244 
THE TERTIARY VOLCANOES 
BOOK VIII 
and later writers as intermediate between pitchstone and basalt . 1 A 
chemical analysis of the rock by Mr. Barker North , 2 gave the following 
composition : — 
Silica . . ■ • • 65 - 81 
Alumina . • • • ■ 14 ’01 
Ferric oxide . . • • • 4'43 
Lime . 2 '°1 
Magnesia .... 0'89 
Soda . 4-15 
Potash ..... 6-08 
Loss in ignition . . • ■ 2 ' v0 
100-08 
The grey devitrified bands, which occur as a subordinate part 
of the mass of the Scuir ridge, are usually somewhat decomposed. Where 
a fresh fracture is obtained, the material shows a fine-grained, some- 
times almost flinty, grey felsitic base, containing clear granules of quartz, 
and facets of glassy felspar. In some places the rock is strongly porphy- 
ritie. Examined under the microscope it presents a more thoroughly 
devitrified groundmass, with the minutest depolarizing microlites, large 
porphyritic crystals of plagioclase and sanidine, grains of augite, and some- 
times exceedingly abundant particles of magnetite . 3 
Although the line of separation between the grey dull felsitic sheets and 
the more ordinary glassy pitch- 
stone is usually well defined, 
the two rocks may be observed 
to shade into each other in such 
a manner as to show that the 
lithoid material is only a de- 
vitrified and somewhat decom- 
posed condition of the glassy 
rock. This connection is 
particularly to be observed 
under the precipice at the east 
end of the Scuir. At that 
locality the pitchstone is under- 
lain by a very hard flinty 
band, varying in colour from 
white through various shades 
fflJMb 
’■A "id 
tifTmfl 
Fig. 282. — Section at tlie base of the Scuir of 
Eigg (east end). 
of flesh -colour and brown into 
black, containing a little free 
Where it becomes black it passes 
into a rock like that of the main mass of the Scuir. Such vitreous parts 
quartz and crystals of glassy 
1 Mineralogy of the Scottish Isles, vol. ii. p. 47. See also Macculloch, Western Isles, vol. i- 
p. 521, and Hay Cunningham, Mem. Wern. Soc. vol. viii. p. 155. 
2 Quart. Journ. Geol. Soc. vol. xlvi. (1890), p. 379. 
3 The microscopic structure of the identical pitchstone of Hysgeir is given on p. 247. 
