_General Features._ This system is considered as a complex assemblage of metamorphosed igneous rocks whose common and marked characteristics are pronounced alteration and deformation, accomplished in pre-Huronian times. A limited amount of sedimentary material, such as the iron ore formation, is also represented. These old diabases, porphyries and related types are much altered and have developed a more or less uniform schistosity through the secondary development of micaceous minerals, but in other respects the complex shows extreme inconstancy and variety from point to point. For this reason the various localities are separately described. By inspection of the map three fairly well defined areas are distinguishable, in addition to which are portions of several others. _Obushkong area._ Much of the country between Obushkong and Firth lakes is underlain by Keewatin. On the east, south and west sides, these rocks disappear beneath the Huronian or are interrupted by masses of diabase, but on the north they merge into Laurentian gneiss, the contact with which is ill-defined. Although some of them retain much of their original massive character, well defined schistosity is the dominant feature. The schists stand vertically or at angles little less than 90° and trend in a general east and west direction. A series of compass observations made at points over the whole area show the strike to vary from N. 65° E. to S. 75° E. One of the most abundant rock types is a stratiform, finely speckled hornblende gneiss or schist, the black hornblende cleavage faces giving it a glistening appearance on newly broken surfaces. It is quite fresh, perfectly crystalline and usually eminently fissile, but sometimes grades into a nearly massive dioritic form of undoubted igneous nature. Extensive exposures occur around Gould lake where the gneiss is traversed by numerous stringers of quartz, rusty in colour from the oxidation of pyrite. It is also well exposed near McLaughlin and McIntosh lakes and to the east and south-east of Foot lake. A fine grained chlorite schist of dull greenish black colour is common in this and all the other areas in the district. To the north-east of Serpentine lake it appears as a sheared phase of a weathered diabase, but it has also been derived from porphyry, exposures being seen between Foot and Obushkong lakes, where feldspar phenocrysts appear on weathered surfaces of the schist as pale, oblong spots. What is probably iron formation was observed at points 25 chains south of Gould lake, and 10 chains south of a little pond just east of Serpentine lake. Both outcrops consist of banded, grey quartzite interlaminated with chlorite schist, but magnetite-bearing bands were not found. Occasionally, narrow bands of pale grey, felsitic schists may be seen among the more common darker rocks. From evidence obtained at various points, these appear to have