A further examination of these iron-bearing rocks was made by the writer during the summers of 1898 and 1899, and more closely during the summers of 1901 and 1902 when engaged in this work for a private company.
These iron-bearing rocks of the east side of Hudson bay have a close resemblance to those of Lake Superior, so famous for the amount and quality of their associated ores of iron. They consist of bedded sandstones, cherts, shales, graywackes and dolomites, associated with great outflows and sills of trap. The following general section of the rocks of the Nastapoka islands will give a good idea of the rocks there, while on the mainland other strata, free from or poor in iron, are found.
Descending order:—
| feet. | ||
| 1. | Rusty weathering, dark gray, siliceous rock containing ankerite (carbonate of iron and magnesia) and magnetite | 20 to 100 |
| 2. | Dark gray siliceous rock, containing magnetite with small quantities of ankerite | 50 to 250 |
| 3. | Red jaspilite rich in hematite ore | 10 to 100 |
| 4. | Red jaspilite poor in hematite ore | 5 to 20 |
| 5. | Purple or greenish weathering, dark-green graywacke shales | 10 to 70 |
| 6. | Red jaspilite poor in hematite ore | 0 to 5 |
| 7. | Light greenish-gray sandstone and shale | 10 to 300 |
| 8. | Fine grained dolomite | 0 to 50 |
The iron ores have a greater thickness and are richer on the islands in the middle of the chain than elsewhere.
The rusty weathering, dark-gray siliceous rocks of division I. are found on all the islands from Flint to McTavish, being wanting only on Cotter island. The typical rock is a dark-gray chert made up of finely divided silica showing under the microscope small grains of quartz filled in by later accessions of that material in a finely divided state. It contains minute crystals of magnetite scattered through the mass, and also patches of crystalline carbonates. At the southern end of the chain it is cherty and sometimes light-green in colour. These rocks are usually in thin beds, the parting between the beds filled with brownish ankerite, which also occurs in flat lenticular masses inclosed in the cherts; many of these masses are several inches in thickness and several square feet in area, so that the rock usually contains from twenty to fifty per cent of ankerite. These ores are too much broken and too intimately mixed with the cherts for profitable mining. The rusty character of the rock is due to surface decomposition of ankerite to limonite. The beds increase in thickness as the islands are followed northward, and reach their maximum development on Davieau island and northward to McTavish island, where they have a thickness of fifty feet. These measures can be traced southward from the Nastapoka chain in the outer islands lying along the coast for upward of 150 miles, being last seen on Long island just north of Cape Jones, where they are overlaid by a considerable thickness of trap.
The second division of the section is an arbitrary one, and was made to embrace all the beds containing important deposits of magnetite. The upper beds of the division grade into those of division I, while the lower pass gradually into division III.
The typical rock of these measures is a dark-gray, fine-grained variety of quartzite chert, containing considerable magnetite scattered through it in minute crystals; it also contains small quantities of carbonates of iron, magnesia and lime. The beds are usually thin (from one to twelve inches) and the partings between them are filled with a mixture of silica and magnetite with small quantities of ankerite. These partings vary in thickness, but are generally thin between the upper beds of the division, and quite thick (six inches to forty-eight inches) towards the bottom, where they form important ores of iron; as the beds of chert are often quite thin between two or more thick partings of ore, they might easily be neglected in mining. The mixture of silica and magnetite in the ore is an intimate one, with the silica usually in a finely divided state.
The proportion of these substances is not constant, so that the ores vary from a lean ferruginous chert to a rich ore containing upwards of sixty per cent of iron. Large quantities of the better ores occur in the lower beds of the division. The occurrence of these ores between the beds of gray siliceous rock, and their intimate association with finely divided silica, point to their deposition and enrichment from the infiltrations of waters carrying solutions of iron and silica which were deposited in the waters in cracks and between the bedding of the already-formed siliceous rocks. This mode of formation has been described by Van Hise for similar ores in the Lake Superior region.
On the three southern islands of the chain there is a gradual change in the nature of these measures. They pass into a brownish-black siliceous shale, rich in iron and containing considerable carbon as small scales of graphite. This is the form in which they are found to the southward on the islands as far as Long island. The thickness of the division is very constant on the islands northward to McTavish, but it does not occur on Cotter island.