X-Ray Analysis of the Gingin Upper Greensand. 241 
DISCUSSION. 
Coarse Fraclions.—X-ray evidence indicates that in the coar.se fractions 
the glauconite content decreases and the <|uartz content increa.ses in pa.ssing 
from greensand to soil. The microscopic e.vamination confirms this. X-ray 
evidence also indicates the pre.spnce of montmorilloniti' and a little kaolinite 
in the subsoil, a possible trace of montmorillonite in the greensand and of 
kaolinite in the soil. In view of the fact that the sample.s e.vaminetl contain 
& fibrous mineral with .sphernlitic structure which lias not been iihmtifiod and 
which readily alters in a characteristic manner to give a variety of particles 
.some of which have optical proportic.s .similar to clay minerals it has not been 
possible to verify beneath the mioro.scope the pre.sence of any clav minerals in 
these fractions. 
Fine FracHons.^Fov the fine fraction.s X-ray evid.>nce indicates that 
‘glauconite is the main constituent of the uppcT green.sand, montinorillonito 
the main constituent of the subsoil, and kaolinite the main constituent of 
the soil. Optical data relating to pure clay minei’als havi' been summari.sed 
by the author elsewhere (19). Members of the montmorillonite and mica 
groups have similar optical properties which are markedly diftei'ent from the 
•optical properties of members of the kaolinite grouj). In Table III. it i.s 
seen that the upper greensand and subsoil have .similar optical propei-tie.s, 
both sample-s being more birefringent than tlu' soil sample which is non- 
birefringent. The non -birefringent character of the soil sarnjile could he 
due to the presence of kaolinite or to the presence of coatings of iron oxi.les 
■or hydroxides on the soil colloid particles which tend to mask the hirefringence. 
It is believed that both play a part. It must he realised that the ojjtical 
properties of aggregates of the fine fractions dt^jjend on all the con.stituents 
present, so that only broad generalisations may be drawn when the main 
•constituents alone are considered. Generally speaking the o])tical properties 
■of the fine fractions are consistent with the composition as determined bv 
X-ray analysis. 
CONCLUSIONS. 
With regard to the composition of the .sample.s X-rav data lead to the 
■conclusions summarised in Table IV. Some modilk-ations, the extent of 
which it IS difficult to estimate, may be necessary in the light of the following 
(a) The relation between relative intensities of lines and relative (piantity 
of corresponding; diifraoting^ niatt'rial. All estimates of relativ<^- 
quantities of the ditfenml constituents are based on visual obserA-a- 
tion of line intensities. ^rh('S(' estimated relative (juantities may 
correctly indicate tlie actual relative (|uuntities only if the ratio of 
the intensities of patterns of differimt constitiK'uts' is ecpial to the 
ratio of the proportions of tlie constituents. This may not l)e true 
and may (lepart widely fn)m tlie truth if quartz is oim of the con- 
stituents since the diffracting power of <)uartz is markedly greatca- 
than that of any clay mineral (9). 
(b) Microscopic evidence. 
From Table IV. the following generalisations as regards mineral changes 
an passing from greensand to soil may be drawn : — 
(а) The content of glauconite decreases. 
(б) The content of kaolinite, cpiartz and oxides and hydroxides of iron 
increases. 
^c) The montmorillonite content is concentrated in the sul)soiI. 
