Thesé serpentinized rocks aro commonly traversed by numerous shearing planes along which light groon serpentine has been developed. Between the shearing planes the rock is broken and rounded into ollipsoidal fragments, = inch to 3 feet in diameter, each cllipsoid consisting of dark green serpentine free from shearing. This feature is best scon on the weathered surfaccs.e In the area lying botween Trembleur Lake and Mount Williams about 10 per cont of the serpentinized peridotites have been altered to carbonate-quartz-mariposite, and carbonate-tale rocks. These altered rocks occur in fractured zones ranging in width from 10 to 500 fect, separated by zones of wmaltored serpentinized peori- dotites of approximately the samo width. The largest quantity of such rocks was observed alorg Mount Williams ridge whore they were traccd for more than 2 miles, The carbonate-quartz=mariposite rocks are buff to yellow- browm, and medium to coarsely crystalline. They are composed of buff-coloured ankeritic carbonate, quartz, and the bright green chrome=bearing mica mariposite, crroncously thought by many of the prospectors in the region to be a copper mineral. These rocks are intersected by a network of quartz stringers. The carbonate-tale rocks are less abundant than the carbonate-quartz=emariposite rocks, They have a groasy feeling, are grconish buff in colour, and are composed of 40 per cont ankcrite carbonate, and the romaining 60 per cent of tale. The high por cent of tale imparts to them a dense appearance. Both types of altered rocks contain specks of pyrite and chalcopyrite. The gabbros and diabase are mcedium-to coarsce-graincd, dark greon rocks consisting essentially of pyroxens (dinllage) and a basic plazioeclase feldspar. Tho pyroxene is usually partly altered to hornblende, and the feldspar to zoisite and sausserite. The amphibolites are dense to fine-grained, pistachio to dark green rocks composed of hornblende, epidote, and sausscrite. They normally have a crude foliation. rocks, In the Middle River Range the basic intrusives are cut by a pody of granite, and south of Stuart Lake they are cut by stocks of augite diorite, and syenite. In no place wore the basic intrusives seen to cut the Mesozoic (?) acid intrusives or younger rocks. The Mesozoic (7?) basic intrusives intruce the Carboniferous