State Museum of Natural History. 3 J 



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

 sionally marked by Scolithus linearis (Hall), supposed to be ancient 

 worm-holes, by which the Potsdam is characterized^ 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 further light upon the forms of the animals 

 which made these impressions; but in thinning a large specimen with 

 some of the tracks on it, for the purpose of placing it in the museum 

 of the Geological Survey, it was ascertained that the surface on which 

 the traces were impressed must have been subject to the ebb and flow 

 of tide. The surface on which the tracks are impressed and the one 

 immediately beneath, show ripple marks; the next in succession, 

 which is about an eighth of an inch below, shows wind marks 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 

 familiar to every person who has examined blow 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. 



" Proverbially 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 approximately 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 difference between the ebb and flow of the sea in the 

 locality, or perhaps 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 equivalents, 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. The 

 Potsdam sandstone terminates some twenty miles to the north at a 

 very low angle against the foot of the Laurentian hills, which rapidly 

 rise up 500 or 600 feet above the Silurian plain. There is little doubt 

 that we have in the flank of those hills the ancient limit of the Lower 

 Silurian sea, the shore of which is thus traceable from Labrador by 

 the northwest, to the Arctic Ocean, a distance of 3,000 miles. But, 

 though we have thus evidence of a Lower Silurian dry land and can 

 scarcely suppose that it was wholly destitute of vegetation, we have 



