O. A. Derby — Association of Argillaceoios Rock, etc. 343 



Art. XXXYIII. — On the Association of Argillaceous Rocks 

 ivith Quartz Veins in the Region of Diamantina, Brazil / 

 by Orville A. Dekby. 



In a recent excursion in the region about Diamantina, Minas 

 Geraes, Brazil, the frequent association of the decomposition 

 products of argillaceous rocks with the quartz veins with which 

 the underlying metamorphic series of the district is threaded 

 attracted attention. The phenomenon was the more striking 

 because of its absence in a long stretch of road over a newer 

 series in which quartz veins are equally if not more abundant. 



The most frequent appearance is that of a selvage to the 

 veins and of intercalations in the mass of the quartz of a red- 

 dish foliated clay apparently resulting from the decomposition 

 of a micaceous rock. Less prominent in the surface exposures 

 but quite frequent in the artificial openings that have exposed 

 the inner portions of the veins, are nests and stringers of a 

 structureless white clay (kaolin) that may be either farinaceous 

 or indurated (lithomarge). The red clay was observed in a 

 sufficient number of instances to warrant the assertion that it 

 is a general, if not a universal, feature of the quartz veins of 

 the region and that when it occurs it is constant throughout 

 the entire extension, both lateral and vertical, of the vein. 

 The distribution of the white clay, on the other hand, is patchy 

 and considerable portions of a vein in which it is known to 

 occur may be free from it. 



A magnificent exposure of a vein of this character is afforded 

 by the eastern wall of the great cutting of the diamond mines 

 of Sao Joao da Chapada, of which an excellent view, repro- 

 duced from a photograph, is given in Boutan's monograph on 

 the diamond (Encyclopedic Chemique de Fremy), and in a 

 recent paper by the present writer in the Journal of Geology 

 (vol. vi, p. 121). Contrary to the description previously given, 

 based on a rapid examination made under unfavorable circum- 

 stances and with a limited experience in the identification of 

 decomposed material, this cutting is made along the junction 

 of a conglomeritic diamond-bearing quartzite, and an older 

 fine-grained quartzite (itacolumite), both being profoundly de- 

 composed and in many places indistinguishable except on the 

 closest scrutiny. The eastern wall, which on account of its 

 inaccessibility in the wet season, when it is defended by a 

 frontage of quicksand, and of its superficial staining from the 

 highly-colored overlying soil- cap has been described as a com- 

 plex of various schists, proves to be a uniform mass of itacol- 

 umite with only a single argillaceous member that presents in 

 a striking manner the above-mentioned association with vein 



