COAKSELT SPHEKULITIC ROCKS. 191 



analogy to that occurring in the large spheroidal bodies met with 

 in the felsitic lavas of Skomer Island, and also in the forms which 

 Mr. Cole had so well described. 



Prof. Maskelyne remarked that the solvent would more probably 

 have been in the form of alkaline thermal infiltration than acid; 

 but he did not understand how the black mineral was produced by 

 simple decomposition of the white spherulitic rock. He doubted 

 the data being sufficient to explain the action of alteration. For 

 instance, what was the 4-37 per cent, loss by ignition ? Was it all 

 water or part of it carbonic acid ? He suspected that while rocks 

 like pitchstones differ in different districts, not merely in chemical, 

 but in mechanical composition, the original formation of spherulitic 

 structure was in most cases the result of contraction, owing to the 

 minerals formed in the globules being denser and tending to shrink 

 from the surrounding mass while solidifying. 



Mr. Cole, in reply, said he had been greatly indebted to Prof. 

 Bonney's papers. With reference to the alteration of the white 

 spherulitic rock into the black product, the rock itself was not now 

 in its original condition, and the additional alkalies of the product 

 may have accumulated from the surrounding mass. Doubtless dis- 

 integration and infiltration had both taken place. He agreed that 

 alkaline waters may have shared in the action. As to the hollowing 

 of the spherulites, he referred to evidence put forward on the 

 continent. 



