78 
Keith R. Miles. 
The red opaque material is limonite , obviously a product of weathering 
and decomposition of other iron bearing minerals in the nodules, viz., glaucon- 
ite and magnetite- hematite. Although usually concentrated in seams or 
veinlets, in some of the more weathered specimens the limonite also occurs 
as a very finely divided material staining the collophanite matrix in irregular 
patches. In such cases estimation of the relative quantity of limonite present 
becomes rather difficult. 
The collophanite cement where fresh is pale fawn to light yellow-brown 
coloured and completely isotropic. It is for the most part massive and struc- 
tureless except where it has replaced fragments of woody tissue or where, 
particularly surrounding quartz grains and vugs in nodule A, it displays dis- 
tinct colloform structures. Under high powers apparently clear collophanite 
is seen to be filled by microscopic inclusions in which limonite, glauconite 
and gas bubbles can be recognised. Under high powers the colloform areas 
in nodule A and to a lesser extent in B, D, and E, reveal crusts with banded 
subradiating structure, of a pale grey coloured very weakly birefringent 
mineral. This has straight extinction. Columns of crusts are sometimes 
length slow and sometimes length fast so that though some of this may be 
collophanite it is probably in part the secondary dahllite, or francolite, or a 
related mineral. 
In the more weathered portions of specimens examined the collophanite 
matrix is usually altered to a red-grey cloudy opaque material often heavily 
stained with limonite. 
Accessory detrital minerals noted were zircon in rare broken grains in 
nodules E, A, and B and one or two grains of rutile and pink garnet in A. 
It may be noted here that no calcium carbonate minerals and no iron or 
aluminium phosphates were seen in any of the slices examined. 
Relative Composition . — To determine the approximate mineral compo- 
sition of the phosphate nodules micrometric analyses were made of four speci- 
mens — A, B, and C from Cook’s Deposit and nodule E from Minyulo. The 
results of these analyses are set out in the following table : — 
Table 1. 
APPROXIMATE MINERAL CONTENT OF PHOSPHATE NODULES. 
Mineral. 
Cook’s 
Deposit. 
Minyulo 
Deposit. 
Nodule 
A. 
Nodule 
B. 
Nodule 
C. 
Average. 
Nodule 
E. 
0 / * 
/o 
% 
0 / 
/o 
% 
0 / 
/o 
Quartz 
24* 
17 
32* 
42 
Collophanite 
31* 
45* 
61* 
46 
32 
Glauconite 
4* 
20 
13 
12* 
15 
Iron Ore ... 
O 1 
7 
6 
5i 
9 
Felspar 
5 
3 
2* 
3* 
2 
100 
100 
100 
100 
100 
* All percentages are by weight. 
In carrying out the micrometric measurements, the author probably 
tended to overestimate the iron ore, especially limonite — particularly in the 
more weathered specimens B, C, and E — since he was inclined to class brown- 
ish, iron-stained glauconite granules as wholly limonite at times, and it is 
considered that the figures for iron ore shown above may be a little high. 
