1393. Geology and Paleontology. 735 
pre-Cambrian continent. The geographic distribution of pre-Cam- 
brian land is based upon the evidence afforded by the absence of 
Cambrian deposits upon known pre-Cambrian rocks; the existence of 
shore lines during earlier Cambrian time; and the presence of deep- 
water deposits. "The features of the surface of the pre-Cambrian land 
are indieated by the relation of the known Cambrian and post-Cam- 
brian formations where it is exposed. 
Mr. Walcott considers the prevailing view of the geographic distri- 
bution and extent of continental area at the beginning of Paleozoic 
time too restricted. The present Appalachian system was outlined by 
a broad, high range that extended from the present site of Alabama to 
Canada, with subparallel ranges to theeastand northeast. The paleo- 
Adirondacks joined the main portion of the continent, and the strait 
between them and the paleo-Green Mountains opened north into the 
paleo-St. Lawrence Gulf, and to the south extended far along the 
western side of the mountains and the eastern margin of the continental 
mass to the sea that carried the fauna of the Olenellus epoch around 
to the paleo-Rocky Mountain trough. 
It is highly probable that ridges of the Algonkian Continent rose 
above the sea to the east of the present continent. On the east and 
west of the continental area the pre-Cambrian land formed a mountain 
region, and over the interior a plateau existed much as it does to-day. 
In late Middle Cambrian time, the Cambrian Sea began to invade 
the great Interior Continental area and extended far to the north 
toward the close of the period. 
At the close of the Cambrian time the Cambrian Sea had extended 
over the broad interior continent and had submerged the low ground 
along the line of the barrier ridges and some portions of the northern 
nucleal V of the Archean Continent. 
Two hypothetical maps based upon columnar sections, and the pres- 
ent knowledge of the distribution of the sediments, represent the con- 
tinent at the beginning of Lower Cambrian and of Ordovician time. 
These maps in connection with one showing the relative amount of 
sedimentation within the typical provinces of North America during 
Cambrian time, and the theoretic sections across the continent, are 
valuable adjuncts tothe text. (Extr. Twelfth Ann. Rept. Director 
U.S. Geol. Survey, 1890-91). 
Lower Silurian Brachiopoda of Minnesota.—The report of 
Mr. N. H. Winchell and Mr. Charles Schubert on the Brachiopoda 
found in the Lower Silurian deposits of Minnesota comprises descrip- 
