No. 381. A HALF-CENTURY OF EVOLUTION. 6 
39 
expanse of land upon the continental platform between the Atlantic and. 
Pacific oceans. The agencies of erosion were wearing away the surface of 
this Algonkian continent and its outlying mountain barriers to the eastward 
and westward, when the epoch of the Lower Cambrian or Olenellus zone 
began. The continent was not thennew. On the contrary, it was approach- 
ing the base level of erosion over large portions of its surface. The present 
Appalachian system of mountains was outlined by a high and broad range, 
or system of ranges, that extended from the present site of Alabama to 
Canada, and subparallel ranges formed the margins of basins and straits to 
the east and northeast of the northern Paleo-Appalachian or the Paleo- 
Green Mountains, and their northern extension toward the Precambrian 
shore line of Labrador. The Paleo-Adirondacks joined the main portion 
of the continent, and the strait between them and the Paleo-Green Moun- 
tains opened to the 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. (Loc. cit., 
p. 562.) 
Remarking on the habitat, or nature and extent of the sea 
bottom tenanted by the Olenellus or Lower Cambrian fauna, 
Walcott remarks : 
One of the most important conclusions is that the fauna lived on the 
eastern and western shores of a continent that, in its general configuration, 
rudely outlines the North American continent of to-day. Strictly speaking, 
the fauna did not live upon the outer shore facing the ocean, but on the 
shores of interior seas, straits, or lagoons that occupied the intervals between 
the several ridges that rose from the continental platform east and west of 
the main continental land surface of the time. (Loc. cit., p. 556.) 
Dana had previously (1890) claimed that the earth’s features, 
even to many minor details, were defined in Archzan time 
(evidently referring to all Precambrian time), and that «‘ Archean 
conditions exercised a special and even detailed control over 
future continental growth.” May not this idea be extended to 
include the life of the Precambrian, and may we not suppose 
that biological variations and evolutions were predetermined, to 
some degree at least, by the geological conditions of these 
primeval ages? The continental masses were then foreshadowed 
by submarine plateaus covered by shallow seas, the deeper 
portions of the ocean basins not being affected by these 
oscillations, extensive as they were. 
