The crystalline core of the Sawatch Range is composed of coarse schist and gneiss, largely of sedimentary origin. Two distinct periods of batholithic intrusions occurred in pre-Cambrian time. Wide zones of mixed supercrustal and granitic material border the outcrops of the batholiths and show gradational facies on one side into the sedimentary schists and on the other side into the plutonic rocks. From field relations and microscopic study of thin sections these migmatite zones are interpreted as being due to regional granitization or replacement, involving the carrying out and bringing in of material by emanations, gaseous or liquid, from the invading batholiths.