1879.] ^'5< [Derby. 



considerable distance horizontally, but whether it is everywhere underlaid 

 by the diamantiferous gravel or not, I cannot state. Other washings have 

 been opened, some twelve or fifteen miles below Tibagy, and it is probable 

 that there are many other localities in which diamonds may be found. 



As I saw no work in progress, I could form no idea of the richness of 

 these mines. The diamonds are said to be rare, and small and poor, in com- 

 parison with those found in the river. The workings have been conducted on 

 a very small scale and very carelessly, so that, although the mines are cer- 

 tainly not extremely rich, it is impossible to affirm that they would not re- 

 pay better, and more systematic management. A small quantity of gold 

 also occurs in these washings, and this metal is quite generally distributed 

 throughout the region. 



In the river, the best stones are found in the deposits in the pot-holes, 

 which contain gravel, firmly cemented by ferruginous matter. Rarely pot- 

 holes are found with a very hard, bluish cement, and these are reported to 

 contain the most diamonds, which are of the best quality both as regards 

 size and perfection. Not having seen this cement, I can form no idea of its 

 character. The miners note as a curious fact, that in a group of pot-holes 

 close together, one may have the bluish cement, while all the rest have the 

 ferruginous, the pebbles of the one being quite different from those of the 

 others. 



Many of the stones shown me were broken and worn, but a fair pro- 

 portion were perfect crystals. The largest ones I saw were about 

 the size of a small grain of corn, but were irregular and broken. The 

 most valuable stone found here, of which I could obtain an authentic 

 account, was sold for a conto of reis ($500). The stones are in general of 

 good color and brilliancy. 



Coming now to the question of the origin of the diamonds, it seems to 

 me to be very evident that they are washed out of the Devonian sandstone. 

 As already remarked, the Tibagy is almost exclusively a river of the De- 

 vonian plain. The lower portion, in the trap region, is not known to be 

 diamantiferous, and if it is, since the stones occur throughout the whole 

 course of the river, before it enters the trap formation, the latter may be 

 eliminated from the problem. There remain then the Devonian rocks and 

 the diorite. Having passed around the head of the river, and crossed it at 

 three different places, I have become satisfied that these are the only rocks 

 that come to the surface, to the eastward of Tibagy, that is to say, in the 

 diamantiferous region. It is of^course possible that the river may have cut 

 down, in certain places, to the underlying metamorphic rocks, but of this 

 there is no evidence, and it is not probable that any considerable area of such 

 rocks are exposed, or if so exposed that it could have furnished diamonds to 

 so widespread a region. Two considerable tributaries, the Yapo and the 

 Pitangui, flow from the metamorphic region, and might be supposed to have 

 brought the diamonds from the rocks cut through in that region, but I was 

 unable to obtain any notice of diamonds, found in those rivers, before en- 

 tering the sandstone district, and the Tibagy is diamantiferous above, as 

 well as below, its confluence with them. 



PROC. AMER. PHILOS. SOC. XVIII. 103. 2g. PRINTED JUNE 10, 1879. 



