Postelsia 343 
and its .ings. Usually the concretions present 
the same hardness from circumference to center, 
but the large ones have a softer central portion. 
When worn off by the waves and weathering, 
they form hollows upon the surface and become 
tide-pools, later developing into many and diver- 
sified forms. An interesting concretion partially 
buried in a vertical rock wall is seen in front of 
the Station buildings. Its shape suggests that 
it was once somewhat spherical but subsequently 
by a reduction in thickness of the sandstone for- 
mation as a whole through dissolving and re- 
moving a portion of its calcium carbonate, the 
concretion had become considerably flattened 
(PT abe SXUxex TTT): 
The question arises: How are these con- 
cretions formed? ‘They consist of sand grains 
aggregated by a cement of calcium carbonate. 
This can be shown by dropping a fragment of 
fresh material into a test tube containing dilute 
chlorhydric acid. After a violent effervescence 
the sand grains lie in a loose mass in the bottom. 
Often in breaking a fresh concretion and holding 
the fractured surface in the light, a reflection 
o b) 
