128 ORVILLE A. DERB Y 



visited and minutely described by Helmreichen 1 who confirmed 

 Eschwege's views of the genetic relation of the diamond with 

 itacolumite, although he confessed a doubt as to whether the 

 rock, which had a conglomeratic aspect, might not be of clastic 

 origin. Subtantially the same view was taken by Heusser and 

 Claraz 2 who also visited the locality and who considered itacolu- 

 mite as a quartzose phase of hornblende-schist and attributed the 

 apparent pebbles of the diamond-bearing bed to concretionary 

 action. In 1882, the present writer showed 3 that in the 

 Diamantina region the massive itacolumite of Eschwege really 

 constitutes an independent formation resting unconformably on 

 the upturned edges of a lower series to which the schistose type 

 belongs, and containing elements derived from it, the diamond 

 being probably one of these derived elements. Several of the 

 " dry diggings " of the vicinity of Diamantina were cited as being 

 probably disintegrated masses of this ancient and metamor- 

 phosed conglomerate and the Grao Mogol deposit, which was 

 not seen, was referred to as being presumably another example 

 of the same kind. Professor Gorceix who afterward visited the 

 Grao Mogol locality, and who for the Diamantina and other 

 regions accepted my view of the dual character of itacolumite 

 as originally described, agreed with Helmreichen and Heusser 

 and Claraz in uniting the diamond-bearing bed with the lower 

 schistose itacolumite, but, in opposition to their view, he con- 

 sidered the whole series as clastic. 4 The pebbles, or pebble- 

 like bodies of the old writers, of the diamond-bearing bed were 

 thought to be derived elements, while the mica, pyrite and 

 martite of the same rock were considered as authigenic. The 

 question as to which of these two groups of elements the 

 diamond should be referred was, on theoretic grounds, decided 

 in favor of the latter, a view that was rendered necessary by that 



1 Ueber das Geognostische Vorkommen der Diamanten und ihre Gewinnungs- 

 methoden auf der Serra do Grao-Mogor. Vienna, 1846. 



2 Zeitschrift d. deutsch. geol. Gesellschaft, XI, 1859, p. 448; Petermann's Mitth., 

 1859, p. 447. 



3 Am. Jour, of Sci., 1882, XXIII, p. 97 ; XXIV, p. 34. 



4 Bulletin de la Society Geologique de France, XII, 1884, p. 538. 



