240 
W. F. Cole 
The SolL 4’h(‘ most noticcahh' featinv of tiio soil sample is the small 
amount of crystHlliiu' material of whieh fiuart/. is the main constituent. This- 
mineral is h<‘avily coatial with iron oxides. Oc.casional grains of glauconite 
and the alteration t)i’oducls previously described (of which some of the brown 
grains si)ow traces of a, splierulitie cross) occur but the main fraction of the 
sample consists of opa(|iie grains yellow-brown in reflected light. Reddish- 
brown iron oxid(‘ grains ai‘(‘ frc(]uent. 
The Fine Fnietions. 
In the det(^rmination of the opti(*al properties of the 2 ^ fractions use 
was made of tlie fact (HI) that indtvidunl particles existing iJi a soil colloid 
suspt'Tision tend to orient themsc'lv'es after drying into aggregates which 
possess uniform optical properties. This takes place even wht'n two or more- 
clay minerals anj 2 )resent so that the o 2 )tical projaa-ties of the aggregates- 
depend upon the pro])orlion of the different constituents forming the aggi’e- 
gates. 
Jn the soil colloids oxamiiuMl in this study there was no marked tendency 
for the aggi'ogates to show a crystallographic orientation of the individual 
particles. No aggregatf^s were found sufficiently well oriented to give an 
intorf(n*ene(i ligia*e from whicii the oj3tioal character could bo determined. 
However, enougli oriented aggregates were found to determine tlie oj 3 tical 
constants. 
I he uj> 2 >er gn'ensand fratdiou wlien viewed beneath the microscope was 
seen to bo in the form of light gi-een aggregates up to 50 /x and greater in di- 
amet(*i‘. About 2i) p(‘t’ cent, of these aggi'cgates show fairly uniform crystallo- 
grapfiic oi’ientatiou of individual compoiieiits and for these y — a ^ 0*01.. 
Tlie remnind('r of tin* aggregates are non-liirefringent. 
I he subsoil fi*action whcai vknved b<‘neath tlie mieroseopo was S('en to be 
in tlie form of light yellowish-brown aggregates uj) to 50 fi and greater in di- 
auK'ter. I lu're is aljout the same jiroportion of bindVingfnit material as in 
the uppi'r gretaisfuid sampk*. The l)ir(dringetLco as measured appears to be 
about 0-005. I’lu's figm*(‘ is probably low due to tlie fact that tlie colloid 
particles ]3i*ol3ably coat('d witli iron oxides or hydroxides whicli obscure 
tlu' birt'fring(ni( (‘, 
I 1k^ soil fraction when viewed beneath the microscope was seen to bo in 
tlie form of dark lirown non-bir('fi’ing('nt particles up to 50 jx and greater in 
dianu'tcu*. 
The t)pti(',al jirojierties of aggn'gates of the soil colloid fractions from the 
upper grec'nsanti, subsoil, and soil are summarised in Table ITT. 
Tahle Til. 
OPTICAL 
DATA RELATIN(; TO THE FINE 
FRACTIONS. 
Sample. 
a 
± 0-002 
Mean 
Refractive 
Index 
+ 0-005 
r 
+ 0-002 
y — a 
Ippcr greensand ... 
1-018 
1-028 
0-010 
Subsoil 
l-OOO 
1-005 
0-005 
Soil ... 
1-020 
... 
