30 R. W. ELLS ON THE POTSDAM FORMATION, ETC. 



been carefully studied bj' Prof. Walcott, who after a long search found Primordial fossils in 

 certain strata of this series. It is probable, therefore, that this group of strata represents in 

 Canada, in part at least, the extension northward ot the old groups of Vermont and N'evv 

 York, known as the Georgia slate, the granular quartz rock and the red sandrock. They 

 are however, entirely distinct from the typical Potsdam formation which we have been 

 describing and are not likely to be confounded with it in any sense whatever. 



The circumstances of deposition must also have been widely dilFerent during the laying 

 down of the Potsdam sandstone and Calciferous rocks from those which prevailed during the 

 deposition of the slates and associated strata east of the St. Lawrence, inasmuch as in the 

 former area we find no trace of Primordial remains while in the eastern area thousands of feet 

 pertaining to the Cambrian system have been recognized. It would seem that the Laurentian 

 area west of the St. Lawrence was permanently above the water during the whole of the 

 Cambrian time, and that at its close, a gradual subsidence took place during which the 

 sandy beds of the Potsdam were deposited, the submergence continuing gradually and 

 quietly throughout the Ottawa and St. Lawrence basin to the close of the Cambro-Silurian 

 period. 



It would apj)ear, therefore, from all the evidence at our disposal, that the real line of 

 division between the Cambrian and Cambro-Silurian systems should be placed at the close 

 of the Georgia slate and red sandrock divisions, and that the series from the base of the 

 typical Potsdam sandstone to the summit of the Utica and Hudson River formations should 

 constitute the system known as the Cambro-Silurian or Ordovician, in view of the fact that 

 there is no stratigraphical break in the sequence of these formations nor any want of 

 harmony in the succession of organic life as furnished by the evidence of the contained 

 fossils. 



