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Page:Popular Science Monthly Volume 21.djvu/634

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THE POPULAR SCIENCE MONTHLY.

As great, also, is the disappointment when, as often happens, the probe strikes the rock without meeting the diamonds—they having been already taken away by the miners of the previous century, all traces of whose former presence have been destroyed by the river. This is another risk that the diamond-seekers have to run.

These gravel-beds are not the only treasure-bearers in the diamond yielding region. The mineral elements of which they are composed have been washed by the waters from more ancient rocks. Now, what are these rocks? Do they still exist, or have they been wholly destroyed? To answer these questions, I have carefully endeavored to determine the group of minerals which I have called the satellites of the diamond, persuaded that, wherever their primitive bed should be found, there would also be met that of the diamond. Now, all around the city of Diamantina, and for more than twenty miles west of it, the dominant rocks are quartzites with green mica, and beds of schists of the same nature and the same age as those of the auriferous formations. They are traversed by numerous veins of quartz containing oxides of iron, titanium, and tourmalines, the satellites of the diamond in the river-gravels. The origin of the latter is evidently due to the destruction of these rocks by the action of the waters; and we may, therefore, conclude that they ought to contain the primitive bed of the diamond.

The study of the geographical distribution of the diamond-bearing streams leads us to the same conclusion. All the streams, the sands of which have been found to be richest in diamonds, depart from this zone.

These deductions are confirmed also by the two facts of the discovery of the diamond in place in the sandstones with green mica, two hundred miles from Diamantina, and the discovery of clay-beds, formed from the decomposition of the schists intercalated in the quartzites, twenty miles west of the same city, where rise two rivers, the Rio Pardo and the Caethe Mirim, celebrated in the annals of the miners for their richness. The idea that the Brazilian diamonds were found only in alluvial deposits was so firmly rooted that at first no one attached importance to these discoveries. I was myself incredulous respecting them till I was able to verify with my own eyes the existence of the diamond in the rocks in place. I distinguished three formations: one white, with considerable quantities of crystals of quartz; a second gray, composed almost entirely of oxide of iron; the third, the strongest, of mottled clay, with considerable quantities of the same crystals, of rutile and oligist iron, which I have already pointed out as occurring in the river-gravels. All the formations are strongly inclined toward the east, and are intercalated with micaceous quartzites, the turns of which they follow; and were, therefore, formed at the same time with them in remote geological epochs, which, in consequence of the total absence of fossil remains, can not be precisely