Age of Pleocliroic Haloes. 655 



assumption as to their uranium content is excessive. What 

 is significant is the fact that in many cases we can find 

 nuclei so small surrounded by well darkened haloes. 



In many examples given in the table the age must exceed 

 that which is given in the last column. This is so because 

 only in some of the last observations, and in those in which 

 the halo happened to match closely the staining of one of the 

 spots, is there any numerical evaluation of the relative dark- 

 ness of spot and halo. The results are numbered in the order 

 in which they were obtained. The highest of the measure- 

 ments point to an age not less than four hundred millions 

 of years as the time required to generate these haloes : in 

 other words, as the age of the early Devonian. The treat- 

 ment of the subject throughout this paper has been such as 

 to render this a minor limit. 



Sources of error must now be inquired into : — 



(a) The nuclei are the complete nuclei. The possibility 

 that the nuclei measured were detached fragments of larger 

 nuclei, the greater part being cleaved oft' in opening the 

 mica, was investigated by comparing the contiguous faces of 

 cleaved flakes and making sure that the part of the halo- 

 sphere removed contained nothing that could be regarded as 

 part of the nucleus, or that could suggest the loss of any part 

 of it. Not a single case of a divided nucleus was met with. 

 The fact is the cleavage of zircon is very imperfect, and even 

 if it lay just at the surface of cleavage it would almost 

 certainly remain bodily in the one flake or the other ; or, 

 possibly, drop out. Again, many of the observed nuclei 

 were demonstrably beneath the surface of the flake. This 

 was rendered quite certain by observations on thick cleavage 

 flakes, using a very strong light. 



(b) The crystallographic direction in which a part of the 

 rays traverses the mica in the case of the halo, is not the same 

 as that in which the rays move in the mica when generating 

 the experimental darkening. The fact, however, that in 

 sections of haloes there is uniform darkening in all crystallo- 

 graphic directions, negatives the idea that error can arise 

 from this source. 



(c) Nothing is known of any mode by which the mere 

 passage of time can intensify a halo or increase the effects of 

 the original ionization. In the case of light-sensitive salts 

 the ionization set up by exposure appears to weaken by the 

 passage of time. No intensification of the latent image is 

 known. A recombination of the products of dissociation and 

 weakening of the halo with time would rather be expected. 

 The halo is probably very stable. 



