Abstract: Evidence that the extrusion history involves a younger and an older phase lies in the difference inbulk chemical composition of certain series of glassy rocks, such as perlite, rhyolite from the Cumaovasi domes.Frequency distributions constitute evidence for two groups of rocks on the Cumaovasi domes. The most obviousdifference is in the Rubidium content, one group containing 195--230 ppm Rb the other 300-350 ppm. All of the samplesthat possessed low Rb content were obtained from the Murtat, Mezarkaya, Karakaya, Yongah and Gülyaka, and allthe high Rb samples from the Çamköy, Dikmendağ, Karadağ- and Pilâvtepe.The cluster analysis demonstrate the presence of two entirely different groups of samples. For group I the meansfor A12O3, TiO,,,, MgO, CaO and K2O are higher than in group II, the means for MnO, Na_O and Rb are higherthan in group I.This difference strongly suggests that, if the Cumaovasi domes are the products of granitic magma chamber, thedifferences in composition of the rocks are the resault of magmatic differentiation.