654 



Profs. J. Joly and E. Hutherford on the 



shape, and the mean diameter is taken as that of the equi- 

 valent sphere. In No. 30 the nucleus is rectangular in out- 

 line and is some distance down in the mica. It is apparently 

 a flat, rectangular crystal ; but its volume has been calculated 

 on the assumption that it is cubical. It was easier to photo- 

 graph this halo owing to the fact that the cleavage flake in 

 which it is contained is so thin as only to be of a pale straw- 

 yellow. Notwithstanding its thinness the halo is not less 

 than twice as dark as spot A. 



No. 

 1 



Diam. of 



nucleus. 



cm. X 10 5 . 



Mass of 



Zircon. 



grm.X 10 14 . 



Radius 

 of Halo, 

 cm. X 10 4 . 



Comparison of Halo with 



intensity of spots 



A, B, or C. 



No. of 



et-rays for 



whole halo. 



xio- rf . 



Mass of 

 trans- 

 formed U. 

 grm.X 10 15 . 



Age, . 

 years. 1 



10 



243 



18 



Dark as 0. 



30 



14 



400X101 



2 



38 



13481 



20 



Nearly opaque. 



9-3 



43 



> 20 „ 



3 



16 



990 



16 



Darker than A. 



59 



28 



>200 „ 



4 



12 



418 



16 



>> ?> 



59 



28 



>470 „ 



5 



33 



8800 



16-5 



>) ;» 



64 



30 



> 30 „ 



6 



37 



12320 



16-5 



)) ?> 



64 



30 



> 20 „ 



7 



20 



1980 



16 



Between A and B. 



4'2 



20 



80 „| 



8 



23 



3080 



16 



Very much darker than A. 



59 



28 



^■60 „ 1 



9 



12 



418 



16 



Dark as B. 



2-6 



12 



200 „ 



10 



14 



660 



16 



Darker than A. 



5-9 



28 



>270 „ . 



11 



21 



2420 



16 



Much darker than A. 



5-9 



28 



^90 „ I 



12 



22 



2640 



16 



Darker than A. 



5-9 



28 



> 80 „ 



13 



26 



4400 



16 



Very much darker than A. 



5-9 



28 



^ 50 „ 



14 



13 



506 



16 



Dark as A. 



5-9 



28 



390 „ 



15 



30 



6600 



16 



Very much darker than A. 



5-9 



28 



^ 30 „ 



16 



17 



1210 



16 



Darker than A. 



5-9 



28 



>160 „ 



17 



25 



3740 



16 



Very much darker than A. 



5-9 



28 



^ 50 „ 



18 



15 



792 



16 



Dark as A. 



5-9 



28 



250 „ 



19 



27 



4840 



18 



>5 >> 



7-4 



35 



50 ,1 



20 



27 



4840 



17 



5) )> 



6-8 



32 



50 „| 



21 



24 



3388 



17 



Darkness=Ax2. 



136 



64 



1-10 „ 



22 



26 



4400 



17 



Darkness=Ax7/3. 



15-8 



75 



120 „ 



23 



23 



3080 



17 



5> }> 



15-8 



75 



170 „1 



24 



57x18x18 



8580 



17 



Dark as A. 



6-8 



32 



30 ,1 



25 



48x32x32 



22000 



17 



Darkness=Ax2. 



136 



64 



20 „1 



26 



40x22x22 



9020 



17 



Dark as A. 



6-8 



32 



30 „ 1 



•27 



12-5 



462 



17 



Dark as 0. 



2-7 



13 



200 „ 



28 



21 



2420 



18 



Dark as A. 



7-4 



35 



100 „ 



29 



29 



5984 



16 



Darker than A. 



59 



28 



> 40 „ 



30 



21X21X21 



5720 



19 



Darkness about Ax2. 



16-5 



78 



100 „ 



It must be borne in mind that the higher values of the 

 age are those of most significance. Low values of course are 

 deduced whenever the mass of the nucleus is large and the 

 surrounding halo faint. Although the darkness of the halo 

 generally varies with the size of the nucleus^ there is no 

 definite proportion of uranium in the zircon or other minerals 

 forming nuclei, and cases of apparent low age are plentiful. 

 They are explained if we suppose that in these cases our 



