MINERALS AND GEOLOGY OF CANADA. 187 
argillaceous sandstones, occur in Thunder Bay, and especially near 
the Grand Falls of the Kaministiquia River, and probably belong to 
the Potsdam period. They overlie the Huronian rocks in uncon- 
formable stratification with these, and hence belong to a succeeding 
geological epoch. If of Potsdam age, the question again arises as 
to whether they represent.a distinct series, older than the sandstone 
beds of the east, or whether they are to be considered of the same 
period of deposition. If older, they might be arranged as in the 
above table, under the name of the Kaministiquia formation. They 
are more or less altered by metamorphic action, and contain native 
copper, iron pyrites, and other metallic matters. 
As the sandstones or shore-line deposits of the Potsdam Group 
form the most characteristic and widely-spread rocks of the period, 
as exhibited at least in Canada, it is necessary to refer to them in 
somewhat greater detail. In the table given above, they are desig- 
nated as the Beauharnois Formation, from their especial development 
in the county of that name. They consist essentially of beds of 
sandstone of various colours, but chiefly white, green, red, brown, 
or yellowish; and of conglomerates of different degrees of coarse- 
ness. Many of the sandstones are fine-grained and of a purely 
silicious character, and some exhibit bands or stripes of different 
colours. With these beds, a few layers of dolomite or of more or 
less impure limestone are occasionally interstratified. Fossils, with 
the exception of fucoids, are of rare occurrence. In addition to the 
problematical Scolithus (see Pant IV., page 97),* the most common 
is a species of lingula (Z. acuminata, fig. 155), a genus which thus 
occurs in the very lowest of our fossiliferous rocks, 
and which, passing npwards through the entire 
series of geological formations, is still found in the 
seas of the existing age. Some remarkable fossil 
tracks occur also in our Potsdam beds. These be- 
nee long to two distinct types or genera. The oldest, 
in point of discovery, were first made known by the 
late Mr. Abraham, of Monireal, in 1847. They were observed on 
the surface of a sandstone bed on the St. Louis River, m the County 
of Beauharnois, and were cousidered to be the tracks of a tortoise 
or some related chelonian. The examination of other examples, 
*The Scolithus cavities figured on this page appear to differ from the common Canadian 
forms i being longer aud wore regula: ly cylind:ical. The Canadian type is named S. Cana- 
densis by Mr. Billings. (See Revised Report on the Geology of Canada.) p. 101, 
