18 F. P. Mennell—Pleochroic Halos. 
found in many rocks. It is as yet scarcely possible to judge how 
the recently discovered radio-activity of potassium affects the question, 
though the abundance of that element renders the fact of obvious 
importance. 
Various Features. —When the halos are fairly intense, they usually 
have quite well-defined boundaries, much more so than one is inclined 
to think before making a series of observations on the point. Where 
they are clearly perceptible in the direction of minimum absorption 
they show no increase of diameter when placed in the position of 
maximum absorption. Sometimes, however, a halo appears in the 
latter position, whereas none was perceptible in the direction at right 
angles to it. This is especially the case with such faint halos as those 
sometimes seen round epidote or sphene, or those in cordierite. The 
more intense halos are usually uniform in tint from inside to outside, 
or slightly lighter towards the outer edge. Irregularities in dis- 
tribution are, however, fairly frequent, and the halos are sometimes 
dark and light in uneven patches. In certain cases, too—always, 
apparently, round rather large inclusions—the halos may have a rim 
darker than the interior. ‘Thus, in the granite to the north of » 
Kahlele’s Kraal, in the Matopo Hills, Rhodesia, there are well- 
marked halos round good-sized orthite crystals enclosed in biotite. 
These halos are usually very sharply defined on the outside, and 
have the margins distinctly darker than the parts nearer the orthite. 
A similar feature may be noticed round some of the zircons in a 
biotite granulite from the neighbourhood of Rhodes’ Drift on the 
Limpopo River, Rhodesia. It is also well seen round the larger 
zircons in the well-known cordierite granulite of the Bodenmais, 
Bavaria. 
The halos round the minute zircons in the tourmaline of the 
Cornish granophyre (quartz-porphyry) in the list given above are 
interesting as extremely good examples of a phenomenon distinctly 
rare in tourmaline, though noticed long ago by Michel Lévy.’ 
That this mineral does not readily lend itself to the formation of 
pleochroic halos is obvious from the evidence of such rocks as the 
Cape Town granite. My slides of specimens from near the contact 
show several instances of small zircons at the junction of biotite 
flakes and tourmaline crystals, and whereas the biotite shows a semi- 
circle of intense pleochroism, the halos are incomplete, owing to the 
tourmaline being entirely unaffected. The same fact was noted at 
several junctions of biotite and cordierite, though in one case a good 
example of a composite halo, half in biotite and half in cordierite, 
was observed. Several fairly distinct halos were also seen round 
zircons, entirely enclosed in tourmaline, and we may perhaps infer 
that in these particular cases the enclosures were more strongly 
radio-active than usual. It is noticeable that where the cordierite 
of this rock has been altered into the so-called ‘ pinite’ pseudo- 
morphs, the halos are much more intense than in the fresh cordierite 
itself. Halos in hornblende are rather rare, though this is perhaps 
due partly to the much greater scarcity of inclusions than in biotite, 
' See Minéraux des Roches, p. 288. 
