318 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
to be remarkably uniform, in some cases four-hundredths of a milli- 
meter and in others three-hundredths, about. These are the meas- 
urements in biotite. In other minerals the measurements are not 
quite the same as in biotite. Such minute objects are quite invisible 
to the naked eye. In some rocks they are very abundant, indeed 
they may be crowded together in such numbers as to darken the color 
of the mineral containing them. They have long been a mystery to 
petrologists. 
Close examination shows that there is always a small speck of a 
foreign body at the center of the circle, and it is often possible to 
identify the nature of this central substance, small though it be. 
Most generally it is found to be the mineral zircon. Now, this 
mineral was shown by Strutt to contain radium in quantities much 
exceeding those found in ordinary rock substances. Some other 
mineral may occasionally form the nucleus, but we never find any 
which is not known to be specially likely to contain a radioactive 
substance. Another circumstance we notice. The smaller this 
central nucleus the more perfect in form is the darkened circular area 
surrounding it. When the circle is very perfect and the central min- 
eral clearly defined at its center we find by measurement that the 
radius of the darkened area is generally 0.033 millimeter. It may 
sometimes be 0.040 millimeter. These are always the measurements 
in biotite. In other minerals the radii are a little different. 
We see in the photograph (pl. 1, fig. 1), much magnified, a halo con- 
tained in biotite. We are looking at a region in a rock section, the 
rock being ground down.to such a thickness that light freely passes 
through it. The biotite is in the center of the field. Quartz and 
feldspar surround it. The rock is a granite. The biotite is not all 
one crystal. Two crystals, mutually inclined, are cut across. The 
halo extends across both crystals, but owing to the fact that polar- 
ized light is used in taking the photograph it appears darker in one 
crystal than in the other. We see the zircon which composes the 
nucleus. The fine lineated appeararice of the biotite is due to the 
cleavage of that mineral, which is cut across in the section. 
The question arises whether the darkened area surrounding the 
zircon may not be due to the influence of the radioactive substances 
contained in the zircon. The extraordinary uniformity of the radial 
measurements of perfectly formed haloes (to use the name by which 
they have long been known) suggests that they may be the result of 
alpha radiation. For in that case, as we have seen, we can at once 
account for the definite radius as simply representing the range 
of the ray in biotite. The farthest-reaching ray will define the 
radius of the halo. In the case of the uranium family this will be 
radium C, and in the case of thorium it will be thorium C. Now 
