 same effect on such mobile materials. 



In general, we find but little difference in the size of the tracks 

 on successive layers ; yet, upon the whole, the tendency is rather 

 to enlarge downwards. Decidedly the most striking example of 

 this which I have noticed is represented in Plate 17, figs. 3 and 

 4, which are one half the natural size. Fig. 3 shows a track of 

 Ornithopus gallinaceus, or of Tricenopus Emmonsianus, I am not 

 certain which, on an upper surface ; fig. 4 shows the same, as it 

 appears in relief, only one inch lower. The latter is the most dis- 



