mineral substance as composes the nucleus, guess at the amount of
uranium, or it may be thorium, which may be present.

This method has been applied to the uranium haloes of the mica of
County Carlow.[1] Results for the age of the halo of from 20 to
400 millions of years have been obtained. This mica was probably
formed in the granite of Leinster in late Silurian or in Devonian
times.

The higher results are probably the least in error, upon the data
involved; for the assumption made as to the amount of uranium in
the nuclei of the haloes was such as to render the higher results
the more reliable.

This method is, of course, a radioactive method, and similar to
the method by helium storage, save that it is free of the risk of
error by escape of the helium, the effects of which are, as it
were, registered at the moment of its production, so that its
subsequent escape is of no moment.

[1] Joly and Rutherford, _Phil. Mag._, April, 1913.

22

REVIEW OF THE RESULTS

We shall now briefly review the results on the geological age of
the Earth.

By methods based on the approximate uniformity of denudative