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of rather less than 2 x 10-4 cms. diameter.  Such nuclei would expel one ray in five years.  And even lesser nuclei will generate in these old rocks haloes with their earlier characteristic features clearly developed.  In the case of the most minute nuclei, if my assumption as to the uranium content is correct, an alpha ray is expelled, probably, no oftener than once in a century; and possibly at still longer intervals.

The equilibrium amount of radium contained in some nuclei may amount to only a few atoms.  Even in the case of the larger nuclei and more perfectly developed haloes the quantity of radium involved is many millions of times less than the least amount we can recognise by any other means.  But the delicacy of the observation is not adequately set forth in this statement.  We can not only tell the nature of the radioactive family with which we are dealing; but we can recognise the presence of some of its constituent members.  I may say that it is not probable the zircons are richer in radium than I have assumed.  My assumption involves about 3 per cent. of uranium.  I know of no analyses ascribing so great an amount of uranium to zircon.  The variety cyrtolite has been found to contain half this amount, about.  But even if we doubled our estimate of radium content, the remarkable nature of our conclusions is hardly lessened.

It may appear strange that the ever-interesting question of the Earth’s age should find elucidation from the

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study of haloes.  Nevertheless the subjects are closely connected.  The circumstances are as follows.  Geologists have estimated the age of the Earth since denudation began, by measurements of the integral effects of denudation.  These methods agree in showing an age of about rob years.  On the other hand, measurements have been made of the accumulation in minerals of radioactive debris—­the helium and lead—­and results obtained which, although they do not agree very well among themselves, are concordant in assigning a very much greater age to the rocks.  If the radioactive estimate is correct, then we are now living in a time when the denudative forces of the Earth are about eight or nine times as active as they have been on the average over the past.  Such a state of things is absolutely unaccountable.  And all the more unaccountable because from all we know we would expect a somewhat lesser rate of solvent denudation as the world gets older and the land gets more and more loaded with the washed-out materials of the rocks.

Both the methods referred to of finding the age assume the principle of uniformity.  The geologist contends for uniformity throughout the past physical history of the Earth.  The physicist claims the like for the change-rates of the radioactive elements.  Now the study of the rocks enables us to infer something as to the past history of our Globe.  Nothing is, on the other hand, known respecting the origin of uranium or thorium—­the parent radioactive bodies.  And while not questioning the law