62 
its former existence. The waves work their will in the very throat 
of that of St. Ivilda, and have cnt huge lanes through the once 
continuous plateau of the Faroes. 
Aside of the smoke-associated lai^illi and ashes of a volcano, 
which may he spattered anywhere, its lava-flows are divided hy 
chemical rec|uiremcnts into well-marked classes ; of these one is 
characterised hy an excess of those ingredients which we term 
acid, while another is deficient in the normal amount of that 
ingredient, and is termed basic. Of these two classes of rocks 
the former is much the lighter, and, in the seething and boiling 
vortex of the volcanic heat, floats upon the surface of the latter. 
When, therefore, an eruptive-throe disgorges the liquid contents, 
the superior rock, being closer to the vent, is the first to be thrown 
out, while some residual portion at least of the latter must remain, 
or be thrown back by gravitation into the volcanic throat, to form 
a solid plug to its central and vertical shaft, upon the expiry and 
the abstraction of the heat. Such a central plug must be the last 
2)ortion of a volcanic cone to be assailed by assaults from without : 
and, as it is not only the densest, but the hardest of the materials 
of the structure (from the 2>rolonged continuance of its vitrification), 
it frequently long remains to vouch for the former existence of those 
other rocks which had performed for it the function of a mould. 
The throats of former volcanoes may therefore present themselves 
to us either by a more or less circular vertical and highly vitrified 
shaft of dense and heavy rock, or by a somewhat more amjfie 
circlet of rocks which have moulded themselves against the retain- 
ing cliff-slopes of a crater. 
Such a cruier-cast the Shiant Islands seem to be. They may 
represent the first of a line of orifices between Skye aird the 
Faroes. Tachylite is the most jrerfectly vitrified form of a basic 
igneous rock ; basalt is the next. The Shiants consist of basalt. 
But they also contain rocks other than those of volcanic origin. 
Some small j^oi’tions of the land over whose planes the volcanic 
cone was thrust, remain, but they remain only because held in the 
grasp, and preserved i'rom total ruin and removal by the durable 
material which now sheathes and almost envelopes them. Those 
rocks consist of Liaosic limestones and shales. Tlicy, however, 
repose in fiat sheets upon igneous rock, though they are also over- 
flowed by it. That igneous rock, therefore, formed the sea-bottom 
