WANLESS— LITHOLOGY OF WHITE RIVER SEDIMENTS. 193 



cent, of the whole. Another sandstone sample, not directly connected 

 with a channel, was nearly uncemented and crumbled readily in the 

 hand. This showed only 2 per cent, of calcareous cement. 



As for mineral composition, the grains show very definitely 

 their derivation from the pre-Cambrian schists and pegmatites of the 

 Black Hills. The quartz grains are ordinarily between 80 and 90 

 per cent, of the total sand and show frequent mineral inclusions, 

 mainly biotite and tourmaline, which, according to Sherzer,"* are, as 

 a general rule, characteristic of gneisses and schists. The coarser 

 sandstones contain a larger proportion of fresh, non-kaolinized feld- 

 spar than is found in the finer silts and clays of the series. Of the 

 grains determined, microcline is the most abundant species; pink 

 orthoclase is next in abundance and the plagioclase feldspars are quite 

 rare, albite and oligoclase-andesine being determined. The feldspars 

 averaged from 5 to 15 per cent, of the sands. Of the micas, white 

 mica and brown mica (muscovite and probably biotite) are the most 

 abundant, but green and black micas also are frequently present. The 

 micas are most abundant in sands with small grains and evidently 

 were laid down in quieter shallows away from the main current of 

 the streams. They may total i or 2 per cent., but rarely more. 



The heavy residues are more abundant and of larger grain in the 

 coarser conglomeratic phases of the sandstones which contain occa- 

 sional large quartz and feldspar pebbles. These were evidently 

 formed where the current was strongest, as the finer sediment could 

 not come to rest. The most abundant of the heavy minerals is a 

 pink garnet which generally occurs in good rhombic dodecahedrons 

 or combinations of these with tetragonal trisoctahedrons, or in sharp 

 angular fragments of these crystals. In one sample where the heavy 

 concentrate constituted 3 per cent, of the total sand, the garnet was 

 nearly 90 per cent, of the concentrate. The other abundant heavy 

 mineral is tourmaline, either the black massive variety of pegmatites 

 (schorl) or very small, long prisms of dark brown and green color. 

 The tourmaline never occurs in rounded grains in these sediments. 

 Magnetite is present in some sands to o.i per cent, or occasionally 

 more. It is usually in very fine well-rounded grains resembling in 



» Bull. Gcol. Soc. America, Vol. 21, 1910, p. 638. 



