DESCRIPTION AND NAMING OF SEDIMENTARY ROCKS 657 
of thin sections made therefrom. Each description, though brief, 
includes (1) locality, position in stratigraphic column, and descrip- 
tion of field specimen; (2) texture, list of constituents, and relative 
proportion of more important minerals; (3) description of the 
chief mineral or minerals; (4) brief description of minor miner- 
als; (5) name. Items (12), (2), (3), (5) are never omitted, and 
here each is given a separate paragraph; (4) is sometimes 
omitted. Terms logically connected are hyphenated, as fine- 
grained; glauconite-limestone; ferruginous-calcareous. 
The descriptions are, as stated, purely illustrative. It may be 
added, however, that the writer has ventured partly to base upon 
these and other thin sections certain conclusions upon Middle 
Cambrian paleogeography rather at variance with the accepted 
account for Wyoming. 
Wy 16: Taken in unnamed creek on south side of Duncom 
Mountain, + mile east of Devil Canyon Road, 20 ft. above 
granite. 
Description of hand specimen: Arkose, massive, gray-yellow 
to buff, weathering brownish-white to gray, with grayish-pink 
irregular lenses, cross-bedded; pebbles quartz and feldspar, usually 
about + in., in finer but arkosic matrix; quartz angular to sub- 
rounded, iron-stained; feldspars pink angular cleavage fragments; 
traces of basic material ? 
Texture granular-fragmental. Constituents quartz, 80 per cent; 
microcline and a plagioclase near oligoclase, 15 per cent; small 
amounts of orthoclase, biotite, ilmenite, zircon, apatite, as acces- 
sories, and kaolin, sericite, leucoxene, muscovite, limonite as 
alteration products. Liquid and gas inclusions in quartz. Cement 
quartz and limonite. 
Two marked groups of quartz grains. Larger average 0.4 to 
0.5 mm. in diameter, smaller 0.06 to0o.o8 mm. Larger grains sub- 
rounded to oval, smaller angular. Vein quartz suggested by wavy 
extinction. 
Microcline fragments quadrangular, fresh; plagioclase same. 
Either rarely over 0.08 mm. Muscovite apparently not primary. 
Classed: granitogene fine-grained arkose. 
