34 
HAROLD’S DISCUSSIONS. 
conform in position with the older rocks, neither 
would they be of uniform thickness. 
THE CAMBRIAN PERIOD. 
The new strata have received the name Cambrian. 
They are exposed around Archaean areas both in 
America and Europe. They were first carefully 
Fig. 13. — A cross-section of highland. a, Archaean; b, pre- 
Cambrian ; c, Cambrian; d, Silurian. 
studied in AYales, which was formerly known as 
Cambria, whence their name. 
When the Cambrian age began, only a few small 
areas of rocky land stood above the almost universal 
dominion of the sea. Everywhere in this sea, espe¬ 
cially near the shore-lines, sediments were being de¬ 
posited. These were of three kinds: those obtained 
from the rapid erosion of the land surfaces, those 
resulting from chemical combinations in the water 
itself, and the remains of animal and plant forms. 
The rocks of this period are very thick in some 
places, and consist mostly of limestone. This is natu¬ 
ral, for the atmosphere must have been full of such 
acids as carbon dioxid, hydrochloric, and sulfurous, 
and the sea water must have contained large quan¬ 
tities of lime, magnesium, silicon, sodium, and alu- 
