68 
MUSEUM BULLETIN NO. 27. 
another, gives rise to a black cross which revolves as the section is turned 
on the stage. Frequently these wedges are traversed by a series of fine 
lines parallel to their base, a feature which is well shown in Plate XII. 
These indicate that colerainite probably has a basal cleavage, although 
it is also possible they may be due to the parallel superposition of very 
thin plates of the mineral. 
This relatively coarsely crystallized material forms a zone of fairly 
uniform width around the margin of each druse. Below this, the 
structure abruptly becomes very fine-grained, although under a higher 
power the material is seen to be similar and to display the same radial 
arrangement. Within this, the clay-like substance is dispersed as dusty 
particles, or in blotches and streaks, and it is found especially at the 
centres of the circular radial groups of crystals. Throughout the fine- 
grained material, also, there are many more coarsely crystallized patches 
and veinlets, representing small druses and crevices within the rock 
which have been completely filled. 
The following analyses of the matrix (column 1) and crystals 
(column 2) were made by M. F. Connor: 
Analyses of Colerainite. 
Matrix 
Crystals 
Molecular ratio 
Percentage com- 
position, calcul- 
ated 
% 
SiOa. ....... ... 
26*98 
24*40 
406*6. . .2 
25*42 
21*61 
AlaOs. 
16-10 
22*77 
223 *2\ , 
FeaOi. - - 
0*22 
0*45 
3-0/-- 1 
FeO. . 
None 
Not det’d 
MgO 
36*56 
32*70 
817*51 
CaO. 
0*12 
0*10 
2 * 0 1 . .4 
33*90 
(NaK)jO 
0*28 
0*30 
4-0 1 
MnO. 
0*20 
0*09 
1 -oj 
HiO. ............ 
19-91 
19*63 
1090*5.. .5 
19*07 
100*37 
100*44 
100*00 
The mineral is thus a hydrated magnesian aluminous silicate. In 
the case of the crystals the molecular ratios approximate to 4Mg0.Al 2 0 8 . 
2Si02.5H s 0 and the percentage composition calculated for such a 
compound is given in the last column above. The calculated values 
are in fair agreement with those given in the analysis, and the formula 
4 MgO.Al 2 O 3 . 2 SiO 2 . 5 H 2 O or H$Mg 2 AlSiOs is, therefore, adopted for 
colerainite. 
The massive character of the matrix naturally renders it impossible 
to be certain that the material selected for analysis was free from foreign 
admixture, but the analysis at least serves to indicate that it has essenti- 
