Postelstia 207 
The sandstone pools present the greatest 
variety of structure, and it is to be expected 
that the greatest variety of causes also exists. 
Biere;, as invethe -shalesaithe water stirikine 
against a line of weakness, causes erosion, and 
as sandstone is softer than shale, erosion occurs 
on a larger scale. This eroded surface may 
be: the beginning of the pool. - he action is 
continued by the water, by the bits of broken 
rock, and by the agency of plants and animals. 
he roots of plants strike into the rock, and 
both by the spreading of the cracks, caused by 
the increase of the size of the holdfast, and by 
the dissolving action of the acid formed by the 
plant, the depression increases in size. 
Similarly, boring animals, as the rock clam, 
and the sea-urchin,, serve to increase the size 
of a depression made by erosion or solution, 
or, they may originate the depression by boring 
into solid rock. A steep sandstone cliff, just 
west of “The Point,’ shows the face of the cliff 
honeycombed into such depressions. 
A rather insignificant cause for the forma- 
tion of tidepools is the change of temperature 
