50 



Biological Conditions 



are given for the ash from the uppermost layers formed during the 

 first eruption (a) and for the light coloured pumice, etc. (b) : 



The soluble substances are derived chiefly from sea- water which 

 percolated through the surface to the reservoir of lava or was driven 

 into the crater at the time of the explosion, and in the course of 

 subsequent eruptions became mixed with the volcanic ash. Possibly 

 some of the gypsum has not come from sea-water but from older 

 strata of the volcano ; this hypothesis derives support from the fact 

 that v the insoluble portion of the ash contains a considerable amount 

 of anhydrite. Among the other components of sea- water magnesium 

 chloride is not represented in the soluble constituents of the ash ; it 

 is probable that by contact with aqueous vapour at a high tempera- 

 ture this was converted into the insoluble oxide of magnesium. The 

 small amount of bromine and iodine in sea-water (about / o ) may 

 well explain the absence of these elements both from the ash and the 

 pumice. 



Some of the inorganic salts required for the nutrition of plants 

 were present in the material produced by the eruption in more than 

 sufficient quantity and in a soluble and available form. Such sub- 

 stances as are unrepresented in the volcanic material may have been 

 brought in the form of dust in the same way as organic germs are 

 carried to islands by water and by wind. Moreover, tidal waves and 



