IP s t e I s 1 a 301 



are composed of rock much harder than the 

 sandstone. Between the surrounding sandstone 

 and the imbedded concretions, is a layer of very 

 hard calcite. About half way to the shore line, 

 the calcite has disappeared from the sides, but 

 the concretion is still firmly attached to the 

 underlying rock. Still further down, the con- 

 cretion is loosened, and the surrounding sand- 

 stone is worn away to such an extent, that the 

 concretion lies like a huge, round pebble in the 

 depression. Up to this point the depression is 

 circular, and the sides are smooth. The con- 

 cretion is hurled about by the tides, and con- 

 tinually wears away the sides and bottom of 

 the pool. When two or more pools lie close to 

 one another, as may be seen in several of the 

 photographs, the separating walls are beaten 

 against by the concretions, until the partition 

 breaks, and an irregular pool begins. The 

 action of the waves and the tides, and the grind- 

 ing of the rocks, continue until at last many 

 small pools have united to form a large pool; 

 the concretion itself is by this time broken into 

 small pieces, or is dissolved. Added to these 



