THE FOSSIL FOOTMARKS OF THE UNITED STATES. 145 



cease to be fractured, and come out in regular curves, when they 

 are split asunder. As far, indeed, as the foot did sink, we should 

 not expect the rock would cleave in curved layers. But may not 

 the narrow toes have bent down the layers so much, beneath where 

 they reached, that they (i. e. the layers) would meet in an angle 

 at the bottom so acute, that, when the rock was split open, they 

 would break across rather than cleave asunder? In such a case, 

 we should infer by this rule that the animal sank deeper than was 

 the fact. And, indeed, I have sometimes found the print of a lateral 

 toe, for instance, showing a perfectly continuous lamination across its 

 depression, while that of the middle toe, nearly an inch deeper, was 

 fractured. Although, therefore, this principle does help us somewhat 

 in determining the layer on which the animal trod, it cannot be 

 implicitly followed. If possible, we should obtain dissections of the 

 track from top to bottom ; and by combining the impressions on 

 the successive layers, we shall probably get an accurate view of the 

 entire foot. On one layer we may find a mere digitigrade, and on 

 another or higher layer a plantigrade, impression ; on one a heel, 

 or a fourth toe, and on another neither. I think it true in general, 

 however, that the layer on which the animal trod was usually 

 nearer the bottom of the impressions than the top. 



Those who have seen the manner in which successive layers of 

 copper, deposited in the process of electro-metallurgy, retain the 

 slightest markings upon the surface, will readily conceive that fine 

 mud would do the same ; less perfectly, indeed, but still so as 

 to preserve the form of a track through many successive layers. 

 On this ground, they will not be surprised that several layers often 

 present the track with so nearly equal distinctness, that the one 

 originally impressed can no more be distinguished, than the film 

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