334 Ornithichnology. 



tance than their situation, in respect to the formation generally. In 

 point of fact, they occur only a few feet below the immediate sur- 

 face of the rock, where the excavations are made. But they are 

 found on the western margin of a formation some miles in extent, 

 reckoning across the strata ; and those strata dip to the east several 

 degrees ; so that in fact, all those strata whose edges crop out to the 

 east of the quarries containing the tracks, were deposited above the 

 Ornithichnites, making a perpendicular thickness of rock of several 

 hundred feet, over these relics, instead of six or eight feet. Indeed, 

 at the locality in the south west part of Montague, the layers con- 

 taining the Ornithichnites pass laterally under Mount Toby, which 

 rises six or seven hundred feet above the spot, so that it is perfectly 

 fair to say, that these foot marks are found several hundred feet deep 

 in the rock. But this statement, although adapted to make a popu- 

 lar impression, is by no means as striking to the geologist, as the fact 

 that they occur in the new red sandstone at all ; for he knows, that 

 since the deposition of that rock, there has been time enough for the 

 formation of those vast masses of rock, constituting the oolitic, cre- 

 taceous, and tertiary groups, each of them many thousand feet in 

 thickness, and formed by slow processes ; and the only reason that 

 they are not piled immediately above the Ornithichnites is, that the 

 causes, by which those particular sorts of rock have been formed, 

 have not here operated. In other words, after the new red sand- 

 stone was deposited, no new rocks were added, in this part of the 

 world, during the immense periods in which the groups above named 

 were in the process of formation in Europe. 



Admitting that these tracks were originally produced by birds, 

 travelling upon mud, let us enquire in what manner the process of 

 covering them up, and of their consolidation, would take place. Al- 

 luvial deposits, it is well known, are arranged in layers, brought on 

 by the successive charges of mud and sand, diffused in the waters ; 

 and these will be finer or coarser, according to circumstances. If a 

 bird be quite heavy, its foot would sink considerably deep into these 

 layers, either breaking through them, or, if plastic, causing several 

 of them to bend downwards. Yet, I apprehend, that the lighter 

 birds would rarely make any such indentation, that would sensibly 

 affect the layers of mud more than an inch deep. But as success- 

 ive layers of mud were deposited, after the impression had been 

 made, if the movement of the water were very slight, they would 

 be scarcely thicker where the track existed, than in other places ; 

 and consequently, the impression would be continued upward for a 



