224 
THE TERTIARY VOLCANOES 
book vi i r 
colour, and sometimes cracks tend to run in radiating fashion, but these are 
the only indications of radial structure. The outer boundary is sharply 
defined, and where the slice is shattered the spheroids have separated from 
the matrix. The matrix is darker than in the normal rock, being obscured 
by iron-oxide which we may conceive as having been expelled from the 
spaces occupied by the spheroids. The little crystal-fragments are enclosed 
in the spheroids as well as in the matrix, hut there is no appearance of their 
having served as starting-points for radiate growths. The flat elongated 
bodies are like the spheroids, with merely the modifications implied in their 
different shape. 
« The identity of the clear colourless substance seems to he rather 
doubtful. It is sensibly isotropic and of refractive power distinctly lower 
than that of felspar. These characters would agree with analeime, which is 
not unknown as a contact-mineral; hut it is difficult to understand how 
analeime, even a lime-hearing variety like that of Plas Newydd, 1 could be 
formed in abundance from palagonitic material. An alternative supposition, 
perhaps more probable, is that the clear substance is a glass, modified from 
its former nature, especially by the expulsion of the iron-oxide into the 
remaining matrix. A comparison is at once suggested with certain types of 
‘ Knotenschiefer,’ but respecting the thermal metamorphism of fine volcanic 
tuffs there seems to be little or no direct information.” 
Lenticular interstratifications of shale and mudstone make their appear- 
ance even in the coarser parts of the conglomerate, as may be observed on 
the beach below Canna House where, as shown in I ig. 269, some shales and 
tuffs («) full of ill-defined leaves are surmounted by a conglomerate (&). The 
deposition of this overlying bed of boulders has given rise to some scooping- 
out of the finer strata underneath. Subsequently both the conglomerate 
and shales have been over- 
spread by a stream of dolerite 
(c), the slaggy bottom of which 
has ploughed its way through 
them. 
Before discussing the prob- 
b able conditions under which 
c 
the group of sedimentary de- 
posits now described was formed, 
we may conveniently follow 
the upper conglomerate band 
of the Compass Hill, and note 
Fig. 269.— Lava cutting out conglomerate and shale. 
Shore below Canna House. 
the variations in structure and composition which its outcrop presents. 
This yellowish conglomerate can be traced along the cliffs for more than 
a mile, when it descends below the sea-level at the solitary stack of Bod an 
Stol. A few hundred yards further west, what is probably the same band 
1 Henslow, Trails. Cavil. Phil. Soc. (1821), vol. i. p. 408 ; Mr. Harker, Geol. Mag. (1887), 
ji. 414. Mr. W. W. Watts suggests a comparison with the hexagonal bodies figured by Mr. 
Monckton in an altered limestone from Stirlingshire : Quart. Journ. Geol. Soc., vol. li. p. 487. 
