188 Derby — Accessory Elements of Itacolumite, and 



evidence of clastic origin. They are sharp and angular with 

 jagged outlines and perfectly fresh surfaces, as in the crystal- 

 line schists generally and in many eruptive rocks as well. The 

 clearest indication of a clastic nature, that of a difference in 

 the size of the grains in some specimens, is of doubtful value, 

 since metamorphosed eruptives of original porphyritic struc- 

 ture might present it as well. No cases of the secondary 

 enlargement of worn grains such as have been described by 

 Tornebohm, Irving and others, could be detected in typical 

 samples of the rock, as those showing such enlargements present 

 macroscopically the aspect of sandstone rather than of itacol- 

 urnite. These are from the diamond district of Lencoes in 

 the state of JBahia, where both types of rock occur (the itacolu- 

 mite very abundantly as rolled pebbles in the sandstone or 

 conglomerate), and one of the specimens is highly charged with 

 tourmaline in a manner suggestive of contact matamorphism. 



The mica (or mica-like) mineral which is almost invariably 

 present in greater or less abundance is also perfectly fresh and 

 is undoubtedly authogenic. Formerly referred to talc or 

 chlorite, it proves in most places to belong to some of the 

 varieties of muscovite. The predominant type is white with 

 a pearly luster, but a green variety suggestive of fuchsite is 

 quite common. That of a specimen representing the diamond- 

 bearing bed of Grao Mogol (massive itacolumite of Eschwege 

 and Helmreichen) is a green brittle mica of high specific grav- 

 ity and apparently a member of the clintonite group. A 

 specimen of sandstone with a small stringer of vein quartz 

 from the Lencoes region in Bahia shows green mica in the 

 vein and in the microscopic fissures of the rock for a centi- 

 meter or so in its immediate vicinity, but not in the general 

 mass of the rock, which is a typical clastic sandstone. From 

 this case it appears that the production of mica in these rocks 

 does not necessarily indicate an advanced degree of meta- 

 morphism. 



Among the rarer accessories zircon is the most constant and 

 the only one that, in most cases, presents unequivocal evidence 

 of an allothogenic origin in the rounded angles and dulled luster 

 of the faces due to wear in transportation. (As the angles of 

 zircon are often rounded in eruptive rocks in which it is un- 

 doubtedly an authogenic element, this character alone cannot 

 be taken as an evidence of wear.) In a few cases the mineral 

 is rare and extremely minute and in these the signs of wear 

 are indistinct. In the most of the specimens examined, how- 

 ever, the zircons are more abundant than would be expected 

 in a corresponding volume of a zirconiferous eruptive, showing 

 an apparent concentration which of itself is an indication, 

 though not an unequivocal one, of clastic origin. When of 



