=Deposits of the Sulphide Type.=--The Sudbury district is situated in southeastern Ontario, _Canada_. In its broader outlines the geology of this district is relatively simple. An immense mass of nickeliferous rock was intruded as a “laccolithic sheet” or sill along an unconformable plane of contact between flat-lying sediments and an underlying complex of ancient rocks. During the intrusion and cooling, or perhaps soon thereafter, the underlying rocks of the central part of the laccolith, covering the reservoirs from which the magma came, subsided. Long periods of erosion then planed down the region until all that is left of the sill occupies a synclinal basin nearly 40 miles long and 10 to 15 miles wide. All of the rocks involved are of pre-Cambrian age. The laccolithic sheet is approximately 10,000 feet thick. It differentiated on cooling into two kinds of rock--micropegmatite, a rock of the granite group, now forming the upper part of the sill, and norite, a gabbro rock, the lower part. The gradation between these two rocks is rather abrupt. At the bottom of the sill, in some places lying between the norite and the underlying rock and at other places entirely within the underlying rock, are bodies of ore consisting of pyrrhotite, pentlandite and chalcopyrite. These are segregated products of the norite, which in some places solidified at the base of the sill and in others were intruded as dikes in the underlying rocks or in previously solidified portions of the norite. They constitute commercial ore bodies where the sulphides form a preponderant part of the rock. The ore bodies are classified as “marginal” and “offset.” The marginal deposits occur along the contact of the norite with the underlying rock. Frequently they lie entirely within the rocks adjacent to the norite. The offset deposits occur where faults cut across the limbs of the fold, forming zones of weakness into which ore or mineralized norite was intruded. The nickel-bearing mineral is pentlandite; the copper-bearing, chalcopyrite. The other sulphide, pyrrhotite, is a sulphide of iron. The ores mined to date average roughly 3.5 per cent. nickel and 2 per cent. copper. From the opening of the district in 1887 to the end of 1916 nearly ten and one-half million tons of Sudbury ore had been mined and smelted; and, from this, about 285,000 tons of nickel had been produced. It is estimated that there are probably fully 100,000,000 tons of ore reserves. Over a million and a half tons of ore were mined and smelted in 1916.