_Age._—The time of intrusion of these rocks can only be defined as post-Huronian or post Middle Huronian, there being no younger formations with which to correlate them. They are identical with the diabase in other parts of the Montreal River region and of Cobalt. As already noted the larger masses have produced very limited metamorphic changes in the adjoining rocks, developing incipient adinole zones in the Huronian greywackes, besides hardening and fracturing them. Usually the contact edge of the diabase has been deeply eroded along with the adjoining rock to form narrow ravines, but wherever it persists a slight chilling is perceptible. The fine grained edges are much more apparent in dikes where the mass of hot material being much less was more susceptible to the influence of cold surroundings. The intrusive nature is also demonstrated by angular blocks of Huronian materials enclosed within the diabase, instances of which may be seen on Wapus creek. _Olivine Diabase._ At the first rapid above Kenisheong lake the Huronian is cut by a broad dike of different character from those belonging to the quartz-diabase intrusion. It is a remarkably fresh compact rock of dark grey colour and medium texture in which an ophitic structure is prominent, acicular prisms of glassy feldspar penetrating the dark main mass. Its density is 2·991. Under the microscope it is holocrystalline and the constituent minerals occur in only one generation. The constituents as determined by linear measurements were found to be plagioclase, 67·5 per cent; olivine, 14 per cent; augite, 1·5 per cent; iron ore, probably ilmenite, 5 per cent; apatite, 1·8 per cent; biotite, 1·3 per cent; and zircon, 0·03 per cent. The rock is remarkably fresh, even the olivine showing no signs of decomposition. The plagioclase was determined by optical methods to be a labradorite of the composition Ab 1 An 1. It forms laths, twinned according to albite, pericline and carlsbad laws, which penetrate the ferromagnesian minerals. A few large crystals show fine zonary lamellation. Olivine is in idiomorphic or rounded grains, occasionally bordered by a little rim of biotite, possibly a product of reaction during the period of crystallization. The augite is reddish brown in colour and later than either olivine or plagioclase, filling the interstices between the feldspar laths. Irregular masses of black iron ore, probably ilmenite, are scattered throughout the section, in some cases showing good crystal forms. A deep brown strongly pleochroic biotite with a very small optical angle occurs accessorily in small shreds. Apatite is mostly in slender but sometimes stout hexagonal prisms. Minute crystals of zircon are rare. The rock may be designated an olivine diabase of markedly fresh aspect. It penetrates the Huronian, but judging by its unusually fresh condition it must be comparatively young. PLEISTOCENE.