Page:Popular Science Monthly Volume 21.djvu/635

Rh identified. A comparison of them with other strata of the São Francisco Valley, which are characterized by the presence of palæozoic corals, permits me to affirm that they certainly ascend as far back as the Silurian period. Other travelers, previously to myself, have mentioned this formation, and my friend the geologist Dorville-Derby, who supports my view, has carefully described it. The washing of these clays is performed in the same manner as that of the gravels, and I have myself extracted diamonds from them. In the washing, the minerals which we found rolled and rounded in the river-sands, and presenting an entirely different aspect, came out in perfect crystals, without a trace of wearing. They are the same satellites of the diamond, but still in their primitive bed. The diamonds also of São João da Chapada are characteristic: like the crystals of oxide of iron, their angles are whole; their wrinkled faces and their uniform color have suffered no modification by friction. Should not the same conclusion be adopted for the diamond, and may we not assume that it also is here found where it was formed? It is true that I have not found diamonds actually in the little veins of quartz that traverse these beds, nor in the schists, the decomposition of which has produced the diamond-bearing clays with these same crystals of oligist iron and rutile. But hardly one diamond exists to a million crystals of oligist iron; more than thirty thousand pounds of clay have furnished only ten little diamonds weighing about a carat. It would have been a rare chance to perceive, even with a strong glass, one of these stones, no larger than the head of a pin, in the midst of an enormous mass of sterile substances. Objections may indeed be made to the views which I have presented. The formations in which the diamond is found have not always been in the condition in which we now see them. They are not eruptive rocks, that have come already formed from the center of the earth, but have originated from the destruction of more ancient formations, and have undergone metamorphic action under the influence of which new crystalline elements have been formed within them, and they have assumed the aspect they now present. Why, it may be asked, may not the diamond also have been derived from these primitive formations? And then the problem, instead of having been resolved, is. only put back another step.

I may reply to this by asking if the processes of trituration effective enough to reduce crystals of feldspar and quartz to mud and sand would not also have modified the diamonds by smoothing their angles and wearing away their surface; especially, since we have seen that much lighter rubbings have been enough to produce such effects in the streams? I could likewise answer other objections that my mind suggests; and I believe I have shown that the diamond of the alluvial formations of Diamantina comes, like the iron and titanic oxides, the tourmalines and the phosphates, its faithful companions, from the destruction of the quartz-veins intercalated into the palæozoic rocks of