CATALOGUE OF GEMS. 503 



The diamond deposits of Brazil were discovered in 1728 and were at 

 one time very productive, although the 3Meld is at present small. Near 

 Diamantina in ]VIinas Geraes the diamonds are obtained in part from 

 river washings and in part from prairie washings. The river deposits 

 consist of rolled quartz pebbles, mixed with or united by a ferrugi- 

 nous clay which rests usually on talcose clays. The more common 

 associated minerals are rutile, hematite, ilmenite, quartz, k3'anite, 

 tourmaline, gold, garnet, zircon, etc. In the prairie washings the 

 diamonds occur in a conglomerate consisting of quartz fragments 

 covered with a thin lied of sand or earth. This deposit affords the 

 finest stones. Other Brazilian localities are those of Bagagem, at 

 which place a 247^ carat stone was found, and at Abaethe, Minas 

 Geraes. In Bahia diamonds are found at Lencaes; along- the river 

 Cacholira, especially at Surua and Sinorca; they occur also on the 

 Salobro and other branches of the Pardo River. 



Derby, ^ in a recent paper on the occurrence of diamonds in Brazil, 

 says that for the question of genesis the most significant of the Bra- 

 zilian localities is that of Sao Joao de Chapada, near Diamantina. The 

 diamonds occur here in thoroughly decomposed material, no fresh 

 rock being found. He regards the various claj's as representing a 

 group of phyllites, of varied character, of principally, if not exclu- 

 siveh', clastic origin threaded with dikes of pegmatite, the clastic 

 origin of the schists being indicated by the worn character of the 

 zircons found in the clay derived from them. The diamond t^treaks 

 appeared to Derb}^ to contain distinct bands composed in part of 

 quartz with plates of hematite, and these bands, he suggests, may be 

 pegmatite dikes. The primary tourmaline and zircon and the sec- 

 ondar}^ hematite and rutile found in the heavy residue after washing 

 the cla}' are supposed to have originated in the pegmatite, the hema- 

 tite, octahedrite, and rutile having formed from the original iron and 

 titanium minerals now gone. The staurolite and kyanite, found in 

 the heavy residue, are supposed to have come from the schists where 

 they may have formed as a result of contact metamorphism induced 

 by the intrusive pegmatyte. The diamonds have come from the schist 

 along the borders of the pegmatite dikes and not from the dikes 

 themselves; that is, he regards it possible that the diamond is a prod- 

 uct of contact-metamorphism. 



By far the greatest portion of diamonds now obtained come from 

 South Africa, their discovery dating from 1867. The diamond work- 

 ings are of two kinds, river diggings and dry diggings. The river 

 diggings are in the gravel of the Vaal River from Potchefstroom down 

 to its junction with the Orange River, and along the latter as far as 

 Hopetown, the principal workings being along the Vaal between Klip 



* Brazilian evidence on the genesis of the diamond, Journal of Geology, VI, 1898, 

 pp. 121-146. 



NAT MUS 1900 35 



