Page:Popular Science Monthly Volume 50.djvu/402

386, the curtain is once more drawn, as it were, over the modern world.

At Lac à la Tortue, twenty-one miles from Three Rivers, we are again in the dim past. We can stand upon the shore of this lake and see a sight that might have been the original for an illustration of iron-ore gathering in Scandinavia one hundred years ago.

This Lac à la Tortue (Turtle Lake) is our curious Canadian iron mine. It is a body of water about four miles long by a mile and a quarter in average width, occupying the center of a large area of swampy land. The surrounding land is largely composed of sand formed by the wearing down of the Archæan rocks by glacial action.

It is well known that decaying vegetable matter yields acids that dissolve the oxide of iron. Evidences of this solvent action of vegetable acids on iron are frequently seen in pieces of slate. The slate is colored by iron, but frequently white or light-colored spots occur. These are points where a leaf or a fragment of bark has been deposited with the fine mud in which form the slate was deposited. The leaf or bark has decayed, the vegetable acids thus formed have dissolved the iron oxide to which the color of the slate was due, and of course a white or colorless patch is formed.

In the sandy area around Lac à la Tortue we find the most favorable conditions for the action of vegetable acids on iron oxide. The sandy land produces a rank vegetation, and its decay furnishes abundance of organic acids. These acids are in solution in the drainage waters, which on their way to the lake percolate through the sand. They thus come into contact with the iron oxide in the finely divided materials, dissolve it, and carry it along to the lake. Here a new chemical action comes into play. The solution of iron in vegetable acid (in which the iron is in what the chemist calls the form of a protosalt) is oxidized by the action of the air on the surface of the lake into a persalt, which is insoluble, and appears on the surface in patches that display the peculiar iridescence characteristic of petroleum floating on water. Indeed, not infrequently these films of peroxide of iron are incorrectly attributed to petroleum. These films become heavy by addition of new particles, they sink through the water, and in this manner, in time, a large amount of the iron ore is deposited on the lake bottom. It must not be supposed that the ore is deposited as a fine mud or sediment. On the contrary, in this lake ore, as it is called, we have an excellent illustration of what is known as concretionary action—that is, the tendency of matter when in a fine state of division to aggregate its particles into masses about some central nucleus, which may be a fragment of sunken wood, a grain of sand, or indeed a preformed small mass of itself. Precipitated in