434: VOLCANIC ROCKS OF THE NOETH-EAST OF FIFE. 



site and porphyrite. The presence of so much water in a glassy rock, 

 such as that cited by the President, would, if general, account for 

 the development of a vesicular structure in certain obsidians when 

 heated. The instance of perlitic structure was singularly beautiful. 

 He inquired if the larger and smaller perlitic structures shown in 

 the diagram differed in age, his impression being that, in this case, 

 the different series of perlitic cracks represented periods of con- 

 traction following so rapidly that the process of fission was approxi- 

 mately continuous. 



Mr. Teall said that the facts described by the President would 

 supply another argument, if such were needed, in support of the 

 view that volcanic rocks of precisely similar composition and structure 

 had been produced at widely separated geological epochs. Eeferring 

 to the enstatite-andesite he remarked on the rapidly accumulating 

 evidence as to the wide distribution in space and time of the rhombic 

 pyroxenes. He had quite recently detected these minerals in 

 plagioclase-augite rocks from Eatho, Kilsyth, and Arran. As in 

 these cases the rocks were granular in texture and basic in compo- 

 sition, he was inclined to call them enstatite-dolerites rather than 

 enstatite-andesites. 



The President, in reply to Mr. Rutley, observed that the smaller 

 perlitic structure is probably slightly posterior to the larger. 



