F. P. Mennell—Pleochroie Halos. 117/ 
Rounp SPHENE. mm. 
In biotite of granulite, Amazon Mine, Rhodesia : 02 
In hornblende of granite, Umguza River, Rhodesia . 03 
In augite of syenite, Hillside, Rhodesia! . 5 é 03 
RounpD OrrHITE (ALLANITE). 
In biotite of granite, Matopos, Rhodesia. : . *03-"04 
In biotite of eranite, Zimbabwe, Rhodesia a‘ "035 
In hornblende of granite, near Blanket Mine, Rhodesia 03 
In hornblende of © tonalite’, Adamello, Tyrol los ‘ 035 
In hornblende of diorite, J ahonda, Rhodesia ; : 04 
‘ Rounp Eprpore. 
In chlorite of granite, Matopos, Rhodesia : : 08 
In biotite of granite, Gadara, New South Wales ? 025 
The above measurements are necessarily somewhat rough, as it is 
difficult to keep an eye on the halo and the micrometer scale at the 
same time. However, the general result is to emphasize the uniformity 
of size and the fact that there does not seem to be any definite 
relation between such variations as are noticed and the minerals 
concerned. . 
Halos and Radio-activity.—Professor Joly has pointed out that the 
penetration of the a rays emitted by radium compounds is about 
‘04mm. in the case of aluminium, and having regard to the slightly 
ereater density of the minerals examined, the results are in close 
agreement with the theory that the halos are due to the alteration of 
the surrounding minerals by those rays. Some of the smaller halos 
were indistinct, and it seems possible that im several cases where 
rather low values were obtained, the radio-active substance was 
present merely as an inclusion in the interior of the mineral to which 
the halo appeared due. This is the more probable as such minerals as 
apatite and sphene cannot be radio-active in virtue of their normal 
constituents, or even their usual impurities.* Epidote, too, may 
owe its occasional activity to minute inclusions of orthite. As bearing 
on this point, may be noted a granite from the Zimbabwe Ruins in 
Rhodesia, which contains apparently primary epidote enclosed in 
biotite. The epidote in turn encloses irregular patches of orthite, and 
wherever the latter approaches within ‘035 mm. of the edge of the 
epidote, the biotite within that limit shows the usual darkening. 
It is noteworthy that zircon may always be expected to contain 
traces of thorium, and orthite invariably contains that element in 
appreciable amounts, and probably uranium as well in some cases. 
From a petrological point of view it seems as if thorium should be 
a far more potent cause of radio-activity in the earth’s crust than 
uranium and radium. We know of no widely distributed uranium- 
bearing rock-former, while zircon is found everywhere, and orthite 
is far from rare in many regions. Over 15 per cent. of my slides 
of granites contain it. Monazite, always rich in thorium, is also 
Nature of enclosed mineral somewhat doubtful. 
Radio-actiwity and Geology, p. 68. 
See, however, Strutt, Proc. Roy. Soc. A., 1908, p. 275. 
e See, for instance, Geo. Mae., 1903, Dec. IV, Yol. X, p. 347, and Geology of 
South Rhodesia, pp. 29-82. 
DECADE V.—VOL. VII.—NO. I. 2 
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