F. P. Mennell—Pleochroic Halos. 15 
witness the hitherto unsuspected occurrence, November, 1908, of an 
old lake-bottom at Perranzabuloe,' 200 feet O.D. Though much of 
this chara-marl has been carted away by agriculturists as fertilizing 
agent, enough remains to yield a most curious limnic molluscan fauna, 
scarcely to be paralleled according to Mr. A. Santer Kennard, F.G.S., 
among the existing British freshwater fauna of the present day. 
T have to thank Mr. W. H. Griffin and my wife for botanical help ; 
and my friend Dr. Henry Woodward, F.R.S., for this opportunity of 
placing my notes for revision, if needed, before abler and more 
competent minds who have given thought to this fascinating subject. 
EXPLANATION OF PLATE IV. 
Fic. 1. Dolmen-sous-marin near Carnac Plage. 
», 2. Granulite rocks at Cap de Garde. The Chaudronnier, or Pierre d’Etel, 
surmounted by a beacon. The Lighthouse stands somewhat inland ( (see 
, next view). 
», 3 tel, viewsouthwards from close to the Raised Beach. The Lighthouse and 
storm signal station are to the right of the Chaudronnier. 
LV.—P.eocurotc Hatos. 
By F. P. Mennett, F.G.S. 
(PLATE Y.) 
(J\HE subject of ‘ pleochroic halos’ has become endowed with peculiar 
interest since Professor Joly suggested that they are due to the 
radio-activity of the inclusions round which they occur.” On looking 
into such petrographical literature as I possess, it appears that very 
few precise observations have been made regarding these halos, and it 
has occurred to me, therefore, that some of my own notes on the subject 
may be of general interest. 
Minerals causing or showing Halos.—The usual type of halo, as seen 
in rock sections, is a dark spot of roughly circular outline surrounding 
a small centrally situated enclosure in another mineral. The enclosing 
mineral may or may not itself be pleochroic, though it usually is so, or 
would appear so in thicker slices. The following is a list of minerals 
which have, to the writer’s knowledge, been found to show halos, 
though it must be stated that in some of them the phenomenon is very 
rare: biotite, augite, hornblende, muscovite, chlorite, tourmaline, 
cordierite, staurolite, and andalusite. As to the minerals producing 
the halos, identification is not always easy, owing to their minute size. 
The following have come under the writer’s notice: zircon, sphene, 
apatite, orthite (allanite), and epidote. It is needless to point out 
that all these latter minerals (except, perhaps, epidote) are known to 
be, comparatively speaking, strongly radio-active. As far as the rocks 
are concerned, halos are far more common in those of igneous origin 
than in the other classes, and are especially noticeable in the plutonic 
types, particularly the granites. It may be noted that even zircon, 
round which halos are most frequently met with, may occur enclosed 
in such a susceptible mineral as biotite without the slightest trace of 
1 Proc. Malac. Soc., vol. viii, pp. 247 and 374. 
2 Phil. Mag., 1907, p. 381. 
