32 CONNECTICUT RIVER SANDSTONE. 



Falls; and, receiving the turbulent floods of the adjacent regions, its ordinary 

 level was thereby disturbed, while at the same time it received fresh depositions 

 of pebbles, sand, and mud, which, upon the subsidence of the agitated waters, 

 were in turn subjected to the action of solar heat, and became the common ren- 

 dezvous of multitudes of birds and reptiles. 



Classification of the Footprints. — There are numerous difficulties in the way of 

 satisfactory classification of the footprints, owing partly to the unstable condition 

 of the rock at the period of the animal's passage over it, and partly to the 

 presumed fact that footprints of dissimilar magnitude were impressed by identical 

 species during the progressive stages of their growth, and also to the circum- 

 stance that many of them present no analogies to the feet of living animals. 



As regards the first of these conditions, there are numerous modifications of 

 the footprints, as has already been remarked, that result from the unequal resist- 

 ance oifered by the strata when in a wet condition, or when not sufficiently 

 consolidated to retain the form of the foot. A very large proportion of the 

 footprints have, for this reason, undergone changes that have destroyed their 

 symmetry and structural markings. The modified forms under which the foot- 

 prints occur are innumerable; and the attempt to reduce such of them as do 

 not present the unequivocal impress of the foot to specific arrangement would 

 be impracticable and useless. When, however, it may be briefly repeated, the 

 stratum was sufficiently consolidated to resist the penetration of the foot, yet 

 yielded slightly to its pressure, a well defined impress resulted, that distinctly 

 retained the rounded forms of the digital articulations, and such only can be 

 subjected to systematic analysis. In well defined examples of footprints, their 

 differences and then individuality may, to a certain extent, be recognized ; and 

 these distinctions offer the only ground of classification. 



A general resemblance of the footprints not differing essentially except in 

 magnitude, offers a perplexing obstacle to classification. This prominent feature 

 of the footprints can only be explained upon the presumption that the various 

 impressions were due to kindred species. This is illustrated by Plate 2, the 

 figures of which are the footprints of the common hen and chickens. If the 

 same disproportion were found to characterize the fossil footprints, it might be 

 extremely difficult to say whether they were specifically distinct, or were due 

 to the same species. 



