EXPLANATION OF PLATE III a. 



Fig. 



1. Eadium haloes in cleavage plate of biotite (Oo. Carlow) ; enlarged about 



76 diameters. Two overlapping haloes are present, as well as a few 

 under-exposed haloes. 



2. A radium halo (lower right-hand part of the field) and a thorium halo (upper 



left-hand part) in brown mica in a granite. The mica is cut across the 

 cleavage. Enlargement about 114 diameters. The thorium halo shows 

 an inner sphere due to Th X. The ratio of the diameters of inner and 

 outer spheres will be found to be as 2-6 : 4-0. (See p. 83.) 



3. Eadium haloes in Biotite (Oo. Carlow) x about 95 diameters. The more 



intense ionization within the sphere due to Ea A is conspicuous in many of 

 them. Careful scaling will show that the ratios of the diameters of the 

 inner and outer spheres are as 23 : 33. (See p. 83.) Seen on cleavage. 



4. A single radium halo from the Carlow biotite. It is enlarged to about 500 



diameters. The inner dark disk is due to emanation. The Ea A sphere 

 succeeds and appears to be less developed than that of Ea C. Viewed on 

 cleavage. 



5. Developing Eadium haloes in Carlow biotite. Magnification about 76 



diameters. Various stages of development are shown. Four haloes are in 

 the field. Viewed on cleavage. 



6. Same as fig. 5. Two haloes in very different stages of development are 



present. The faint early indications of Ea C in this and the last 

 photograph are delicately shown. Viewed on cleavage. 



