298 IP s t e I s i a 



of the water in the pools. In the Port Ren- 

 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 



