This account is the result of mapping and studying the geology of the Cariboo Gold Quartz Mine over a period of 20 months with the ultimate object of defining the ore-controls and so indicating the areas to which major development should be directed. It is believed that this objective has been attained to a large degree, although additional geological evidence that may be uncovered during future work might modify some of the present conclusions.The geological setting of the ore deposits is briefly as follows: The sedimentary country rocks known as the Rainbow Series were severely overturned, intensely drag-folded, and then cleaved. A series of major north-south faults dipping to the east broke the folded rocks into a number of segments, each of which was sliced up by faults parallel to the cleavage. One set of tension fractures developed at right angles to the intense compression during folding and another set perpendicular to the north-south faults were opened up during the later stages of faulting to receive mineralization of pyritic auriferous quartz. The veins so produced are of commercial grade adjacent to the two sets of faults where the fractured ground provided the best channelways for the mineralizing solutions. A third type of vein was formed in strong shear zones developed along or parallel to the axial planes of the major drag-folds.The high-grade replacement ore that has been the mainstay of the neighboring Island Mountain Mine is very limited in its occurrence in the Cariboo Gold Quartz Mine but it is hoped that appreciable amounts will be found in the unexplored ground on each side of Island Mountain Mine.The Cariboo district has yielded about 600,000 ounces of lode gold to date and the indications are that ultimately the total lode production from the known ore-bearing sections and their extensions will equal the placer production of over 2,000,000 ounces of which the Cariboo Gold Quartz Mine should produce three-quarters.