The New Quebec Crater is a circular depression in the acid Archaean gneisses of northern Quebec, about 2 miles in diameter and 1 300 ft deep. The crater is roughly bowl-shaped, with a prominent rim elevated 300 to 500 ft above the surrounding barrens. The rim structure is a relatively recent deformation resulting in systematic outward dip of rock sheeting, and radial distribution of other curviplanar structural elements. Rocks of the rim are strongly mineralized with epidote and hematite, and are altered to sericite. These minerals are not found in the same rock units outside the rim. Glaciation of the rim appears to have removed little or no material. No evidence of impact was found.The crater may have arisen either by extraterrestrial impact or by volcano-tectonic collapse. The impact theory is implausible because of absence of distinctive criteria of shock and failure to explain the characteristic structure and mineralization of the crater. The volcano-tectonic hypothesis explains the structural and petrographic evidence but appeals to a type of volcanism not presently observed in active volcanic areas. Geophysical measurements are not likely to resolve the dilemma. The diagnostic evidence lies in the rocks hidden beneath the crater, concealed by 800 ft of water.