THE CAMBRIAN PERIOD. 
35 
minum. The silicon in the water, no donht, took 
oxygen from the air and formed the qnartz, not so 
much during this age as during the previous one. In 
the same manner the aluminum formed our clay, and 
the lime in the water united with the carbon dioxid 
from the air and 
formed the lime¬ 
stone. Magnesium 
formed the gypsum 
beds. The sodium 
of the water united 
with the chlorin in 
the hydrochloric acid 
and formed salt. The 
crinoids, and particu¬ 
larly the corals, must 
have taken large 
quantities of lime 
out of the sea water, 
and thus have puri¬ 
fied it, while the salt 
was left, making it 
briny. 
The best way to 
become acquainted 
with formations of 
rock is to go to some 
valley or cut where 
the different layers are visible; the typical forms 
of life, however, may be more easily studied and 
better understood by examining the fossils in a mu- 
Fig. 14.—A trilobite of the Cam¬ 
brian period. 
