618 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 al- 

 ready taken away by the miners of the previous century, all traces of 

 whose former presence have been destroyed by the river. This is an- 

 other 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 de- 

 stroyed ? 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 forma- 

 tions. 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 bv the two facts of the dis- 

 covery 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 quartz- 

 ites, 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 at- 

 tached importance to these discoveries. I was myself incredulous 

 respecting them till I was able to verify with my own eyes the exist- 

 ence of the diamond in the rocks in place. I distinguished three for- 

 mations :" 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 quartz- 

 ites, the turns of which they follow ; and were, therefore, formed at 

 the same time with them in remote geological epochs, which, in con- 

 sequence of the total absence of fossil remains, can not be precisely 



