190 Derby — Accessory Elements of Itacolumite^ and 



A frequent, though not constant, accessory is tourmaline in 

 minute prisms, or prismatic fragments, with lustrous faces. 

 The appearance of the grains is that of an authogenic ele- 

 ment even when, as in two cases, there is evidence of a new 

 formation about allothogenic grains. These two cases are in 

 the residue from the rocks of the Jurumerim cutting on the 

 Minas e Rio Railroad in southern Minas Geraes", where the 

 observations on the flexibility of itacolumite recorded in this 

 Journal (vol. xxviii, 1884, p. 103) were made, and from a de- 

 composed rock of a cutting on the Mogyana Railroad close to 

 the station of Rifaina, near the Rio Grande bridge on the 

 boundary between Sao Paulo and Minas Geraes. In these 

 residues a considerable number of the larger grains of tourma- 

 line show a dark, sometimes completely opaque, nucleal portion 

 enclosed in lighter colored transparent material having the 

 same optical orientation. At first sight the appearance might 

 be taken for zonal structure, but with proper illumination, the 

 dark nucleus is seen to be either well rounded with a fretted 

 surface indicative of wear, or angular with outlines that do not 

 correspond with those of the completed individual, as would 

 be the case with zonal structure. The accompanying figures, 

 kindly drawn by Dr. E. Hussak, show some of the most inter- 

 esting cases observed. Figs. 1 to 5 are from Jurumerim, fig. 6 



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from Rifaina. Basal sections (1 and 2) are nonpleochroitic. In 

 sections parallel to the c-axis the nucleus is black when this 

 axis is perpendicular to that of the polarizer, and dark brownish 

 red when parallel to it, though it is often necessary to concen- 

 trate a very strong light on the dark center for it to appear 

 translucent. The rounded nuclei represented by figs. 1, 2, 3 

 and 6, show a roughened surface indicative of wear which is 

 less distinct, or imperceptible, in the angular fragments like 

 fig. 4. In one case the light-colored tourmaline substance of 

 the outer shell was seen filling a deep Y-shaped embayment in 

 the rounded surface of the dark nucleus. The lower dark 

 grain in fig. 5 is an iron ore, probably magnetite. 



The whole appearance of these dark-centered tourmalines is 

 that of clastic grains, either well rolled or recently broken, 

 that have been enlarged by a new formation of the same sub- 

 stance which, as in the well-known case of enlarged quartz 

 grains, takes on the same optical orientation as in the fragment 

 that serves as a nucleus. The amount of tourmaline in the 



