Whether there is also a natural fractionation and improvement of the first heavy oils as they undergo repeated migrations is not known.
"The Economic Aspect of Geology"
C. K. Leith
The big varieties came first, the little ones arose later by fractionation, although now the devotees of continuity could arrange them in a graduated series from white to deep purple.
"Hormones and Heredity"
J. T. Cunningham