THE ORIGIN OF PLANT LIFE ON THE LAND. 91 



devoted many days of fruitless labour to the examination of the 

 large areas of Potsdam sandstone exposed in some parts of 

 Canada. But as these rocks \, .re evidently formed along the 

 borders of a Laurentian continent capable of supporting 

 vegetation, we may still hope for some discovery of this kind, 



Fig. Zi.—Protannularia //arinessit (Nicholson). A Lower Silurian Plant, from the 



Skiddaw series. 



more especially if we could find the point where some fresh- 

 water stream ran into the Cambrian sea. 



The oldest plants, probably higher than Algae, known to me 

 by their external forms, are those described by Nicholson 1 from 

 the Lower Silurian Skiddaw slates of the north of England 

 ^ Geological Magazine, November, 1869. 



