X-Ray Analysis of the Gingin Upper Greensand 
239 
The Coarse Frnctiojis. 
Upper OreensayuL — This sample contains an amount of crystalline material 
intermediate between the amounts occurring in the soil and subsoil. The 
main constituent minerals are glauconite and quartz. The glauconite occurs 
in yellow-green to brown non-pleoehroic grains which shoY’ aggregate polarisa- 
tion. The refractive indices lie between 1 • (500 and 1 ■ 020. This gives a 
maximum birefringence of 0*()20. The cpiartz content is not liigh. The 
grains arc subangular and are usually coated with glauconite granules. 
There oc^curs in addition to glauconite and (juartz a sjuall amount of 
each of the following particles : 
(а) A light green fibrous mineral which in its clear unaltered form has a 
refractive index greater than, but close to, metliylene iodide (1-74). 
The birefringence- is variable but not high. Tlu^ fibres, which have 
positive elongation are alnnys arranged in a radial mannei’, giving 
a spherulitie struct ure. The fibres when viewi'd end on are greenisli- 
blue. non-pleochroic. witli the birefringence masked by the original 
colour. In this oritntation the mineral is granular in appearance, 
frequently coats other minc'rals and has a j'aiige of refractivn indices 
from about 1 • (iS— 1 • 72. 
Alteration readily takes place at the pt'riphery of the spherul- 
ites, sectors of which then show a plumose structure. Thf' altera- 
tion is accompanied by a reduction in 7 *efractive index and an 
increase in birefringence. As alha-ation advances the fibrous 
structure is lost and the grains take on an oolitic appearanct' (in the 
form of concentric shells) while still retaining tho .s])herulitic cross. 
These grains are markedly birefringent and the refractive indices lie 
between 1 * Of) and 1 * 085. At this stage the grains tend to break up 
into smaller curved laminae formed by the splitting off of successive 
concentrif^ shells. In tlu%se particles the refractive index is de- 
creased but does not <lrop below about 1 -58. The birefringence is 
also often decrease! I . 
(б) Brownish-coloured grains, somt^ of whicli show traces of a spherulitie 
cross. The birefringence of these particles is obscured by the 
colour, but the refractive indices vary from I -01 1 • (54. 
(c) Particles very similar to the alteration products of (a) but distinguislied 
horn tliem b_\ th(^ absence of a sphfirulitic ci'oss. The particles ar(' 
only of weak to moderate birefringence and the mean refractive 
index is about 1-585. The low birefringence coukl be diu! to lack 
of orientation of aggregating ]>articles, but an occasional particle 
is sufficiently well ori(nit('d to give a biaxial figure of negative 
optical character and of small axial angle. 
{(I) Ihirticles distinguisfM'd from (c) mei'ely by the fact that they are 
only very weakly liinTringent and their refractive indices' vary 
from 1*535- d-555. Tlieir colour is light yellow-green. 
TJie Snhsotl.- Tiu' most notiiu'able featun' of tlu^ subsoil is the large 
amount of crystalline material of which only a small amount is ijuartz and 
glauconite. The (piartz as before forms subangular grains <*oated with other 
minerals. 1 he glauconite is recognised by its possessing similar optical proper- 
ties to the glauconite in the upper greensand. The refractive indices are, how- 
ever, a little lower, the range being from 1 -58- -I •()(). The remaimltn* of the 
sample is made up of the particles a — d considered in the upprr greensand 
section. 
