To fully understand Gentry's hypothesis a basic background in geology, mineralogy, and radiation physics is helpful.
The boxes on the next few pages present a brief tutorial in rocks, minerals, and radioactivity.
Geologists classify rocks into three main categories - sedimentary, igneous, and metamorphic - based on the way in which they form.
Sedimentary rocks are secondary in formation, being the product of precursor rocks (of any type).
Through the cycles of burial, folding, faulting, and subduction of crustal plates, rocks get pushed and dragged down to depths where - under heat and pressure - changes take place.