Biotite granite and gneiss are most prevalent in the northern and eastern portions of the area. No microscopic examination of these rocks has been made by the writer, consequently, little can be said regarding their composition. They are of much the same texture as the hornblende type, but show a somewhat higher degree of gneissification. Biotite is a fairly abundant constituent, but is more or less altered to chlorite. Sometimes, as may be seen on the East branch, two miles below Obushkong lake, chloritized mica forms enclosing films about the large feldspar grains, developing a slight ‘augen’ structure. Stockwork-like quartz veins are frequent, particularly on the West branch between Near and Sedge lakes and at the south end of Kenisheong lake. At the latter point the biotite gneiss is well foliated and steeply tilted. A body of deep red, biotite granite of undetermined extent and relationship was observed on the east side of Kenisheong lake. Pegmatite dikes almost certainly occur in this region, although not actually observed. Near Crotch lake the granite is locally of unusually coarse texture, although not truly pegmatitic. Although the two granites described appear to be the essential constituents of the Laurentian they usually contain a variable proportion of other material, some of which at least is not really Laurentian. Over the entire area, but more noticeably in the vicinity of contacts with the Keewatin, they include narrow ribbons or lenses of a glistening stratiform hornblende gneiss, closely resembling the gneiss of this sort described under the Keewatin system. In some cases these bands are portions of the Keewatin caught up by the Laurentian material at the time of its intrusion and highly metamorphosed by it. Excellent examples of this may be seen on the East branch at the foot of the marshy stretch two miles below Obushkong lake. These inclusions are mapped as Laurentian, being an almost constant feature, and for map purposes inseparable from it. They are to be distinguished—a difficult matter—from other dark inclusions believed to be drawn out, basic segregations of Laurentian magmas; the latter are commonly less sharply defined. Like the other formations of the region, the Laurentian is cut by diabase dikes of post-Huronian age which, owing to their dark colour, are conspicuous among the lighter granite rocks. In the neighbourhood of Zigzag lake they are abundant, and are usually from twenty to sixty feet or more in width, the smaller of which are not large enough to map.