28 ICHNOGRAPHS OF THE 



the Plates have been selected from this prolific field. The region of Turner's 

 Falls, constituting the northern limit of the sandstone beds, appears to have been 

 a common rendezvous of these ancient animals ; while the retentive properties of 

 the strata were singularly favorable to the preservation of their footprints. So 

 numerous were the creatures that resorted to this attractive region, that the 

 strata are often literally covered with their footsteps. When, however, they are 

 thus associated in numbers, it is manifest that they were not all simultaneously 

 impressed. Some of them are deep and imperfect; others are superficial; and 

 when the materials of the strata became, by exposure, too dense to receive 

 articular forms, all that resulted from the transits of the birds is the indentation 

 of their blunt claws. 



The stratified beds of sandstone were evidently deposited upon the rim or 

 marginal limits of the ancient basin or receptacle of water, the materials being 

 transported to the place of deposition by the agency of powerful floods or streams. 

 The strata were of course deposited in a slightly inclined position, and by per- 

 petual succession acquired a depth of many thousand feet, and throughout, foot- 

 prints occur at irregular intervals, even at the bottom of the sands. At the 

 commencement of the sandstone deposition the animals whose remarkable history 

 it commemorates were already in existence, and they flourished, without inter- 

 mission, to its completion. 



It is usual to find several contiguous fossil-bearing strata that are separated 

 by wide intervals of a coarser description of material and stratification, that 

 contain no evidences of organic life. The proportion of the latter to the former 

 is very insignificant; and hence the detection of the footprints is uncertain, and 

 depends as much upon chance as upon exploring discernment. Their search, 

 however, is much facilitated by negative considerations : it is not difficult to 

 determine the lithological conditions under which they do not occur. When 

 the rock is coarse or granular or conglomerated, and is destitute of that delicate 

 glazed deposit that results from earthy precipitation, it is in vain to search 

 for footprints; but when the stratification approaches the condition of shales, and 

 their surfaces are smooth and bright, it may be inferred that footprints exist. 

 The fossil strata are comparatively thin, being at most a few inches only in 

 thickness. 



The elevation of the sandstone beds has been accomplished b}' an irruption of 

 basaltic rock in a melted condition; and in many places at the outbreak of the 



