MICROPEGMATTTE IN PURCELL SILLS. 
9 
Si0 2 ... 
51*92 
Na 2 0 
. 1*38 
Ti0 2 ... 
1*83 
K 2 0 
. 0*47 
A 1 2 0 3 . . 
14*13 
h 2 o+ 
. 0*10 
Fe 2 0 3 . . 
2*97 
H 2 0- 
1*07 
FeO... 
6*92 
P 2 O 5 
0*04 
MnO. . 
0*14 
COo 
. 0*06 
MgO . . 
8*22 
CaO... 
11*53 
100*78 
S.G 
3*000 
The norm of the hornblende gabbro is: — 
Quartz 
6*78 
Orthoclase 
2*78 
Albite 
11*53 
Anorthite 
30*86 
Diopside 
21*07 
Hypersthene.. . : 
19*44 
Ilmenite 
1*52 
Magnetite 
4*41 
Water *1 
Carbon dioxide/ 
1 * Zo 
Hence, in the quantitative classification, the rock is salfemic, 
quadrofelic, percalcic, presodic. 
Towards the centre of the thick sills, there are often con- 
spicuous streaks of light coloured material in the gabbro, which 
generally approximate a position parallel to the upper and lower 
contacts of the sill. This variation strongly resembles the 
banding in the gabbros of Skye, described by Harker . 1 The 
light coloured bands are pegmatitic in character and resemble 
segregation veins. There are two phases of the pegmatitic 
gabbro — an acid and a basic phase. The acid phase consists 
chiefly of quartz and the feldspars, orthoclase and oligoclase- 
albite. The orthoclase occurs as intergrowths with quartz 
forming micropegmatite. In this instance, quartz holds the 
feldspar, which is the reversal of the usual relations of the min- 
1 A. Harker, Mem. Geol. Sur. of the United Kingdom, 1904, pp. 90-92, 117, 121. 
