The passage of time can be charted by the reduction in the number of parent atoms, and the increase in the number of daughter atoms.
You cannot predict exactly when any one particular grain will get to the bottom, but you can predict from one time to the next how long the whole pile of sand takes to fall.
Once all of the sand has fallen out of the top, the hourglass will no longer keep time unless it is turned over again.
Similarly, when all the atoms of the radioactive element are gone, the rock will no longer keep time (unless it receives a new batch of radioactive atoms).
The rate of loss of sand from from the top of an hourglass compared to exponential type of decay of radioactive elements.
Wiens has a Ph D in Physics, with a minor in Geology.
His Ph D thesis was on isotope ratios in meteorites, including surface exposure dating.It has become increasingly clear that these radiometric dating techniques agree with each other and as a whole, present a coherent picture in which the Earth was created a very long time ago.Further evidence comes from the complete agreement between radiometric dates and other dating methods such as counting tree rings or glacier ice core layers.When the glass is turned over, sand runs from the top to the bottom.Radioactive atoms are like individual grains of sand--radioactive decays are like the falling of grains from the top to the bottom of the glass.He was employed at Caltech's Division of Geological & Planetary Sciences at the time of writing the first edition.