In a report of his findings published in 1913 in the journal , Holmes expressed the less-than-ecstatic reception his findings received: "The geologist who ten years ago was embarrassed by the shortness of time allowed to him for the evolution of the Earth’s crust , is still more embarrassed with the superabundance with which he is now confronted."The Earth's age continued to be hotly debated for decades afterward. In the 1920s, Earth's age crept up toward 3 billion years, making it for a time even older than the universe, which was then estimated to be about 1.8 billion years old.The best estimate for Earth's age is based on radiometric dating of fragments from the Canyon Diablo iron meteorite.
Of course, Kelvin formed his estimates of the age of the Sun without the knowledge of fusion as the true energy source of the Sun.Without this knowledge, he argued that, “As for the future, we may say, with equal certainty, that inhabitants of the Earth cannot continue to enjoy the light and heat essential to their life, for many million years longer, unless sources now unknown to us are prepared in the great storehouse of creation.” This last statement was prophetic.In 1898, Marie Curie discovered the phenomenon of radioactivity, in which unstable atoms lose energy, or decay, by emitting radiation in the form of particles or electromagnetic waves.By 1904 physicist Ernest Rutherford showed how this decay process could act as a clock for dating old rocks.There were indeed powerful and unknown sources of energy fueling the Sun’s energy output.
The same is true of the basis of Kelvin’s estimate of the age of the Earth.The solar estimate was based on the idea that the energy supply for the solar radioactive flux is gravitational contraction.These two independent and agreeing dating methods for of the age of two primary members of the solar system formed a strong case for the correctness of his answer within the scientific community.The study of geology grew out of field studies associated with mining and engineering during the sixteenth to nineteenth centuries.In these early studies the order of sedimentary rocks and structures were used to date geologic time periods and events in a relative way.The answer of 25 million years deduced by Kelvin was not received favorably by geologists.