280 On the Track of an Animal lately found 



be distinguished. In addition to these upward affinities in the 

 only preserved forms, there are beds of passage between the Pots- 

 dam and Calciferous formations, in which the strongly marked 

 distinctive lithological characters of the two are well preserved, 

 and at St. Timothy on the Beauharnois Canal those beds of the 

 inter-stratification which are allied to the lower rock are occasion- 

 ally marked by Scolithus lineai'is (Hall), supposed to be ancient 

 worm-holes, by which the Potsdam is characterised in many parts. 



Immediately beneath these beds of passage are the celebrated 

 foot prints of Beauharnois, to which Professor Owen has given 

 the name of Protichnites. Since these were described by Owen, 

 nothing has been discovered to throw farther light upon the 

 forms of the animals which made these impressions ; but in thin- 

 ning a large specimen with some of the tracks on it, for the purpose 

 of placing it in the museum of the Geological Surv^ey, it was 

 ascertained that the surface on which the traces were impressed 

 must have been subject to the ebb and flow of a tide. The 

 surface on which the tracks are impressed and the one immedi- 

 ately beneath, shew ripple-mark ; the next in succession which is 

 about an eighth of an inch below, shews wind-mark, in a number 

 of sharp and straight parallel ridges from two to four inches long 

 and an eighth or a quarter of an inch wide. These characterize a 

 considerable surface, and are precisely similar to the marks so famil- 

 iar to every person who has examined blown sand. The surface 

 must thus have been alternately wet and dry, and the organic 

 remains of the formation being marine, we have thus pretty clear 

 evidence of a tide. 



Pi'overbially unstable as water is, the mean level of the sea, 

 that is the point which is half-way between high and low water, 

 is supposed to be the least changeable level on the face of the 

 globe, and taking it to be now pretty much as it was during the 

 Lower Silurian period, we establish the means of knowing approx- 

 imately how much the position where the tracks are found, is 

 higher than it was when these were impressed, the limit of 

 error being the number of feet which would represent the diflfer- 

 ence between the ebb and flow of the sea in the locality, or per- 

 haps not more than fifty feet. "We have thus a bench-mark to test 

 the rise not only of these strata at Beauharnois, but of their equi- 

 valents, wherever else they may be met with. 



Finding that this ancient sand bank was exposed at the ebb of 

 tide we naturally look out for some coast to which it was related. 



