16 rar Mennell—Pleochroie Halos. 
any halo being seen. However, a halo round zircon is certainly the 
normal state of affairs, and the same may be said of orthite. It is 
curious to notice that large crystals of these minerals may fail to give 
rise to halos, though these last may be conspicuous round the smaller 
crystals in the same slice. Round apatite and epidote halos are only 
occasionally seen, and are usually faint, while round sphene they are 
extremely rare. 
Shape of Halos.—It is evident that, if three dimensions are considered, 
halos are usually spherical, and thus give circular sections, except in 
cases where the enclosed crystal is distinctly elongated and of sufficient 
size for the difference of length and breadth to be appreciable. In the 
case of large irregular grains or granular aggregates the halo may be 
quite irregular in shape, but extends to a uniform distance from each 
part of the margin of the substance which it surrounds. 
Size. — A feature revealed by measurement is the remarkable 
uniformity in size of the halos. The distance to which they extend 
from the edge of the enclosed crystal appears practically uniform in 
nearly all cases. Of course there are numerous instances where halos 
apparently smaller than usual are observed, owing to the dark sphere 
not haying been cut centrally, but in these no nucleus can be seen. 
There do, however, appear to be some small halos which cannot be 
thus accounted for. On the other hand the alteration of minerals like 
orthite tends to produce stains which spread principally along cracks 
or cleavage planes, and must be carefully distinguished from real halos. 
The following are some approximate measurements, made by means 
of an eyepiece micrometer, of the breadths of fairly well-marked halos. 
seen round various distinctly visible minerals in a variety of rocks :— 
Rounp Zircon. 
mm, 
In biotite of granite, Haytor, Dartmoor . : .  *03-"04 
In biotite of granite, Rubislaw, Aberdeen : , “08 
In biotite of granite, Cowra, New South Wales . : 035 
In biotite of granite, Gadara, New South Wales : "02 
In biotite of granite, Cape Town : : F : 035 
In biotite of granite, Matopos, Rhodesia. c . 703-"04 
In biotite of granulite, Rhodes’ Drift, Rhodesia. : “04 
In hornblende of granite, Weinheim, Baden! . : “08 
In hornblende of granite, Bulawayo, Rhodesia? . P 08 
In hornblende of diorite, Jahonda, Rhodesia ‘ ‘ 035 
In augite of granulite, Amazon Mine, Rhodesia ' 3 03 
In chlorite of granite, Mountsorrel, Leicester . : 08 
In chlorite of granite, Matopos, Rhodesia . : 2 03 
In chlorite of granite, Cowra, New South Wales : “04 
In tourmaline of granophyre, Cornwall . 3 - 108-7035 
In tourmaline of granite, Cape Town. - : : 03 
In cordierite of granulite, Bodenmais, Bavaria . . "045 
In cordierite of granite, Cape Town . 2 . : “035 
In ‘pinite’ of granite, Cape Town. : 4 : 035 
Round APATITE. 
In biotite of granite, Cape Town : ¢ é - 03 
In biotite of diorite, Hillside, Rhodesia! . : ; “03 
In biotite of granulite, Amazon Mine, Rhodesia - 02 
In biotite of granulite, Rhodes’ Drift, Rhodesia : 03 
1 Nature of enclosed mineral somewhat doubtful. 
