ICHNOLOGY 155 



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 conso- 

 lidate the sand layers into a fissile rock : it will split in the 

 way it was formed, and the cleavage will expose the old moulds 

 on one surface and the casts on the other. 



Another condition for fixing the impressions on a sandy 

 shore is the following : — When an extensive level tract is 

 left dry by the retreating tide, as at the estuary of the small 

 rivers entering the Bay of Morecombe, on the Lancashire 

 coast, those rivers occasionally overflow the sands at low-water, 

 and deposit in the footprints made previous to such overflow 

 the fine mud which sudden heavy rains have brought down 

 from the surrounding hills. Again, those sudden " freshets," 

 as they are locally called, sometimes as quickly subside, and 

 the thin layer of argillaceous mud is left dry on the sand 

 before the returning tide. Such layer of mud readily receives 

 and retains the footprints of the many birds that course over 

 the fiat expanse ; and as the tide returns, it deposits in such 

 footprints a layer of the fine sand which the rising waters 

 hold in suspension. 



