136 ORVILLE A. DERBY 



quence whether the series as here limited by the exclusion of 

 the massive itacolumite of Eschwege is a simple or a composite 

 one, since the evidence as to origin applies very directly to the 

 beds in question exposed in the diamond mine itself. 



As recent studies in various parts of the world on schists of 

 doubtful character have proved that the schistose structure is not, 

 as was long supposed, an unequivocal proof of a clastic origin, an 

 attempt has recently been made to find in the rocks themselves 

 internal evidence for or against the hypothesis of such an origin. 

 As is well known, most of the mineralogical elements of a meta- 

 morphosed rock, whether clastic or otherwise, are authigenic ;: 

 others which in certain cases may be presumed to be allothi- 

 genic may have been broken up, recrystallized, enlarged by sec- 

 ondary additions, 1 or etched, so that all traces of the original 

 worn surface of clastic grains may have been lost or so obscured 

 as not to be recognizable with certainty. The hopes of finding 

 such internal evidence are therefore limited to the rare acces- 

 sories, and among these practically to zircon, not only on account 

 of its almost universal distribution in sedimentary deposits, but 

 also of its resistance to chemical changes. In the examination 

 of the heavy residues of a considerable number of the rocks of 

 the series in question, it was found that in rocks of their charac- 

 ter and degree of metamorphism, zircon is the only element that 

 can be depended upon to give unequivocal evidence as to the 

 mode of origin. All the other constituents, principal or accessory 

 (quartz, mica, iron, and titanium oxides, tourmaline, disthene,. 

 etc.) , present the fresh appearance of authigenic elements, as- 

 most of them doubtless are, though in some cases this appear- 

 ance may be due to the fragmentation, regeneration, or etching 

 of original clastic grains. On the contrary, the zircons in the 

 considerable number of residues examined have failed to show 

 evidence of secondary modification by any of the processes 



J As will be shown elsewhere, tourmaline may be regenerated by secondary enlarge- 

 ment in the same manner as in the well-known case of quartz. A remarkably fresh 

 appearance of the surfaces of quartz grains, due to etching, is noticeable in the wash- 

 ings from the clays of Sao Joao da Chapada, but it is to be presumed that this is a 

 phenomenon of decomposition rather than of metamorphism. 



