570 



MR. W. A. RICHARDSON : A MICROMETRIC [vol. lxxix, 



boundaries of the minerals, we may apply to each area the average 

 for its own granite-type, and so mark out those zones in which 

 the mineral is present in an amount greater than its average. 

 These may be called groupings of the second order, so as 

 to distinguish them from first-order grouping : that is, with 

 reference to the mean for the Avhole outcrop. These new zones 

 are shown on the original maps for the chief minerals by dotted 

 lines, and the positive areas (that is, those with quantities greater 

 than the mean) are shaded. The values fall into definite zones, 



Fig. 10. — JSLineral-variation diagram on a ' space base ' across 

 the St. Austell granite-mass. 



fa T£Wt T 5 rf ]: t HENSBARROW AffCA 

 DTEPHOG 



Area 

 Litfiiow te Gilbciiit el Lithionite 



LUXULLYAN 

 Area 



BiotitesMuscoYit e 

 I. 



\o miles 



which, in many cases, do not differ materially from the groupings 

 of the first order. The new mode of arrangement may be briefly 

 summarized. 



In the case of quartz (fig. 2, p. 54S) each area is occupied 

 by a negative group more or less centrally placed. For the 

 St. Stephen's and Hensbarrow areas these groups are well estab- 

 lished ; but, although the same tendency is exhibited in the 

 other areas, it rests on fewer points. A tendency to the formation 

 of a second negative area is also shown in the extreme north- 

 east of the Hensbarrow area. Orthoclase (fig. 4, p. 550) shows 



