xii. a, 5 Heise: Crater Lake of Taal 251 



The results of the most recent analyses of samples taken by 

 me in February and April, 1917, are given in Table I, coupled, 

 for comparison, with the older analyses. The analysis of the 

 water from Lake Bombon is also shown. 



A comparison of the above analyses shows that the crater 

 waters became progressively more salty before the eruption ; that 

 immediately after the eruption, as might be expected, the water 

 entering the crater was much less highly mineralized ; and that 

 the salt concentration has greatly increased since that time. 



The crater lake is at approximately the same level as Lake 

 Bombon, that is, a few meters above sea level, and must be over 

 70 meters deep. There are no indications of any springs, or of 

 ingress of water other than that which would normally occur in 

 an excavation extending below the water table. The crater is, 

 in effect, a huge basin exposed to insolation. 



There is, however, a large volume of water added to the lake 

 through the rainfall, which, in this region, averages about 1,900 

 millimeters (75 inches) per year. The rainwater, running down 

 the inner slope of the crater and into the lake, leaches the soluble 

 salts from the volcanic ejecta forming the crater walls, increasing 

 the amount of salts in the water; the intense tropical sunlight 

 stimulates evaporation. Thus the lake water becomes more 

 concentrated. This alternate addition of salt and evaporation 

 of water is sufficient to account for the increasing concentration 

 of the lake water. 



The quantity of soluble salts in the soil of the volcano may be 

 inferred from the fact that large areas on the outer slopes are 

 still practically unvegetated, except for occasional tufts of coarse 

 grass, and that the inside of the crater is barren, except in 

 isolated places. On the outer slope of the volcano, the soil, 

 5 to 20 centimeters below the surface, still shows, six years 

 after the eruption, large quantities of soluble matter. Analyses 

 of the water-soluble constituents of two soil samples — one from 

 a typical unvegetated area on the west slope of the volcano and 

 one from a typical grass area from the west shore of the island — 

 are given in Table II. For comparison, two analyses (5) of the 

 water-soluble material in fine ash and ejecta, collected soon 

 after the eruption, are shown. 



