The post-Huronian diabase is an equally potent topographical factor. Its surface is one of marked irregularity, but the peculiarly distinctive features occur at its contacts with the Huronian. These contacts appear to be zones of low erosive resistance, and are commonly coincident with ravines, walled on one side by diabase, on the other by Huronian. Small lakes may occur at intervals along them as, for example, between Firth lake and the West branch. This erosion feature is well shown by the configuration of Gowganda lake, where diabase bodies are unusually abundant; both of the long arms to the north-west lie in trough-like depressions marking the edges of the eastern diabase mass. The same tendency in an incipient condition is observable on the east side of the large peninsula where a series of three land-locked bays extend along the contact between the eastern diabase mass and the Huronian. Near the middle of Duncan lake, a diabase-Huronian contact which crosses the lake diagonally is marked by two deep bays, one extending to the north, the other southward. While this tendency is an evident one it is not to be understood as invariable; the large island in Duncan lake between the two above-mentioned bays is sufficient to indicate that contacts may lie in high ground, yet even here there are minor features indicating the contact zone to be structurally weak. Another less explicable topographic peculiarity becomes apparent only upon scrutiny of the drainage system. A brief consideration of the map shows that both East and West branches follow peculiar zig-zag courses running north for a short distance, then turning abruptly east, this feature recurring repeatedly. In some instances the east-west portion of both branches lies in the same line. In the case of Zigzag lake and adjacent portions of the West branch this feature is repeated with an almost conventional regularity, which precludes attributing it to chance causes. Many of the smaller lakes—Foot lake, for instance—exhibit the same character on a small scale. This abnormality has been commented upon by investigators in the country to the east, the courses there, however, being N.E.-S.W. and N.W.-S.E. Regional faulting is suggested in explanation. The canyon-like east and west walls of Zigzag lake suggest such conditions, but a discussion of the matter must be deferred until further data can be collected. DRAINAGE. All the drainage water escapes by way of the Montreal river, whose two chief tributaries are the East and West branches, the latter being considerably the larger. The East branch is without feeders of important size, but the West branch receives a large creek, the Wapus, from the south, and a considerable volume of water enters through Duncan lake.