190 
THE TERTIARY VOLCANOES 
BOOK VIII 
The banded character arises from marked distinctions in the texture of 
different layers of a lava-sheet. In some cases (a) these distinctions arise 
from differences in the size of the crystals or in the disposition of the com- 
ponent minerals of the rock ; in others ( b ) from the varying number and 
size of the vesicles, which may be large or abundantly crowded together in 
some layers, and small or only sparsely developed in others. The structure 
thus points to original conditions of the lava at the time of its emission and 
may be regarded as, to some extent, a kind of flow-structure 011 a large scale. 
(a) Where the banding is due to differences of crystalline texture, the 
constituent felspars, augites, and iron-ores may be seen even with the naked 
eye as well-defined minerals along the prominent surfaces of the harder ribs, 
while the broader intervening flutings of finer material show the same 
minerals in minuter forms. The alternating layers of coarser and finer 
crystallization lie, on the whole, parallel with the upper and under surfaces 
of the sheets in which they occur. But they likewise undulate like the 
streaky lines in ordinary flow-structure. 
Banded structure of this type may be seen well developed in the lower 
parts of the basalt-plateaux throughout the Inner Hebrides and the Faroe 
Islands. A specimen taken from the west end of the island of Sunday, near 
Canna, which showed the structure by a conspicuous parallel fluting on 
weathered surfaces, was sliced for microscopical examination. Mr. Harker 
has been kind enough to supply me with the following observations regard- 
ing this slice : — - 
“ In the slice [ 6660] 1 the banding becomes less conspicuous under the 
microscope. The rock is of basaltic composition, and, with reference to its 
micro-structure, might be styled a fine-grained olivine-diabase or olivine- 
dolerite in some parts of the slice, an olivine-basalt in others. It consists 
of abundant grains of olivine, imperfect octaliedra and shapeless granules of 
magnetite, little simple or twinned prisms of labradorite, and a pale brown 
augite. The last-named mineral is always the latest product of consolidation, 
but it varies in habit, being sometimes in ophitic patches moulded upon or 
enclosing the other minerals, sometimes in small granules occupying the 
interstices between the felspars and other crystals. The ophitic habit pre- 
dominates in the slice, while the granulitic comes in especially along certain 
bands. If the former be taken as indicative of tranquil conditions, the 
latter of a certain amount of movement in the rock during the latest stages 
of its consolidation, the banding, though not strictly a flow-structure, may be 
ascribed in some degree to a flowing movement of the nearly solidified rock. 
There is, however, more than this merely structural difference between the 
several bands. They differ to some extent in the relative proportions of the 
minerals, especially of olivine and augite ; which points to a considerable 
flowing movement at an early stage in a magma which was initially not 
homogeneous.” 
1 The figures within square brackets throughout the following pages refer to the numbers of 
the microscopic slides in the Geological Survey collection, where I have deposited all those pre- 
pared from my specimens. 
