Nickel ores occur where there was an unusual segregation of the element in the igneous rock at the time of solidification. In some places further concentration by weathering has been necessary to make an ore of the rock. In still other places, the concentration by weathering has not been sufficient to make the nickel valuable in itself, but has raised the iron content of the rock, and with it the nickel content, to such an extent as to make the ore valuable in the first instance as an iron ore and of additional value because of its nickel content. Nickel is unique among the less rare metals in that a single district contains a quantity of accessible, workable ore, far ahead of that in all other known deposits of the world combined. This is the Sudbury district of Ontario. The nickel ores there were segregated from an enormous mass of igneous rock intruded under conditions favorable for broad segregation and subsequently so eroded that large ore bodies are accessible. In those nickel ores and nickeliferous iron ores where weathering has been a necessary agent in concentrating the nickel and iron enough to make them commercially valuable, the original deposits were nickeliferous igneous rocks--that is, they were similar to the rocks associated with the nickel deposits of the sulphide type, but contained much less nickel. The nickel-ore deposits of the New Caledonia region, which rank next in productiveness to those of the Sudbury district, are of this type. The Cuban nickeliferous iron ores best exemplify iron ore deposits of this type. GEOGRAPHICAL DISTRIBUTION The nickeliferous ore deposits of the world may be divided into two main types--the sulphide type, in which weathering has not been of prime importance, and the garnierite or lateritic type, in which weathering has altered and concentrated the nickel as well as the chromium and iron of the original rock. In addition, nickel occurs in some places with ores of precious and semi-precious metals in veins. Nickel may be recovered from such ores as a by-product, but the ores are never mined primarily for their nickel content. The brief descriptions of the known deposits, grouped according to type, which follow, have been taken from the descriptions of the world’s nickel deposits in the report of the Royal Ontario Nickel Commission. A few direct quotations from the text of the report are given.