298 Postelsia 
of the water in the pools.” In the ori Rene 
frew region the range is small, but it is enough 
to bring about a slight degree of erosion. 
Air is soluble, and is compressed by the 
weight of the water. In pools, or in the inden- 
tations caused by the partitions between the 
pools wearing away, especially under rocky 
ledges, jets of water are sometimes seen spurt- 
ing out, and a booming sound is heard. Both 
the sound and the jet are caused by the sudden 
expansion of air in the water. The expansion 
of air is due to the removal of pressure on the 
enclosed air, probably by the shifting of the 
water. 
Ordinary air currents, beating against rock, 
gradually erode it; storm currents cut out the 
rock rapidly. Air exerting its pressure through 
water acts in the same way, and slowly wears 
out the rock against which its force is directed. 
The most potent cause of sandstone pools 
is the concretions. If the region of the ‘‘ Devil’s 
Billiard Table’? be examined, it will be seen 
that on the land side, the concretions are solid 
masses firmly imbedded in the sandstone, and 
