1908] on Radio-active Changes m the Earth. 149 



whether the granite of Cornwall can be supposed to furnish the 

 uranium of pitch-blende, it is, of course, fundamental to know 

 whether any uranium is present in the rock. It should be said, by 

 way of preface, that the quantity must, at best, be very small, and 

 certainly too small for detection by the methods of chemical analysis 

 as ordinarily applied. We have seen that uranium in nature is 

 invariably accompanied by a proportionate quantity of radium, and 

 as it is in practice much easier to detect minute quantities of radium 

 than to detect the corresponding quantities of uranium, it is best to 

 look for the former only, and to be content to infer the presence of 

 the latter. 



1 have made a large number of experiments to find out how 

 much radium there may be, not only in Cornish granite, but in a 

 large variety of other rocks. In every case the presence of radium 

 has been established, though only to the extent of about one-millionth 

 part of what is found in pitch-blende, and even that, it will be 

 remembered, is not much. If we take into account the very large 

 bulk of the granite and the very small bulk of the pitch-blende 

 veins running through it, there is no difficulty in admitting that the 

 granite was capable of supplying the radio-active material of the 

 pitch-blende. 



Granite, of course, consists of a variety of different minerals, 

 which give it its mottled appearance. These minerals, there is no 

 reason to doubt, have been formed in the successive stages of crystal- 

 lisation of an originally molten mass. There is a mineral called 

 zircon, of which the jacinths sometimes set by jewellers are a variety, 

 which is present in very minute crystals in granite. These minute 

 crystals of zircon have a very characteristic geometrical shape : a 

 square prism terminated at each end by a pyramid, as you see in the 

 photograph. The fact that they have this perfect shape is a proof 

 that they have been perfectly free to assume their natural form, and 

 have not been hampered for want of space by other minerals sur- 

 rounding them. The inference is plain that zircon has been one of 

 the first minerals to crystallise in the consolidation of granite. 



I have found that this zircon is very much richer in radium than 

 the granite generally, though, on the other hand, it is poor compared 

 with pitch-blende. It seems clear that the minerals which crystallise 

 first take an unfair share of the radio-active elements, leaving the 

 rest of the magma impoverished. 



In the light of this observation, Professor Joly, of Dublin, has 

 been enabled to explain a curious appearance which is seen when a 

 section of the granite thin enough to be transparent is examined 

 under the microscope. You see upon the screen one of Professor 

 Joly's photographs of this appearance. This is a minute crystal of 

 zircon, which is imbedded in a large crystal of mica. You will 

 observe that the material surrounding the zircon for a definite dis- 

 tance outwards has become darkened in colour. The altered region 



