Chemical analysis of samples of the Older Granodiorite, Co. Donegal, Ireland, shows that it has a geographical variation in which Si, K and O increase, Na is about constant and all other major elements decrease in amount from east to west. There is a high degree of positive linear correlation between Mg and Fe2 + Fe3 which implies derivation of these elements from some previously existing ferromagnesiam mineral in which the appropriate ratio was maintained. There is a low degree of negative linear correlation between Na and K which implies that these elements are mobile and independent of each other.

The chemical relationships of the granodiorite to its pelitic country rocks are shown and discussed. The relations are such that the preferred hypothesis for the origin of the granodiorite is one of influx of Na, K and Ca ions in an alumino-silicate medium. Country rocks are diluted by this influx and it is unnecessary to assume any great migration of Mg and Fe. With time the nature of the influx changes—Ca decreases in amount whereas Na, K and Si all increase but K and Si to a very much greater extent than Na.

The members of the Rosses Granitic Ring Complex have been chemically analyzed. The data are presented as averages with 95 per cent confidence limits for individual elements. A limited amount of crystallization differentiation is demonstrated when the major units—the biotite microgranite sheets, the porphyry dykes, the ring granites and the grey and pink microgranite sheets—are compared with one another. This involves an increase in Si, K, a decrease in Ti, Al, Fe, Mg, Ca and P, but Na (and Mn) remain constant. The ring granites G1-G4 undergo very little differentiation and variations in their composition, except for Si, are ascribed mainly to heterogeneity.

The origin of the magma which gave rise to the complex is due to the palingenesis of the Older Granodiorite under the influence of a Na, K, alumino-silicate influx. This magma was the source of the biotite microgranite sheets and, after differentiation, the granites G1 and G2, together with some of the grey and pink microgranite sheets. After a period of quiescence a second influx produced the similar magma which was the source of the porphyry dykes and, after differentiation, gave rise to the granites G3 and G4, the Newer Granite bosses and the late grey and pink microgranite sheets.