484 Branner — Minerals Associated with Diamonds and 



in the Lavras quartzites, or whether they originated elsewhere 

 and were washed out of their original matrix and deposited 

 along with the other sediments in the Lavras beds. Perhaps 

 this question might be readily answered under ordinary circum- 

 stances, but in this case it is not so simple. 



It was hoped that a study of the minerals associated with 

 the diamonds might settle the question definitely; and it was 

 in this hope that the present examination was undertaken. 



In order to study the mineralogical association of diamonds 

 and carbonados in the diamond region of Bahia, I have 

 obtained thirty-five samples of the concentrates from different 

 parts of the Bahia fields, and have had the minerals in them 

 separated and identified. 



Some of the samples were collected by myself, but most of 

 them were obtained for me through the kindness of my Bra- 

 zilian friends, Dr. Alencar Lima of Bahia, Col. Dias Coelho 

 of Morro do Chapeo, and of Mr. Arthur R. Turney of Lencoes. 

 The samples consist of the fine heavy materials left in the bot- 

 tom of the wooden bateas at the final "clean-up" in washing 

 for diamonds at various places in the state of Bahia. The 

 different minerals were separated by the use of Thoulet heavy 

 solutions, and the percentages were determined by weighing 

 before and after the separation. The weights are not alto- 

 gether precise owing to slight losses, one of which came from 

 the solubility of the gold in the solutions used. 



Notes on the minerals. — The table shows that the minerals 

 are unevenly distributed, some of them being very abundant 

 in one locality and lacking at another. 



As was to have been expected, quartz is by far the most 

 abundant at all localities. The quartz grains nearly all have 

 secondary enlargement ; that is, the cementing material of the 

 diamond-bearing quartzites is partly quartz in optical continu- 

 ity with the original sand grains. 



* Samples 1 to 5, inclusive, are from Mosquitos near Lengoes, and were 

 given me by Mr. Arthur R. Turney of Lengoes, Estado da Bahia. 



f Samples 6 to 14, inclusive, were obtained for me by Col. Dias Coelho of 

 Morro do Chapeo. The precise localities are not stated, but it is under- 

 stood that they are from washings about Morro do Chapeo, Campinas, and 

 Ventura. 



% 15, from Col. Dias Coelho, is marked Tres Moitas, Jacobina Nova, which 

 is on the west side of the Salitre Valley about 130 kilometers west of Villa 

 Nova. 



§16, from Col. Dias Coelho, is marked Eio Cambao, Jacobina Nova. The 

 locality is within the Salitre drainage west of the Serra de Angico and 160 

 kilometers west of Villa Nova. 



|| 17, from Col. Dias Coelho, has no locality label. 



*T Nos. 18 to 35, inclusive, were sent me by Dr. Alencar Lima of Bahia. 

 The exact localities are given in the table. The Chique-Chique mentioned 

 is near Mucuge and should not be confused with a city of the same name on 

 the Rio Sao Francisco. 



