24 | THE ROYAL SOCIETY OF CANADA amorphous serpentine, and 10 per cent or less of colourless zoisite, chromite, and secondary carbonate with a few scattered grains of unaltered pyroxene. The flakes of antigorite are usually arranged in a rectangular pattern ‘characteristic of serpentine derived from pyroxene, but unfortunately this is not conclusive evidence that the original mineral was pyroxene as serpentine derived from olivine may show a similar arrangement in advanced stages of alteration. The reasons for considering the serpentine surrounding areas of unaltered pyroxenite to be derived from that rock are two: (1) The contacts of the serpentine rock with the pyroxenite are not sharp but gradational across an interval of a foot or more. In this distance the pyroxenite can be seen to become gradually more and more serpentinized; and the outlines of the larger pyroxene crystals are preserved within the serpentine for distances of ten feet and more from the unaltered pyroxenite boundary. The above relationship was best observed in the eastern portion of the ultrabasic mass between Cunningham and Stuart Lakes. In the western part of the same mass a student assistant observed outcrops of pyroxenite in a matrix of serpentine. (2) Thin sections of the serpentine show grains of unaltered pyroxene. As pyroxene grains in all the thin sections of peridotite examined are more altered than those of olivine, unaltered olivine grains might be expected if these rocks had been peridotites. None was observed. Hence it is concluded that no olivine was originally present, and the rock, accordingly, must have been pyroxe- nite. In the western and eastern extensions of the ultrabasic mass between Cunningham and Stuart Lakes, gabbroic rocks occur around the borders of the pyroxenite areas. They are medium-grained, dark green, and composed of 25 per cent or less of labradorite and 75 per cent or more of augite. The large gabbro body underlying Deescius Mountain and the small body cutting the peridotite near the summit of Mount Williams are medium- to coarse-grained dark green rocks. They contain from 20 to 40 per cent labradorite which is turbid as a rule with grains of clinozoisite, paragonite, and calcite. Diallage, the principal mafic mineral, is commonly altered to uralite and chlorite. In two thin sections a few grains of olivine were observed. In many places the gabbro body underlying Deescius Mountain has an ophitic texture. These gabbros differ from the gabbroic rocks at the borders of the large ultrabasic masses in their higher proportion of feldspar and ophitic texture. The ultrabasic body between Stuart and Cunningham Lakes consists of a central core of peridotite approximately 50 square miles