102 PHILOSOPHICAL SOCIETY OF WASHINGTON. 
There is still another group, the St. Johus or Acadian, that ‘occu- 
pies an horizon below the Georgian and may fill in a portion of the 
period of erosion between the pre-Potsdam and Keweenawan and 
the Tonto and Grand Caiion series, or it may represent some of the 
upper portions of the Grand Cafion and Keweenawan series. In 
the geologic sections it is placed beneath the Georgian and as above 
or passing down into the lower groups. For the present both it 
and the Paradoxidian argillites of Braintree must be left in doubt 
with regard to their relations to the lower Cambrian of Wisconsin 
and northern Arizona. 
Of the Canadian survey sections, the one on the north side of the 
Straits of Belle Isle is most interesting as it gives the Georgian 
horizon, but unfortunately an interval of ten miles in width is occu- 
pied by the straits before the section is again continued. In this 
interval the Potsdam group is lost, but farther along the coast 
there occurs, below limestones referred to the Calciferous horizon, 
a mass of sandstone that may be assigned to the Potsdam forma- 
tion—giving, in connection with the Olenellus or Georgian horizon, 
a section not unlike that of Central Nevada. 
No other section that has been determined in the British Provinces 
throws much light on the stratigraphic succession of the Cambrian 
rocks. At Point Levis a curious mingling of the Cambrian and 
Silurian faunas has been said to occur, but this is rather to be at- 
tributed to error in the interpretation of the stratigraphy in a much 
disturbed area than to a break in the sequence of organic remains, 
elsewhere so uniform. I prefer to accept the interpretation given 
by M. Jules Marcou, who says (The Taconic and Lower Silurian 
Rocks of Vermont and Canada, Proc. Bos. Soc. Nat. Hist., Vol. 
VIII, p. 252, 1862,) that the primordial or Cambrian types are 
associated together and occur in a belt of limestone that contains 
no traces of the second or Silurian fauna. 
The accompanying table of sections gives a general outline of the 
Cambrian. Numerous local sections of the Potsdam series are not 
mentioned, as they do not add materially to the general informa- 
tion in regard to the system in its vertical range. 
The geographic range is great, extending as it does from New- 
foundland to Montana on the northern line, and thence south to 
Nevada, Texas, and Alabama. 
