Z. F, Harjper — Evidence of Uplift. 



49 



of blowhole. Wherever the underlying bed of tuff has been 

 subjected to maximum wave action, the lava sheet has been 

 undermined. In one instance, at the landward end of a cave 

 60 formed, a basaltic column dropped out and left an opening 

 about 15 feet deep and 1^ feet in diameter. The column w^as 

 probably loosened both by the chemical action of the sea 



Fig. 1 , 



Fig. 1. Eock shelf crossed by a weathered dike. 

 South Wales. 



Illawarra district, New 



water forced up the joint faces and by the mechanical action 

 of the waves on the unsupported prism of basalt. 



In the case of the horizontal blowhole, which has no vertical 

 opening, an incoming wave compresses the air in the rock 

 chamber, and as the wave subsides the water is forced out in a 

 horizontal shower of spray. In the event of a heavy sea and 

 favorable tidal conditions, the effect produced by blowholes is 

 awe-inspiring as well as spectacular. At such times, a subter- 



Am. Jour. Sci.— Fourth Series, Vol. XLIV, No. 259. 

 4 



-July, 1917. 



