Geology 



acid and a layer of tufa was deposited on the lip. This 

 layer then gradually increased in height, slowly raising 

 the lip, layer by layer, and impounding the waters behind 

 it, until it eventually reached its present height (Fig 25). 



We found the entrance to the lower tunnel without 

 difficulty and soon returned to the outer air, where we 

 were surprised to find that it was already dark. We 

 had been so interested in our work that we had failed to 

 notice the flight of time, but nevertheless felt consider- 

 ably fatigued by our exertions in the confined and damp 

 atmosphere of the caverns. 



Over our evening meal we discussed the chances of 



FIG. 25. Formation and growth of barrier. 



a further exploration of the cavern, and came to the con- 

 clusion that if this were possible it must be by some 

 outlet from the chamber not yet known to us, and 

 decided that a careful examination of the walls for some 

 other opening must be our next endeavour. 



Accordingly, on our next visit, we skirted the shores 

 of the underground lake and examined the walls of the 

 chamber foot by foot, but without success ; finally we 

 landed once more on the platform and made a search 

 along its rear wall. We had almost given up hope 

 when one of us climbed up the cone of stones previously 

 noted, and found that a strong draught of air, which 

 nearly extinguished his candle, proceeded from a crevice 

 between the uppermost stones. He called to us and we 



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