326 PROCEEDINGS OF SECTION C. 



The writer has never seen a diamond in place in the Lavras rocks. 

 The evidence that they are derived directly from this series lies in 

 the fact that they are taken from the disintegrated beds of this series. 

 Many illustrations of this fact might be cited, but only a few typical 

 examples will be mentioned. Near Andarahy, where the rocks of the 

 Lavras series are at the surface, the soil and disintegrated rocks have 

 been removed by the diamond miners with great care down to the 

 hard rock, and the whole has been carefully washed for diamonds. 

 At and about Morro do Chapeo the trenches in which the diamonds 

 and carbonados are found are entirely in the rocks of the Lavras 

 series. Diamonds and carbonados have been found in loose materials 

 resting upon other rocks, but in every instance the stones are readily 

 and directly traceable to the Lavras beds. For example, at Ventura 

 the stones are found resting upon the underlying Caboclo shales, but 

 this is because the streams flowing over and from the Lavras series 

 have carried them down into the channels cut into the underlying 

 beds. Diamonds have also been found well away from the rocks of 

 the Lavras series, but along or in streams flowing over and from the 

 latter beds. 



The writer has seen a large number of both diamonds and 

 carbonados from this Bahia region. Of the stones known to have 

 been taken from the Lavras beds (we mean those not found in stream 

 beds far removed from their original positions), not one showed any 

 signs of wear: of those taken in streams leading away from the 

 Lavras beds, some showed a little wear. Of the carbons nothing 

 can be stated with certainty. So far as examined, their surfaces 

 were always smooth, but it was not clear Avhether this smoothness was 

 due to wear in streams. 



Oeigin of the Diamonds.- — Two theories of the origin of the stones 

 naturally suggest themselves : First, that they may have originated 

 as independent crystals in the Lavras sedimentary beds ; second, that 

 they may have been produced, like the South African diamonds, in 

 connection with peridotite efl'usions, and may have passed down from 

 one series of sediments to another to find their resting-place in the 

 Lavras series. 



There seems to be nothing inherently impossible or improbable 

 in such a theoiy, but it must be confessed that the satisfactory 

 support for it is yet to be found. It is interesting in this connection 

 to note that the diamonds are not confined to any one horizon, and 

 neither are they evenly distributed throughout the Lavras series. 



When the field work was being done in this region it was 

 supposed that support for the second theoiy was entirely lacking. 

 Upon working over some of the rocks collected in the field by th© 

 "writer, it was found that there is an area of seipentine at least 

 3 miles long lying along the eastern margin of the diamond fields of 

 Bahia. The rock is so altered that its true nature was not recognised 

 when it was found, and it was only discovered to be serpentine after 

 a microscopic examination and a quantitative chemical analysis. 

 This serpentine was set down in the field as a part of the crystalline 

 complex underlying all these sediments. It is possible, however, that 



