33° 
THE TERTIARY VOLCANOES 
BOOK TO! 
position of these rocks. It is found enclosing the augite grains and 
behaving like a groundmass between the felspars. Among the dark bands 
there occur narrow lenticular black layers (‘ schlieren ’) composed entirely of 
augite and iron-ore. 
The extraordinary differences between the composition of the pale 
felspathic and the dark ultra-basic bands are well brought out in the 
following analyses by Mr. J. Hort Player, No. 1 being from a light-coloured 
band consisting mainly of labradorite with some augite, uralitic hornblende 
and magnetite; No. 2 from a dark band composed of augite, magnetite and 
labradorite ; and No. 3 from a thin ultra-basic layer mainly formed of augite 
and magnetite. All these specimens were taken from the ridge of Druim 
an Eidhne, on the eastern side of the Cuillin Hills, Skye. 1 
I. 
II. 
III. 
Silica 
52-8 
40'2 
29-5 
Titanic acid . 
■5 
4-7 
9-2 
Alumina 
17-8 
9-5 
3-8 
Ferric oxide . 
1-2 
9-7 
17-8 
Ferrous oxide 
4-8 
12-2 
18-2 
Ferric sulphide 
•4 
•4 
Oxide of manganese . 
•4 
■3 
Lime 
12-9 
13-1 
10-0 
Magnesia 
4-8 
8-0 
8-7 
Soda 
3-0 
•8 
•2 
Potash 
•5 
•2 
■1 
Loss by ignition 
1-2 
•5 
1-0 
99-5 
99-7 
99-2 
Spec. grav. 
2-91 
3-36 
3-87 
se-grained massive 
Gabbros. — These rocks, 
so abundant among 
the great basic bosses of the Inner Hebrides, are characterized by their 
coarse granitic structure, their component crystals being sometimes more 
than an inch long. They occur as sheets, veins and irregular masses 
traversing the varieties of gabbro already mentioned. They consist of the 
same minerals as the banded forms, and indeed are themselves sometimes 
banded. They are more uniform in composition than the typical banded 
gabbros, though showing also some variation in the relative proportions of 
their constituents. The specific gravity of three specimens was found to be 
2-82, 2-97, and 3'06. 
(4) Pah Gabbros of the Veins . — These occur abundantly as irregular 
branching veins, from less than an inch to several yards in width, and cross 
all the other varieties (Fig. 330 2 ). Their whiteness on weathered surfaces 
1 Quart. Journ. Geol. Soc. vol. 1, (1894), p. 653. Banded structures have been recognized in 
many gabbros of different ages. See the references in this paper ; also Mr. W. S. Bayley, 
Journ. Geol. Chicago, ii. (1895), p. 814, and vol. iii. p. 1. 
2 Figs. 330, 336 and 337 are from photographs taken for the Geological Survey by Mr. R. 
Lunn. 
