194 CHAREES D WALCO Ta 
their environment, hence we will first broadly outline the conditions 
under which the known marine organisms of Cambrian time lived. 
NORTH AMERICAN CONTINENT AT THE BEGINNING AND AT THE CLOSE 
OF CAMBRIAN TIME 
The information obtained since the publication of my first map on 
this subject in 1891? has been assembled on the two accompanying 
maps by Mr. Bailey Willis. The first map outlines a central mass 
of pre-Cambrian land, flanked on either side by large barrier 
islands that served to protect straits, sounds, or seas from the open 
ocean. Ocean currents flowed through the sounds with varying force 
and volume, not only from the cold arctic waters to the north, but 
from the warm tropical region to the south. The relative position of 
land and sea is based on the present interpretation of the observed 
characters and distribution of the pre-Cambrian and Lower Cambrian 
rocks. The distribution of Lower Cambrian faunas indicates the prob- 
able courses of the marine currents. A fundamental assumption is 
that the great ocean basins and continental masses occupied their 
present relative positions during at least the Algonkian portion of 
pre-Cambrian time. 
The map of the continent at the close of Cambrian time shows that 
during this period upon the continental area marked changes in the 
positions of the land and sea took place. Broad shallow seas followed 
the transgressing shore-line of Middle Cambrian time, offering most 
favorable conditions for the long-continued development and distribu- 
tion of marine life. There were undoubtedly deep and shallow seas 
and bays, cold and warm waters, strong and weak ocean currents of 
unlike temperatures, protected bays with sandy and muddy bottoms, 
shore lines gently sloping to deep water, and many conditions promot- 
ing the evolution of the faunas through favorable or unfavorable 
changes in environment, temperature, and food-supply. 
The sediments of Cambrian time are mainly those deposited near 
the shore-line and in adjacent relatively shallow waters. There is 
little if anything to indicate deposits of the abyssal sea. If the littoral 
t Bull. Geol. Soc. Amer., Vol. X, 1899, pp. 199-244. 
2 Bull. U. S. Geol. Survey, No. 81, 1891, Pl. III. 
3 See theoretic section at the close of Cambrian time: Bull. U. S. Geol. Survey, 
No: 81; 189r; Pla 
