Sollas — On Soda-Granites and Associated Dykes of Wicklow. 267 



followed, sometimes including previously-formed crystals of zircon, 

 next the highly basic and ferruginous biotite ; the already existing 

 crj'stals of zircon, and soforth, serving apparently as nuclei. The 

 aureole around the included zircon crystals is probably richer in 

 ferrous salts than the rest of the biotite, and may be possibly looked 

 upon as another instance of the early excretion of the more ferru- 

 ginous compounds. Considering the easy fusibility of biotite, and 

 the large extent to "which water enters into its composition, its 

 consolidation at this stage is not a little remarkable. Stable only 

 under the conditions which existed at the time of its formation, a 

 good deal of it, with change of conditions, subsequently became 

 destroyed, leaving clusters of zircon and apatite crystals to mark 

 its place ; the biotite now present in the rock may thus bear but a 

 small proportion to that originally crystallized out. Part of it 

 further gave rise, by re-action with other constituents of the glass, 

 to muscovite ; and possibly epidote and other minerals were formed 

 from it at the same time. The conversion of biotite into musco- 

 vite, during the consolidation of granite, appears to be a process of 

 common and wide occurrence in this district. 



The more calcareous and basic plagioclase then crystallized out 

 (in some cases about fragments of muscovite), as its growth pro- 

 ceeded the lime became exhausted and the successive envelopes 

 became poorer in this constituent, and correspondingly richer in 

 soda. The growth of the felspar was not uninterrupted ; there 

 were periods when solution was substituted for deposition, as is 

 shown by the rounding off of the angles of some of the zones. 

 Solution also took place when the felspar crystals were completed, 

 as is shown by the character of their junction with the succeeding 

 quartz. Muscovite also developed along the cleavage planes, per- 

 haps by a kind of schillerisation. After the plagioclase the excess 

 of silica consolidated as quartz, and finally the microcline, crystallis- 

 ing partly contemporaneously with, and partly subsequently to 

 the quartz, completed the structure of the rock. 



Porphyritic Soda Microgranite of Aughrim.— East of the margin 

 of the soda granite, towards Wooden Bridge, two or more thin 

 dykes of a fine-grained felstone-like rock occur as intrusions in the 

 Ordovician schists ; analysis shows that some of these have approxi- 

 mately the same composition as the adjacent granite, of which they 

 are evidently apophyses. 



