<0 Mesozoic (?) Basic Intrusives Peridotites, dunites, and pyroxenites in all stages of serpontinization are found in the map-arca, Associated with thom are less basic gebbroic, diabasic, and dioritic differentiates, These basic intrusives normally have gradational contacts with one another. Arcas of greenstone and amphibolite, up to 5 square miles in extent, thought to be of Carboniferous age, are so intimately intruded by basic rocks that they could not be mapped separately, Two large areas of these basic intrusives are present in the map-aree, One, covering on arca of 65 square miles, extends northward from Cunningham Lake to near Trombleur Lake, and the other with an arcal oxtont of 160 square miles includes tho greater part of the Middle River Range, and extends northwost from Trembleur Lake to the northern extremity of the mapearea, A body of peridotite 80 square miles in areca underlies the Mowmt Williams group of mountains and extends southward to Trembleur Lako. Another large body of peridotite, 40 square miles in extent, lies between Stuart ond Cunningham Lakes, Bordering this poridotite mass aro three areas of pyroxcnite and gabbro, ranging in size from 5 to 10 square miles cach. The largest body of gabbro and diabaso is 35 square miles in cxtent and underlics Deescius and Tsitsutl mountains. Several bodics of peridotite up to 2 square miles in area occur in this gabbro, diabase body. The peridotites are dark green, medium=grained rocks, that weather to bright orange and reddish brow. They aro composed of olivine, with a minor amowmt of pyroxene, the pyroxene crystals standing out on the weathered surface. Tho dunites form irregular masses, $ to 2 square miles in size, in the peridotitos, They are dark green to black, medium=grained rocks composod of olivine, The pyroxenites are coarse-grained rocks and are composed essentially of pyroxene, although they contain some basic feldspar and grade into gabbro, They aro dark green and weather to brownish and reddish tints, The peridotites, dunites, and pyroxenites have all undergone partial or complete alteration to serpentine, and it is in many cases impossible to ascortain the original composition of the rock, The serpentine is olive or omerald green and is normally of the varicty antigorite, although veins of the colwmar variety picrolite are found in some localities,