in the Potsdam Formation, 



281 



The Potsdam sandstone terminates some twenty miles to the north 

 at a very low angle against the foot of the Laurentide 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 north-west, to the Arc'dc 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 not yet discovered 

 any certain drifted vestige of its plants along many hundred miles 

 of its coast. 



Fig. 1, One-thirtieth nat. size. 

 The crustacean which impressed the tracks at Beauharnois must 

 have been a litoral animal, tracks of which have now been found 

 in several places nearer than Beauharnois to the marginal limit of 



