For example, isotopes with very long half lives are no good for dating rocks younger than about 100 million years.
This is because, in just 100,000,000 years of time, not enough parent will have decayed for daughter concentrations to be reliably measured.
He calculated the modern rate of salt delivery to the oceans, and suggested that the present salinity of ocean water would take at least 100 million years to develop.
In the 1860's, English physicist Lord Kelvin disagreed with Charles Lyells proposition that the earth behaves in a uniform, unchanging manner.
Different isotopes of an element have similar chemical properties (undergo similar chemical reactions) but have different physical properties (such as evaporation rates).