ON THE GENESIS OF THE DIAMOND 1 37 



mentioned so that, when present, their evidence is positive when 

 they show distinct signs of wear, doubtful when these signs are 

 absent or dubious. 1 



Applying the zircon test to the material at hand from the 

 Sao Joao da Chapada mine the evidence for the clastic origin 

 of the greater part of the original rock types from which the 

 clays are derived has proved to be unexpectedly satisfactory. 

 A number of samples of typical clays, including some reputed 

 to be diamantiferous, afforded zircons which in abundance, size, 

 and amount of wear, are comparable with those of the granular 

 quartz rock (itacolumite) that occurs above the schists in the 

 immediate vicinity of the mine. To judge from the number 

 and character of the zircons, these samples represent original 

 grits rather than more purely argillaceous material as was sup- 

 posed from their present character and appearance. This con- 

 clusion is confirmed by the amount and size of the quartz grains 

 (beautifully etched) that are also separated by the washing of 

 the clay. In a miner's concentrate representing mixed material, 

 fresh, prismatic glassy zircons occur mingled with the ordinary 

 rounded, reddish clastic type indicating that other types of rock, 

 presumably eruptive, may be represented among the clays. For 

 the present discussion, however, the essential point is that the 

 generality of the zircons of the clays are worn, thus confirming 

 the assumption, based on stratigraphical evidence, that the 

 clays of the mine represent a series of schists of which the pre- 

 dominant types are of clastic origin. This conclusion, however, 

 does not exclude the possibility of subordinate intercalations, 



1 The rounding of the angles alone cannot be taken as an unequivocal sign of 

 wear, as it is often an original feature of the zircons of undoubted eruptive rocks. 

 When the angles are rounded by attrition the faces are also dulled in a manner that is 

 readily distinguishable from that produced by malaconization. Undoubted elastics 

 occur in which the signs of wear of the zircons are inappreciable, either because the 

 amount of transportation has been too small or the material too fine to produce them, 

 or because they have been involved in other elements, as in the case of arkose and 

 tuffs. In the case of argillaceous rocks the rarity and minuteness of the zircons may 

 be an argument in favor of a clastic origin even when they show no distinct signs of 

 wear, but the evidence is not conclusive as they are minute and rare in some eruptives 

 as well. 



