part 4] study of the st. a.ustell granite. 553 



Near the margins bands of closely-packed, parallel phenocrysts 

 sometimes occur, chiefly in the district near Menacudclle. A good 

 exposure on the east side of Trenail ce Viaduct showed that such 

 bands were thin. They were accordingly avoided in sampling, 

 as there their total effect on the bulk composition is small, and 

 single samples would disturb the result. Finally, there are 

 porphyritic rocks in which the grain of the phenoeiysts is the 

 general grain of the rock, and these porphyritic elements are 

 embedded in a small amount of very fine ground-mass. Careful 

 selection of slides from such rocks ensures good sampling, and 

 they give rise to no uncertainty. 



III. The Minerals and their Distribution. 



The most advantageous way of presenting the distribution of 

 mineral quantities and their peculiarities is by a series of maps. 

 Considering the coarse-grained character of the rock and the 

 fluctuations of sampling, attempts at fine contouring were not 

 helpful ; but the device described below is a rapid method for 

 detecting the existence of definite grouping. Each amount was 

 compared with the arithmetic mean for the whole series (col. I, 

 Table V, p. 554). If in excess of the mean a plus sign, and if 

 in defect a minus sign, was plotted at the locality. Boundaries 

 were then drawn enclosing the 'plus' quantities. These boundaries 

 are shown by heavy chain-dotted lines in the maps, and the plus 

 sign indicates the side on which the quantities are greater than 

 the mean. Exceptions to the groupings are shown by small circles 

 instead of dots. Such groupings of value with respect to the 

 general average will be designated groupings of the first 

 order. 



Quartz. — Throughout the whole district the quartz possesses 

 the characters usually met with in granites, but in places a few 

 peculiarities are developed. Quite large patches of quartz, when 

 present, in most cases are resolved microscopically into aggregates 

 of smaller crystals. Occasionally, however, single crystals are 

 found sometimes so much larger than the general grain as to merit 

 the term ' phenocryst '. 



Slight strain-shadows are usual, and only absent from grains 

 enclosed in orthoclase. In the Luxullyan area frequently, and 

 elsewhere occasionally, the quartz may present idiomorphic out- 

 lines to the orthoclase. Cracks in the quartz are, as a rule, quite 

 irregular ; but some examples exhibit a pattern, with a crudely 

 rectangular arrangement, doubtless due to the imperfect rhombo- 

 hedral cleavage. Examples were seen at Gready and Tregargus 

 Mill, but the feature does not appear to be confined to special 

 localities. 



The larger inclusions in quartz may consist of any other rock- 

 mineral, while the bulk of the more minute inclusions is made up 

 of fluid- tilled cavities (with bubbles, etc.). These may be so small 

 as to be almost irresolvable under high power, but may be as 



2p2 



