482 Brannt r — Minerals Associated with Diamonds and 



these eruptives are dikes of diabase and that they break through 

 the Lavras quartzites. 



It should be noted in this connection that the principal dia- 

 mond deposits extend over a wide area in the interior of Bahia, 

 and that nowhere have eruptive rocks been found cutting the 

 diamond-bearing sediments except at the places imported by 

 Mr. Williams. 



In the crystalline complex, however, that underlies all of the 

 sedimentary beds of the interior of Bahia, there are many 

 kinds of eruptive rocks. So far as observed, the eruptives in 

 the crystalline series are all old; at least they have not been 

 found to pass up into the sedimentary beds in which the dia- 

 monds are found, with only the exception just mentioned. 



Of the sedimentary rocks the Minas (or Jacobina) series, as 

 shown in the Serra de Jacobina, has its beds generally stand- 

 ing at a very high angle. Whether the position of these beds 

 is due to folding or to faulting is not yet entirely clear, though 

 the weight of evidence seems to be in favor of the theory of 

 faulting.* 



The Lavras quartzites. — Of the overlying Paleozoic beds 

 the diamond-bearing Lavras series was most studied. The 

 beds are usually more or less folded. The folds vary greatly: 

 in some places they are so gentle as to be almost imperceptible, 

 while in others they are highly distorted. In the mountains 

 west of Grruna the beds are much contorted, broken, and 

 faulted. Over most of the area, however, the folds are not 

 closely pressed, the dips varying from ten to forty degrees. 

 This structure in connection with the sharp separation of the 

 Lavras series from the underlying and overlying beds, makes 

 the working out of the distribution of the diamond-bearing 

 beds comparatively easy. 



The rocks of the Lavras series are nearly all pinkish quartz- 

 itic sandstones and conglomerates. The pink color appears 

 to be characteristic of the series everywhere except in places 

 where weathering has permitted the leaching out of the color- 

 ing matter. 



Analysis of the Lavras Quartzite 



Collected by J. C. Branner near Andaraby, State of Bahia. 

 L. R. Lenox, analyst. 



Silica (SiO a ) - - - - - -97-94$ 



Oxides of iron and alumina (Fe 2 3 and A1,0 3 ) 1-98 



Lime (CaO) none 



Magnesia (MgO) , trace 



99-92$ 

 * This Journal, xxx, 390-391, Dec. 1910. 



