PROF. J. JOLY ON THE GENESIS OF PLEOOHliOIC HALOES. 
79 
point out that discrepancy between results obtained on radioactive and geological 
data in the evaluation of geological time would disappear if the present views have 
any foundation. 
It is quite evident that we have at present no source but the halo from which 
information of the kind we are discussing can be obtained. At this early stage in 
the study of haloes, it may seem premature to offer such far-reaching suggestions as 
I have ventured upon above. The answer is obvious. The incentive to careful 
study of haloes formed in rocks of different ages increases with the issues at stake. 
Such studies are very desirable. If, in recent materials, the originating uranium 
halo could be found, we might at once decide whether the Silurian or Devonian haloes 
we have been considering showed a real discrepancy between existing and past 
ranges of the a-rays, or whether we must seek some other explanation of the observa¬ 
tions. My own efforts to apply this criterion have so far not been successful, 
although a large number of micas have been examined. The reasons seem to be 
that very small nuclei alone afford the requisite definition. Now small nuclei contain 
little radioactive material, and, consequently, either because the time is too short 
or the rate of decay is now too slow, a uranium-bearing nucleus in a recently formed 
mineral may rarely or never afford the embryonic halo. In tertiary granites—as, 
for instance, that of the Mourne Mountains—the haloes lack the detail and delicacy 
of the more ancient haloes. The nuclei are large and the darkening around them 
fuzzy and indefinite. Here, however, the nature of the medium is partly to blame. 
Similar discouragement has been encountered in other recent materials ; but it may 
be that further search may be rewarded with haloes which will reveal decisive 
evidence on the points at issue. 
DESCRIPTION OF PLATE. 
Fig. 1.—Uranium halo. First stage. 
Fig. 2.—Developing uranium halo. Second stage, showing Rings 1, 2, 3 and 4. 
Fig. 3.—Uranium halo. Third stage, showing development of third ring. 
Fig. 4.—Uranium halo. Fourth stage. 
Fig. 5.—Reversed uranium halo. 
Fig. 6.—Radium emanation halo. First stage. 
Fig. 7.—Thorium halo. First stage. 
Fig. 8.—Thorium halo. Second stage. 
Fig. 9.—Thorium halo. Third stage. 
