424 Scientific Proceedings, Royal Dublin Society. 



micro-pegmatite. Rosenbusch suggests, in the last edition of his 

 work, 1 that they may he formed of a definite chemical compound 

 — micro-felsite — in which the ratio of alkalies to alumina is the 

 same as in the felspars, the silica being in excess of that re- 

 quired to form an alkali-felspar ; whilst Teall 2 " is inclined to 

 regard the spherulites as due to the simultaneous crystallisation 

 of quartz and felspar." 



We have referred to the crypto-crystalline matter present in 

 the groundmass of the rock. It seems well to mention its char- 

 acters before entering upon the general description of the rock, 

 especially seeing that it stands in such close relation to the 

 spherulitic masses. It is noticed, under cross nicols, to consist 

 of ill-defined flecks and granules, which polarise in low tints. In 

 only rare instances are the boundaries of the grains, constituting 

 the double-refracting substance, recognizable. There are then 

 faint indications of micro-pegmatitic structure to be sometimes 

 observed ; but the intergrowth of felspar and quartz is not very 

 definite in character. There is, therefore, both crypto-ciwstalline 

 and micro-crystalline matter in the rock. It is quite possible that 

 superposition may have something to do with the production of 

 the indefinite aspect which crypto-crystalline matter gives under 

 crossed nicols. We can easily imagine the case of double refract- 

 ing particles — small in comparison with the thickness of the 

 slide — so overlapping each other as to interfere with the success- 

 full differentiation into individual grains. 



Orthoclase is the predominating felspar in the rock. Its form 

 is generally well preserved ; but rounding of the angles, through the 

 corrosive action of the acid magma, is often apparent. In fact, the 

 greater part of a crystal may be destroyed by this influence. Still, 

 such an extreme degree of corrosion is rare in this rock. This 

 corrosion of porphyritic crystals is due to chemical and physical 

 changes during consolidation. Lagorio 3 has shown that mere 

 relief of pressure will cause corrosion. But it is still more pro- 

 bable that corrosion is largely due to the changes in the chemical 



1 p. 190. 



2 British Petrography, p. 402. 



3 Ueber cler Natur der Glasbasis, &c, Tschermak's Min. u. petr. Mitt., 1S87, viii., 

 510. 



