Many ways of measuring geological time have been attempted by various geologists of eminence; but however diverse their methods, so far as I know, all finally hinge upon either the rates of denudation or accumulation. It is urged as an objection to this that in past ages such actions have gone on more rapidly, and therefore the calculations based upon present rates are valueless. Physicists on the other hand have sought to put a limit to the age of the earth much below what geologists generally demand. Reasoning from certain data which are necessarily more or less hypothetical, they say, that from the thermal condition of the globe at present, it cannot be more than from ten to twenty million years since it was at a temperature in which life on it would have been impossible. Geologists can hardly be blamed if they attach greater weight to their own observations and data and to reasoning that is more familiar and appears more certain and satisfactory to their minds.