312 
THE TERTIARY VOLCANOES 
BOOK VIII 
especially towards the right hand of the section, they might, on casual 
inspection, he easily mistaken for nodules in the dark shales. Somewhat 
later in the time of intrusion are veins of basalt which, as at c, break across 
the nodular sills, and sometimes expand into thicker beds (V). 
I have never seen such a congeries of minute sills among the Tertiary 
basalt-plateaux as that here exhibited. In a space of about three feet of 
vertical height there must be more than a dozen of roughly parallel leaves 
of intrusive rock. Veins (e) run up from the chief band of eruptive material 
into the overlying finely vesicular basalt (/). The dyke (y) is probably the 
youngest rock in the section. 
The more general and extensive submergence of the base of the basalt- 
plateau on the west side of Skye has for the most part carried the platform 
of sills below sea-level, so that it is only exceptionally where, owing to local 
irregularities, that base has been brought up to the air, that the intrusive 
sheets show themselves. Yet the persistence of the platform on that side 
is indicated by its extension even as far as the southern promontory of the 
island. 
The Trotternish type of sill extends down the west coast under the 
headlands of Diminish. Thus at the mouth of Dun vegan Loch, where the 
underlying Jurassic platform has been ridged up above the surface of the 
sea, it has carried with it the marked sill which forms the islets of Mingay 
and Clett that lie as a protecting breakwater across the entrance of the inlet. 
The intrusive rock rests on shell-limestones full of oysters ( Ostrea hebridicci), 
and referable to the Loch Staffin group of the Great Oolite Series. This 
sill, when observed from a little distance, presents the usual regularly pris- 
matic or columnar structure so well developed among the Trotternish 
examples, but on a closer view shows this structure less distinctly. It is an 
olivine-dolerite of medium and fine texture, which in thin slices displays 
under the microscope a distinctly ophitic structure, the abundant light-brown 
augite enclosing the striated felspars. Its lowest portion, from three to seven 
or eight inches upward from the. bottom, is much closer-textured than the 
rest of the rock and is finely amygdaloidal. Its vesicles are in many cases 
drawn out to a length of three or four inches, and the zeolites which now 
fill them look like parallel annelid tubes or stems of Lithostrotion. It is 
noteworthy also that the elongation of the vesicles has sometimes taken 
place at a right angle to the surface of contact with the underlying strata. 
But the most remarkable feature in this sill is the surface which it presents 
to the oyster-beds on which it rests. The fine-grained dark dolerite has 
there assumed the aspect of a sheet of iron-slag, with a smooth or wrinkled, 
twisted, ropy surface, which displays fine curving flow-lines. Vo one looking 
at a detached specimen of this surface would be ready to admit that it could 
possibly have come from anything but a true lava-stream that flowed out at 
the surface. The contours of a viscous lava are here precisely reproduced 
on the under surface of a massive sill. 
A little further south, the promontory of List, forming the western 
breakwater of Moonen Bay, consists of an important sill or group of sills 
