CHAP. XXXVIII 
THE PLATEAU OF SMALL ISLES 
223 
It is u noteworthy fact that the sedimentary intercalations among the 
Can 11 a basalts generally end upward in carbonaceous shales or coaly layers. 
The strong currents and overflows of water, which rolled and spread out the 
coarse materials of the conglomerates, gave way to quieter conditions that 
allowed silt and mud to gather over the water-bottom, while leaves and 
other fragments of vegetation, blown or washed into these quiet reaches, 
were the last of the suspended materials to sink to the bottom. Good 
illustrations of this sequence in the case of the lower conglomerate zone of 
Ganna may be studied along the shores of Sanday, from the Catholic Chapel 
eastwards. The fine pebbly sandstones, tuffs, and shales, which there overlie 
the coarse conglomerate, are surmounted by dark brown or black carbonaceous 
shale, with lenticles of matted vegetation that pass into impure coal. Imme- 
diately overlying this coaly layer comes a sheet of prismatic vesicular basalt, 
followed by another with an exceedingly slaggy texture. 
Lenticles of shale and mudstone likewise occur in the heart of the finer 
parts of the conglomerate, especially towards the top, as may be seen in the 
section exposed beneath the basalt behind the first cottage west from Canna 
House. One of the most interesting layers in this section is a seam of 
tuff, varying up to about two inches in thickness, which lies at the top of 
the lenticular band of tuffs and shales, and immediately beneath the band 
of basalt-conglomerate, on which a basalt, carrying a vesicular band near 
its bottom, rests. Traced laterally, the dark brown tuff of this seam 
gradually passes into a series of rounded bodies and flattened shells com- 
posed of a colourless mineral which has evidently been developed in situ 
after the deposition of the tuff. Mr. Harker’s notes on thin slices made 
from this band are as follows : — 
“ This is a rusty-brown, dull-looking rock, rather soft and seemingly 
light, but too absorbent to permit of its specific gravity being tested. The 
dark brown mass is in great part studded with little spheroidal bodies, - ] (j 
to yjj inch in diameter, of pialer colour, but the larger ones having a dark 
nucleus. In other parts larger flat bodies have been formed, as if by the 
coalescence of the spheroids, extending as inconstant bands in the direction 
of lamination for perhaps \ inch, with a thickness of inch or less. The 
appearance is that of a spherulitic rather than an oolitic structure. 
“A slice [6658 a] shows the general mass of the rock to be of an 
extremely finely divided but coherent substance of brown colour, which can 
scarcely be other than a fine volcanic dust, composed of minute particles of 
basic glass or ‘ palagonite ’ compacted together. Scattered through this are 
fragments of crystals recognizable as triclinic and perhaps monoclinic 
felspars, green hornblende, augite, olivine (?), and magnetite, usually quite 
fresh. 
“ The curious spheroidal and elongated growths already mentioned are 
better seen in another slide [6658 b], where they occupy the larger part of 
the field, leaving only an interstitial framework of the brown matrix. The 
substance of the little spheroids is clear, colourless, and apparently structure- 
less. The centre is often occupied by an irregularly stellate patch of brown 
