Poste tsita 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 le 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 
