PlfROCRAPHlCAL NOTtS. 83 



Well shaped and have evidently grown freely in the surrounding 

 magma, but they are usually too small to allow of their species being 

 determined, though most of them are probably orthoclase. Occasion- 

 ally one may be found which shows striated twinning. 



In many of the specimens the groundmass breaks up on the 

 application of polarised light into a more or less irregular mosaic of 

 light and dark areas (PI. IX, figs. 4, 5), in which, when in a position of 

 extinction, the felspar microlites appear as dim points of light. That 

 the matrix of each of these areas consists of crystalline quartz is 

 evident when they are in contact with the quartz phenocrysts, for it 

 is then seen that the quartz of the phenocryst is in optical continuity 

 with the substance filling the interstices between the felspar microlites. 

 The original quartz of the phenocryst and the " secondary " quartz 

 of the groundmass extinguish simultaneously, the latter forming the 

 closed areas or " courts " surrounding the phenocryst, already described. 

 It is possible that each of the areas forming this "quartz mosaic," as 

 it may be called, has been built up round a previously existing granule 

 of quartz, introduced, like the larger phenocrysts, from outside, though 

 such a central granule is not always visible in the slide, owing to the 

 direction the section has taken. The size of the areas appears to 

 depend upon the number and distribution of these granules; where 

 they are numerous and close together the mosaic will be proportionally 

 fine grained. The quartz composing any individual of the mosaic 

 appears to have grown outwards until its development was arrested 

 by contact with the surrounding individuals. Thus, in cases where 

 the mosaic is composed of small areas, the " court " surrounding a 

 quartz phenocryst, even though the latter is a large one, is propor- 

 tionately narrow. Conversely, a small phenocryst may be surrounded 

 by a broad " court " if the mosaic composing the groundmass consists of 

 large individuals. 



It is a question whether this mosaic of quartz grains was formed at 

 the time of consolidation of the rock, or whether it is a result of second- 

 ary devitrification of an originally glassy magma. I am strongly 



( 83 ) 



