CONNECTICUT RIVER SANDSTONE. 29 



igneous rock, the sedimentary beds are thrown up into hills of considerable ele- 

 vation. In some instances the elevation has been effected without material change 

 of the natural inclination of the strata. The irruption of the basaltic rock is 

 coextensive with the sedimentary formation, commencing with it at its northern 

 terminus, and accompanying it throughout in low, bleak ridges, throwing up, in 

 eastern or upper aspect, the fossil-bearing strata. 



Causes that have prevented the perfect Preservation of the Footprints. — Several circum- 

 stances have interfered to prevent the preservation of the footprints, the most 

 prominent being that which depends upon the variable density of the stratum 

 at the time of the animal's passage over it. If the foot was planted upon the 

 stratum while yet too soft to retain its form, the impress was of necessity obliter- 

 ated, or modified by subsequent changes of the semifluid mud ; and hence the 

 distinctive marks of organization disappear, each toe being simply represented by 

 a linear depression, that has sometimes been mistaken for the impress of a slender 

 toe (PL 20). All tridactylous footprints, or those ascribed to birds, that do not 

 exhibit the phalangeal divisions of the toes, have been modified by disturbing 

 causes. It was not until my attention was directed to the localities at Turner's 

 Falls in 1842, that examples of footprints were procured that presented the 

 phalangeal outlines of the toes. The perfection of these footprints proves une- 

 quivocally that the feet of the animals making them were distinguished relatively 

 by massive toes, a stout nail, and broad heel. This is the invariable character 

 of those impressions that preserve structural markings; and such as do not are 

 not reliable. I have sometimes seen footprints in which the divisional lines of 

 the middle toe were visible while those of the lateral toes were not, the flowing 

 of the plastic mud not being sufficient to obliterate entirely the forms of the 

 articulations. 



In the imperfect impressions represented by Plate 20, the linear impress of 

 the toes is often repeated upon several contiguous strata, — a phenomenon that 

 is explained upon the supposition that the bird, traversing the unconsolidated 

 strata, penetrated with its foot several layers, and upon withdrawing it, left in 

 each, rude representations of the respective toes. These layers, that are always 

 thin, often multiply as many as five or six linear representations of the same 

 foot. Sometimes there is in the upper layers a prolonged depression, running 

 backward from the heel and joined to it, that is due to the leg sinking with 



