The following observations are the result of an attempt to separate and study such heavy minerals as occur in the sediments, mainly impure limestones and shales, of Upper and upper Middle Ordovician age, in Ontario. The specimens studied are from three localities: Lake Ontario, Georgian Bay, and Manitoulin Island. As was expected, many of the sediments contain very few heavy detrital minerals, but some, notably the Meaford and Dundas formations, gave good results. The characteristics of the heavy minerals are described. In the Dundas a difference is noted between the heavy minerals near Lake Ontario and those at Manitoulin Island, the latter showing characteristics of a nearby pre-Cambrian source. Marked similarity in the heavy minerals of the Meaford and the Dundas of the Lake Ontario district is noted. The presence of authigenic feldspar in the Dundas hard bands is recorded.

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