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Trenton (Sponge beds (Ottawa). 
|Cystid beds (Ottawa). 
| Prasopora beds. 
| Dalmanella beds. 
|Trinucleus beds (Montreal). 
Parostrophia beds (Montreal). 
| 
Black River (Black River. 
{ Lowville. 
| Pamelia. 
Upper Chazy Aylmer. 
Beekmantown { Beauharnois. 
\ Theresa (Ottawa). 
UPPER CAMBRIAN. 
Potsdam Potsdam. 
DESCRIPTION OF FORMATIONS. 
POTSDAM. 
The Potsdam sandstone is a very hard, white to 
yellow sandstone, made up largely of quartz sand, cemented 
by silica. It contains few fossils in Canada, Lingulepis 
acuminata being the only common one. Large and well 
marked trails, supposed to have been made by huge mol- 
lusks, are not uncommon. They were first discovered and 
are still to be seen at Beauharnois, not far from Montreal. 
The Potsdam sandstone is «mployed with good effect as a 
building stone, and examples of its use may be seen in 
the Government buildings at Ottawa. 
BEEKMANTOWN GROUP. 
Theresa.—In some parts of the Ottawa valley the 
oldest beds of the Beekmantown are composed of reworked 
Potsdam sand, with a calcareous cement. These beds are 
therefore softer and weaker than the Potsdam _ beds. 
They contain fossils at several localities, the more common 
of which are Ophileta complanata and Pleurotomaria 
canadensis. The upper part of the Theresa is a thin- 
bedded, gray dolomite, with the same fossils. The 
Theresa is absent from the section at Montreal, and is 
thin near Ottawa, but thickens southward, toward Smiths 
Falls and Brockville. 
