420 E. C. HARDER AND R. T. CHAMBERLIN 



seems best to leave the question whether the diamonds in these 

 chapada conglomerate deposits have, for the most part, been derived 

 directly from igneous rocks of the Archean complex, or whether, 

 coming in any case from that source originally, there occurred an 

 intermediate stage of incorporation in the Caraga quartzite — an 

 open one. 



Perhaps the most noted of the diamond mines of this district 

 is that of Sao Joao da Chapada, which has been rather fully described 

 by Dr. Derby.^ It is located some 30 kilometers northwest of 

 Diamantina on the tableland which represents the old base-leveled 

 surface. Sunk in the solid quartzite of the tableland is a steep- 

 walled trench, perhaps 500 meters long and now about 30 meters 

 in depth (Fig. 23). This trench has the appearance of being the 

 channel of an old stream which became completely filled with resid- 

 ual clay, sand, and gravel when the surrounding region was close to 

 the base level. This alluvial-filled trench has been cleaned out again 

 as the deposits of clay, sand, and gravel have been washed away in 

 the process of diamond mining. 



Dr. Derby in several of his papers has been inclined to the 

 hypothesis that the diamonds of Sao Joao da Chapada are vein 

 minerals,^ derived from a pegmatite vein of which there seems to be 

 some evidence in the cut. Certain masses of pure-white kaolin 

 containing nests of large and beautiful crystals of quartz, which 

 have never been exposed to the wear of running water, look much 

 like the decay products of a pegmatite vein. Unfortunately at the 

 time of our visit in 191 2 the mine had not been worked for twenty 

 years, so that definite and reliable information was not easily 

 obtained, but from the accounts given by some of the people at 

 Sao Joao da Chapada and the published papers which touch upon 

 this point, it would seem that the diamonds were far more abundant 

 and characteristic of the waterworn conglomerates, especially where 

 pebbles of higher specific gravity (iron ore, itabirite, etc.) were 



' O. A. Derby, " Brazilian Evidence on the Genesis of the Diamond," Jour. 

 Geol. VI (1898), 121-46; "On the Association of Argillaceous Rocks with Quartz 

 Veins in the Region of Diamantina, Brazil," Ant. Jour. Sci., 4th Ser., VII 343-56. 



^ O. A. Derby, "Modes of Occurrence of the Diamond in Brazil," Am. Jour. Sci. 

 3d Ser., XXIV, 34-42; "The Genesis of the Diamond," Science, IX (1887), 57-58. 



