294 Postelsia 
quantities. The shale pools contain stagnant 
water oftener than do the sandstone pools, and 
in such pools, no life forms are found. 
Tide pools are caused by a combination of 
conditions, among which may be mentioned: 
cracks ‘in “the rock; ‘concretions: —limesmeon 
stratification; erosion by waves, tides, and the 
wind; the action of carbon dioxide; variation 
in temperature, both of water and of the air; the 
action of plants and of animals. 
The lines of stratification are the chief cause 
of shale pools, since the water, striking against 
the rock, finds the lines of stratification the 
lines of weakness. The rocks are easily broken 
along these lines, and the hard, angular bits 
of shale act as grinding tools, to deepen and 
widen the shallow depressions made in the 
shale by the beating waters. 
Conglomerates afford a variety of hardness 
and the softer parts are easily worn out by the 
water. As soon as the cement holding the 
rock as one mass is removed, the remaining 
harder portions separate, and falling into the 
shallow depression, grind a deeper hollow into 
the= rock: 
