The ultramafic rocks in the Shaw Dome area, southeast of Timmins, Ontario, are divided on the basis of held criteria into two groups for the purpose of comparing their chemical compositions: (1) extrusive ultramafic rocks consisting of peridotitic komatiites and (2) intrusive ultramafic rocks, consisting of dunites and lherzolites.The two groups can be distinguished on a chemical basis by graphical and statistical methods which provide accuracies of extrusive–intrusive classification of about 92 and 96% respectively. Both methods provide qualitative and quantitative aspects of discrimination.The discriminating parameters were developed using samples which show all types and degrees of alteration that are common in ultramafic rocks. Preliminary studies indicate that the genera! procedure for chemical discrimination between extrusive and intrusive ultramafic rocks may be applicable to similar Archean rocks from other areas.In addition, the separation provides an aid to exploration for minerals such as nickel sulphides, asbestos, magnesite, and gold which may be largely restricted to either extrusive or intrusive ultramafic rocks.