hyre, and Leucite lava.

(2) _The Acid_ (Lighter)--rich in silica, poor in iron; containing
silica 62-78 per cent. Examples: Trachyte, Rhyolite,
Obsidian, Domite, Felsite, Quartz-porphyry, Granite.

The Andesite group forms a connecting link between the highly acid and
the basic groups, and there are many varieties of the above which it is
not necessary to enumerate. Durocher supposes that the molten magmas of
these various rocks are arranged in concentric shells within the solid
crust in order of their respective densities, those of the lighter
density, namely the acid magmas, being outside those of greater density,
namely the basic; and this is a view which seems not improbable from a
consideration not only of the principle itself, but of the succession of
the varieties of lava in many districts. Thus we find that acid lavas
have been generally extruded first, and basic afterwards--as in the
cases of Western America, of Antrim, the Rhine and Central France. And
if the interior of our globe had been in a condition of equilibrium from
the time of the consolidation of the crust to the present, reason would
induce us to conclude that the lavas would ultimately have arranged
themselves in accordance with the conditions of density beneath that
crust. But the state of equilibrium has been constantly disturbed. Every
fresh outburst of volcanic force, and every fresh extrusion of lava,
tends to disturb it, and to alter the relations of the interior viscous
or molten magmas. Owing to this it happens, as we may suppose, that the
order of eruption according to density is sometimes broken, and we find
such rocks as granophyre (a variety of andesite) breaking through the
plateau-basalts of Mull and Skye, as explained in a former chapter.
Notwithstanding such variations, however, the view of Durocher may be
considered as the most reasonable we can arrive at on a subject which is
confessedly highly conjectural.

(_j._) _Conclusion as regards the Ultimate Cause of Volcanic
Action_.--Notwithstanding, however, the complexity of the subject, and
the uncertainties which must attend an inquiry where some of the data
are outside the range of our observation, sufficient evidence can be
adduced to enable us to arrive at a tolerably clear view of the ultimate
cause of volcanic action. So tempting a subject was sure to evoke
numerous essays, some of great ingenuity, such as that of Mr. Mallet;
othe