[ 51 ] 
III. The Genesis of Pleochroic Haloes. 
By J. Joly, Sc.D., F.R.S., Professor of Geology and Mineralogy in the University 
of Dublin. 
(Received June 1,—Read June 29, 1916.) 
[Plate 1.] 
Introduction. 
In an earlier paper on the subject of pleochroic haloes^ it was pointed out that 
several features of the halo derived from the uranium family of radioactive elements 
were not easily accounted for. This fact led me to undertake a more careful study of 
haloes. But, although many observations and measurements were made, until recently 
my attempts at obtaining solution of the difficulties were too crude to merit 
publication. They were more especially hampered by the scarcity of haloes derived 
from the thorium family of elements. I could ’find out nothing as to the mode of 
origin of these haloes, and, on the observations available, it even appeared as if there 
were some unaccountable difference in the course of development of the haloes derived 
from the uranium and the thorium families. The initial stages of development 
seemed to be entirely absent in the case of the thorium series. This was not 
satisfactory. A few months ago, however, I found in a Yosges granite, a mica which 
was rich in thorium haloes. Some of these haloes appeared in the earlier stages of 
development; stages corresponding to the earliest in the development of the uranium 
halo. This find showed that the same course was followed in the genesis of both 
kinds of halo. 
The find was also important in another respect. Certain small discrepancies 
between the observed and the theoretic dimensions of the uranium halo had been 
forced on me as the measurements became more refined. This, of course, led me to 
distrust the basis upon which I was going in attempting to define theoretic dimensions. 
But in the developing thorium halo it would appear as if the basis of my predictions 
was uniformly supported. This seems to show that there is something anomalous, 
according to our existing knowledge of the ranges, in the early development of the 
uranium halo. 
' There was another difficulty which I have only lately been able to clear away. 
There were found to exist in the Leinster granite of County Carlow a very large 
number of embryonic haloes having dimensions uniformly and distinctly greater 
than those which I had been led to regard as the initial form of the uranium halo. 
And these larger haloes showed not the least trace of the existence of the lesser 
* ‘ Phil. Mag. ’ April, 1910. 
VOL. CCXVII.—-A 551 . 
I 
[Published January 29, 1917. 
