104 
MUSEUM BULLETIN NO. 2. 
If this correlation is correct the fauna of the Flathead would 
appear to be quite closely comparable to that of the shales in the 
Dearborn River section which are also believed to be referable 
to the base of the Middle Cambrian (See the discussion of the 
age of the Albertella fauna, pages 118-120). Differences of meta- 
morphism between the Flathead and the underlying formations 
are marked and the two series are separated by an unconformity 
which brings the Cambrian Flathead into contact not only with 
many of the different units into which the Pre-Cambrian has 
been divided but with the Archaean as well. Here, as in the 
Grand Canyon region of Arizona, the Pre-Cambrian age of the 
lower series would seem to be demonstrable, though the overlying 
rocks can not be proven to have been deposited earlier than the 
Middle Cambrian. 
Montana, Dearborn River Section. — An unfossiliferous sandstone 
150 feet thick separates a superjacent shale carrying an assem- 
blage of fossils to which the name Albertella fauna has been given, 
from underlying shales having apparently the same strike and 
dip as the base of the sandstone, but which appear, when traced 
on the strike, to occupy an unconformable relation. The upper 
shale and sandstone are referred to the Lower Cambrian by 
Walcott, 1 the underlying shales being referred to the Pre-Cam- 
brian. The Pre-Cambrian age of these underlying shales seems 
to be well attested by sections measured by Mr. Walcott at 
many other points in this area of Beltian rocks. The overlying 
shale and sandstone, however, occupy lithologic positions exact- 
ly analogous to those of the Wolsey shale and Flathead sandstone 
in the Little Belt Mountains section, 100 miles to the east. There 
both the Wolsey and the Flathead carry well developed Middle 
Cambrian faunas, and while that of the Wolsey is noticeably 
different from the Albertella fauna found in the Dearborn River 
section, the fossils occurring in the underlying Flathead in the 
Little Belt Mountains are described by Mr. Walcott as com- 
parable with the oldest part of the Middle Cambrian fauna 
(see page 103). This, the Albertella fauna is now believed to 
represent, see pages 116-120. 
1 Smithsonian Misc. Coll., vol. 53, No. 5, 1908, p. 203. 
