ICHNOLOGY. ] 79 



footprints of birds surpass all others in regard to their num- 

 ber, distinctness, and variety of sorts. 



But how, it may be asked, are such footprints preserved ? 

 A common mode may be witnessed daily on those shores 

 where the tide runs high, and the sea-bottom is well-adapted 

 to receive and retain the impressions made upon it at low- 

 water. 



Dr. Gould of Boston, U. S., first called the attention of natu- 

 ralists to this interesting operation on the shores of the Bay of 

 Fundy, where the tide is said to rise in some places seventy 

 feet in height. The particles deposited by that immense tidal 

 wave are derived from the destruction of previously existing 

 rocks, and consist of silicious (flinty) and micaceous (talcky) 

 particles, cemented together by calcareous (limy) or argillaceous 

 (clayey) paste, containing salts of soda, especially the muriate 

 (common salt), and coloured with various shades of the oxide or 

 rust of iron, of which the red oxide predominates. The perfection 

 of the surface for receiving and retaining an impression depends 

 much upon the micaceous element. Vast are the numbers of 

 wading and sea birds that course to and fro over the extensive 

 tract of plastic red surface left dry by the far retreat of the 

 tide in the Bay of Fundy. During the period that elapses 

 between one spring tide and the next, the highest part of the 

 tidal deposit is exposed long enough to receive and retain many 

 impressions ; even during the hours of hot sunshine, to which, 

 in the summer months, this so-trodden tract is left exposed, 

 the layer last deposited becomes baked hard and dry, and 

 before the returning tidal wave, turbid with the same commi- 

 nuted materials of a second stratum, has power to break up 

 the preceding one, the impressions left on that stratum have 

 received the deposit. A cast is thus taken of the mould pre- 

 viously made, and the sediment superimposed by each suc- 

 ceeding tide, tends more and more surely to fix it in its place. 

 Then, let ages pass away, and the petrifying influences consc- 



