270 Scientific Proceedings, Royal Dublin Society. 



sional apophyses of felspar. Each granule of quartz is the centre 

 of a micro-pegmatitic area : such an area consists of the central 

 quartz granule and a number of surrounding or partly surrounding 

 crystals of felspar, which are " pegmatised " for a variable distance 

 outwards from their junction with the quartz, sometimes the peg- 

 matitic structure extends throughout the felspar crystal ; some- 

 times it stops less than halfway across. When the felspar is in 

 excess of the quartz intergrown with it we may fairly speak of the 

 pegmatisation of the felspar, but sometimes the reverse is the case, 

 and the felspar occurs included, pegmatite fashion, in a granule of 

 quartz, and then one must speak of the pegmatisation of the 

 quartz. 



The quartz of this rock, as of that of the Coolboy granites, has a 

 very dusty appearance, owing to the great abundance of inclusions, 

 some liquid and gas cavities, and some of an indeterminable nature, 

 but certainly solid. 



The history of the consolidation of this rock diverged from that 

 of the preceding varieties at a very early stage, for the highly 

 acidic glass, remaining after the excretion of the biotite, contained 

 so little lime, that the first felspar to consolidate was albite ; soon 

 after the appearance of this, the quartz separated out as granules, 

 and this and the albite continuing their growth simultaneously, 

 at last intercrystallized to form micropegmatite : the microperthite 

 structure seems to belong to a somewhat later phase than that of 

 the normal albite twinning. 



Correlation of the foregoing rocks. — The name given by Haughton 

 to these rocks is an exceedingly appropriate one. The most coarsely 

 crystallised are evidently granites, but distinguished from normal 

 granite by the abundance of plagioclase felspar, the average com- 

 position of which probably approaches Ab 4 Ani or Ab 3 Ani. This 

 involves an excess of soda, and hence the distinctive name. The 

 presence, however, of soda bearing felspars (often of the nature 

 of anorthoclase, which may be present in the soda granites) is a 

 characteristic of the rocks known as keratophyre, rhomben-porphyr, 

 and pantellarite ; and the bulk analysis of quartz-keratophyr does 

 not differ widely from that of some of the soda-granites. I extract 

 the following from " Teall's Petrography," page 371, and place 

 Dr. Haughton's analysis side-by-side for comparison : — 



