Data on the distribution of Ba, Mn, Nb, Rb, Sr, and Zn in granites from the main intrusive units and from greisenized zones of the Rosses complex, northwest Ireland, are reported and interpreted in terms of a process involving progressive partial fusion and partial assimilation of country rock. Greisenization resulted in an increase in quartz and muscovite, decrease in feldspars, and elimination of biotite. Beryl is also present where the greisenization has been severe, and one analyzed specimen, which may be unique, contained 20-30 percent beryl. The greisens also show a general increase in concentration of Rb, Ta, and Mn, and a decrease in Ba and Sr; Zr increases in some and decreases in other specimens. The trace element changes are related to the changes in mineralogy. Chemical analyses are given.