As studies with mass spectrometers succeeded in determining the ages of rocks, investigators began to use them in searches for “isotopic signatures” that they hoped would indicate the sources of magmas. Problems had been encountered in dating wherein the data for the daughter isotope did not conform to the linear relationship with the parent that defined an isochron, clear evidence that the sample had undergone some kind of alteration between the initial solidification of the rock and its collection. Obviously, such alteration would preclude the use of isotopes to identify magma sources as well as falsify ages, so it became imperative to understand what the processes of alteration were and how they affected data. As a secondary matter, the alterations provided some information about the history of the rock during this intermediate period. Because parent and daughter isotopes are different elements, heat, water and pressure can cause their differential diffusion into or out of the various crystals that compose a rock. The mobility of such atoms is enhanced by partial melting, the melting of the crystals in a rock assembly that have lower melting points than their neighbors. Thus research emphasis shifted from pure chronology to searching for signatures of magma sources and to understanding the processes that could confuse the observer. Year Book 62, the year after Stanley R. Hart joined the Department, described the group's efforts as “Geochronology and Isotope Geology,” and in the following Year Book the reference to dating was omitted.