P.G. H. Boswell—Quantitative Methods in Stratigraphy. 107 
constituent minerals are allogenic. In the authigenic group occur 
glauconite, hematite, limonite, marcasite, opal, chalcedony, and other 
forms of secondary silica, calcite, dolomite, gypsum, barytes, etc. 
The cementing materials of rocks come under this head, as do also 
those minerals such as limonite, anatase, leucoxene, secondary silica, 
micaceous aggregates, etc., when they result from the decomposition 
in situ of other minerals. It is not certain to what extent these 
secondary minerals are also detrital, for they may have been themselves 
derived as a result of previous decomposition of rocks. Anatase 
has been frequently observed growing upon, and at the expense of, 
ilmenite and leucoxene.! The opinion has been expressed that the 
tabular form is probably always authigenic, but that the pyramidal 
form may be allogenic. Among the few deposits known to the writer 
in which anatase is really plentiful in fairly large crystals (grains 
“3mm. diameter) is the Pliocene (?) sand of St. Keverne, near the 
Lizard, Cornwall. The sand is full of ilmenite (decomposing to 
leucoxene) derived from the Lizard gabbro, and there is little doubt 
that the blue tabular anatase is a secondary product from ilmenite. 
In the Yeovil sands ( Inferior Oolite) abundant yellow tablets of anatase 
accompany the yellow and red rutile which makes up a large part 
of the non-magnetic residue. Koenigsberger describes rutile pseudo- 
morphs after anatase in the Eastern Aar mass,? and the unexplained 
abundance of rutile in many sediments indicates that much work 
_ remains to be done upon these isomers. 
Glauconite may be detrital in certain cases. Dr. A. Morley Davies 
holds the view, with which the writer agrees, that the glauconite of 
some of the Mesozoic and Cainozoic deposits was derived from older 
- beds and was not formed at the same time as the sediment. In that 
ease redeposition probably took place under similar (i.e. reducing) 
conditions to those of its formation. Certainly much of the glauconite 
of deposits of all ages from Cambrian to Pliocene has no suggestion 
of foraminiferal character about the grains. 
Side by side with the mineral analyses of the deposits, mechanical 
analyses, obtained by elutriation, should also be made. A good 
classification is that suggested by Mr. T. Crook: gravel 10 to 1 mm., 
sand 1 to -1mm., silt (or very fine sand) ‘1 to -01mm., and mud less 
than -01 mm. diameter. The material is, for practical purposes, sifted 
to |mm., and the mud or true clay portion estimated by difference 
in the elutriation with the form of apparatus he recommends.* 
If necessary, intermediate grades can be estimated and inserted later 
in the tables, which are not invalidated if sufficient grades had not 
been estimated previously. In sands (e.g. for glass-making, ete.), it 
is often desirable to know the percentage weights of the portions of 
diameter > °5 mm. and < 1 mm. (coarse sand), > °25 mm. and < ‘5mm. 
(medium sand), and > -1 mm. and < :25 mm. (fine sand); these can be 
1 J. B. Serivenor, Min. Mag., vol. xiii, p. 348, 1908; H. H. Thomas, 
Q.J.G.S., vol. Ixy, p. 232, 1909; W.R. Smellie, Trans. Geol. Soc. Glasgow, 
vol. xiv, p. 267, 1911-12. 
2 Economic Geology, vol. vii, p. 697, etc., 1912. 
3 Hatch & Rastall, Sedimentary Rocks (London, 1913); Appendix by 
T. Crook, p. 350. 
