GEOLOGY. 315 



These it was suggested os possible, may have originated from the edge or lip 

 of the shell of the crawling mollusk, grooving the softened mud. 



After describing, in a general way, the reptilian footprints in the carbon- 

 iferous red sandstone of Pennsylvania, assigning the positions which they 

 occupy in the formation, Professor Rogers proceeded to offer some reflections, 

 showing the bearings of our present knowledge of the footmarks in the ancient 

 strata generally, on certain cardinal doctrines of geology, especially on the 

 theory of a progressive development in the extinct inhabitants of the earth. 

 He called attention to the fact that, associated with the earlier bird tracts, 

 there are none ascribable to quadrupeds, or any mammalian creatures, while 

 in company with those of the earlier reptiles, occur none attributable to birds. 

 The first birds seem to have appeared about the close of the Triassic or dawn 

 of the great Oolitic period of the middle secondary ages, and no mammalian 

 animals have left either their prints or their skeletons in strata of a date so 

 old ; while of neither bird nor mammal is there print or vestige of any kind 

 in the still more ancient deposits of the carboniferous and yet earlier rocks, in 

 which the tracks and bones of reptiles and fishes, and the trails and shells of 

 mollusks are of frequent occurrence. Such successive disappearance of the 

 traces of the higher forms of life with the advance of geological time, is one 

 of the clearest proofs we can have of a progressive elevation in the scale of 

 structure of the races successively created. The law of successive appearance 

 of the footprints, is quite as conclusive as that of the parallel introduction of 

 the actual skeletons and remains of the creatures themselves ; it is perhaps 

 of even more weight, as precluding all discussion upon the differences in de- 

 gree of destructibility of the bones or carcasses of the several classes of 

 animals which peopled the ancient world. Some of the bird-tracks and rep- 

 tilian footprints left on the once soft surfaces of the old rocks, are as clear and 

 legible imprints as any impressed yesterday on surfaces of moist mud or 

 sand, by the creatures of corresponding structure. Once buried and sealed 

 up, their preservation has been independent of the lapse of time. This law 

 of a progressive rise in the character of the footprints, like that so generally 

 recognized in regard to the organic remains themselves, distinctly refutes the 

 view urged by Sir Charles Lyell, and some other disciples of the Huttonian 

 theory of the earth's history. Fancying a uniformity in the series of past 

 changes in the animate and inanimate world, they contend that the evidence 

 in support of the theory of progressive development of organic life is incon- 

 clusive, on the ground that the higher forms being inhabitants of the land, we 

 ought not to look for their remains in strata of marine or aqueous origin, 

 but must suppose they were never entombed. But the negative evidence 

 from footprints is of positive force when it appeals to appearances in the im- 

 printed surfaces of the rocks which show that they were exposed in a moist 

 sta.te to the air, above the level of the waters, and in situations as accessible 

 to mammalia as to birds, and in the case of the still earlier formations, as 

 accessible to both mammalia and birds as to the reptiles, which left their 

 tracks in such numbers upon them. 



Professor Rogers also adverted to the superior value of the evidence 

 afforded by such unequivocal shore marks QS those of the footprints, and the 



