294 P s t e I 6 i a 



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 wliich may be mentioned: 

 cracks in the rock; concretions; lines of 

 stratification; erosion by waves, tides, and the 

 wind; the action of carbon dioxide; variation 

 in temi)erature, both of water and of the air; the 

 action of j)lants 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 alTord 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. 



