The vast number and variety of such impressions, due either to physical or meteoric forces, to dead organic bodies, parts or products, or to the transitory actions of living beings, have at length raised up a distinct branch of palmonto logical research, to which the term "Ichnology" has been given.
In this class of evidences the impressions called " protich nites"* (fig. 64), left upon the " Potsdam sandstones"t of the older Silurian age in Canada, are the most ancient ; but the 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 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 flat expanse ; and as the tide returns, it deposits in such footprints a layer of the fine sand which the rising waters hold in suspension.
The best-defined footprints in the new red sandstone quarries at Stourton, on the Cheshire coast, are found where strata of sandstone are separated by a thin layer of argil laceous stone, which, when exposed, soon breaks up and crumbles away. This layer has, however, received the im pressions when it was plastic, and the superincumbent deposit of sandstone retains those impressions in relief upon its under surface. The observations which have just been recorded, of the circumstances that produce an interposition of a thin layer of claystone between thicker beds of sandstone, and which circumstances the writer has witnessed in the Bay of More combe, explain the formation and the preservation of the best " ichnites" of the labyrinthodont and other reptiles in the new red sandstone of Stourton.