ORIGIN OF PETROLEUM. 
63 
of the Cambrian, Silurian, Devonian, and Carbonifer¬ 
ous periods. It was during these periods that most 
of the dry land was formed. At the close of the 
Archaean age there was a V-shaped continent about 
Hudson Bay, and small islands in the Appalachian 
ridge and in the Bocky Mountain region. Although 
the number of square miles of land was not large 
then, it was enough to fairly outline the great conti¬ 
nent to be. At the close of the ancient-life era the 
American continent was practically complete. Only 
small strips of land have since been wrested from the 
sea along the ocean borders and in the Mississippi and 
Missouri valleys. The land was low and the seas 
wide and shallow; hut a great change marks the 
close of the Carboniferous age. The Appalachian 
Mountains rose to splendid proportions, probably 
higher than the Bockies have ever been. It has been 
called the period of the Appalachian revolution. 
In other parts of the world equally great changes 
must have taken place in land areas. From the 
irregular ridges in the floor of the Atlantic and 
Indian oceans, it has been supposed by some there 
was a great continent stretching eastward and uniting 
South America, Africa, and Australia with the Javan 
group of islands, if not with the Asiatic continent itself. 
It has been given the poetic name of Gondwana. 
However this may have been, and whatever the 
elevations and subsidences, the close of this period 
is the starting-point of new conditions, and there¬ 
fore of new life. The Cambrian strata, as we have 
seen, at once introduce us to primitive types and 
