350 0. A. Derby — Association of Argillaceous 



The most serious objection to bringing the Dattas rock into 

 line with the others is the absence of monazite, but in regard 

 to this it may be noted that this mineral (like the xenotime in 

 the white kaolin) is so rare that it failed to appear in some of 

 the tests made.* 



The association of argillaceous material with quartz veins is 

 also a prominent feature in the diamond mines about the little 

 village of Sopa, about a dozen kilometers to the southward of 

 Sao Joao da Chapada. The diamond-bearing material is here 

 a decomposed conglomeritic quartzite entirely similar to that 

 now on view in the mines of Sao Joao.f This rests uncon- 

 formably on an older series of quartzites (itacolumites) with 

 intercallated schists that is also profoundly decomposed. This 

 lower series is abundantly threaded with small irregular quartz 

 veins that frequently show included pockets and stringers of 

 indurated kaolin and almost invariably partings and selvages 

 of various colored schistose rocks or clays that closely resemble 

 those of the Guia above described. The white kaolin which 

 is less abundant and constant than in the Guia, affords the 

 same residue of rutile and anatase with rare prismatic grains 

 of xenotime. In a washing from the kaolin of the Bamba 

 mine the residue, consisting almost exclusively of anatase with 

 some grains approaching a macroscopic size, was nearly 0*05^ 

 of the quantity washed. In another from Misael's mine the 

 residue is scanty, as in the greater part of the tests from the 

 Guia, and consists principally of rutile with rare grains of ana- 

 tase. In both the xenotime is in rare individuals, mostly in 

 fragments, of comparatively large size showing a tendency to 

 assume macroscopic proportions. 



The schistose accompaniment of the veins at Sopa is, in the 

 few cases observed in which it is well preserved 3 a well lami- 

 nated, almost papyraceous, micaceous greenish schist that gives 

 on decomposition an ash-colored, or reddish, clay, not distin- 

 guishable, except by its residue, from the clays derived from 



* Owing to this circumstance, and to the difficulty of avoiding admixtures, the 

 tests of the material from the Guia in which the rare accessories proved to be 

 infrequeDt and inconstant, were many times repeated and with special precau- 

 tions. The identification based on form, optical properties and behavior with 

 heavy liquids was confirmed by microchemical tests for phosphoric acid and for 

 cerium and yttria. 



f The question of the diamond in these deposits will be discussed in another 

 place.^ The conclusion presented in former papers, that ihe gem occurs in vein 

 material, was based on the examination of amass shown me in 1880 as a diamond- 

 bearing body, but which I am now convinced was a detached section of the Guia, 

 in which, according to the best obtainable information of the present day, no 

 diamonds have been found, and which presents no evidence of having been 

 worked. On the other hand, it is certain that one of the diamond-bearing bodies 

 not now visible was closely similar to it in appearance, though perhaps not in 

 original character and origin. The section of the Guia examined in 1 880 was 

 much richer in tourmaline than the parts washed on the recent visit. 



