11. A study of the volcanic products of a long series of eruptions within the same geographical region may be expected to throw light on the changes that take place during the course of ages in the character of the internal molten magma. In a former chapter ([vol. i. p. 27]) reference was made to the subject of volcanic cycles and to the sequence, observed in various widely separated parts of the world, among the materials erupted from below. Allusion was likewise made in a later chapter ([vol. i. p. 90]) to the remarkable differences in texture and composition noticeable within some large bodies of eruptive material, and to the evidence which these differences furnish of a segregation or differentiation among the constituents of an eruptive mass after it has been injected into its position within the crust of the earth.

Table of the Periods of Volcanic Action in the British Isles and
of the Chronological Distribution of the Volcanic Products.

Granites,
Granophyres,
etc.		Felsites,
Rhyolites,
etc.		Dacite,
"Pitchstone"
of Eigg.		Trachytes.Andesites
(Porphyrites).		Gabbros.Dolerites,
Basalts
(Diabases).		Picrites and
highly basic
lavas.		Tuffs,
acid or
basic.
Older Tertiary
(Plateaux, dykes,
necks, bosses,
sills)		*********
Mesozoic
No volcanic rocks.
Permian...*......*...***
Carboniferous?
Puy type...*......*...***
Plateau type...*...**...***
{Devonian..................*...*
Old Red
Sandstone
Upper..................*...*
Lower**...**...*...*
Silurian
Upper...*..................*
Lower, Bala**...******
" Arenig**...****...*
Cambrian...*......*...*...*
Uriconian...*............*...*
Dalradian..................*...?
Torridonian
Lewisian*...............**...

From the history of volcanic action in the British Isles it is clear that differentiation is effected under three distinct conditions.

In the first place, a notable difference may be occasionally observed between two adjacent parts of the same mass of lava which has flowed out at the surface. Thus, in the Carboniferous picrite of Blackburn, there has been a separation of the heavy basic constituents, which have in great part settled down into the lower part of the sheet, while the lighter felspar has mainly come to the top. In this case the gradual transition from top to bottom suggests that the separation occurred after the lava had reached the surface and taken the form of a stream or sheet.

In the second place, segregation has taken place in the magma within the terrestrial crust after intrusion, for it is frequently observable in large bosses and sometimes in sills, the basic elements having tended to mass themselves towards the margins of the rock, leaving more acid material in the centre. The cases of Garabol Hill among the Dalradian schists of Scotland, of Carrock Fell among the Silurian strata of the Lake District, and of the Cramond picrite among the Carboniferous formations of Midlothian, with others that might be cited from various other regions and geological formations in Britain, prove to what a considerable extent a separation of ingredients may take place in a boss, and even sometimes in a comparatively thin sill before the molten mass consolidates.

In the third place, there is good evidence that already before the magma is either intruded or extruded, and while it still lies within the internal reservoir, it may not possess a general uniformity of composition, but may have become more or less heterogeneous. In regard to intrusive rocks, the extraordinarily banded gabbros of the Tertiary series of Skye obviously proceeded from a magma in which the molten material consisted in some parts mainly of felspar, and in others mainly of the ferro-magnesian minerals and iron-ores. Streams from these differently constituted parts of the magma were simultaneously or successively injected as sills into the older portions of the volcanic series, while, as the process of differentiation within the magma proceeded, still more felspathic liquid was left behind, to be thrust into cracks in the sills previously consolidated.

Moreover, the banded basalts of the Tertiary plateaux show that this heterogeneity was not confined to internal intrusions, but maintained its place even when the molten material was ejected to the surface. The differentiation indeed is not so striking there as among the sills of gabbro; but its presence, even in a less degree, proves that the separation of constituent minerals was not due to any general cooling of an erupted body of igneous rock, but was already developed in the reservoir from which the molten material was propelled to the surface.

Attention has been called to the remarkable similarity of structure between these banded intrusive rocks and some of the ancient gneisses. The resemblance is so close that we may with every probability infer that the gneisses acquired their characteristic banding as intrusive masses of igneous rocks, discharged from heterogeneous magmas, like that which supplied the gabbros of the Cuillin Hills. And as these gneisses belong to pre-Cambrian formations, we are thus led to the interesting result that the tendency to develop heterogeneity was already as characteristic of the magma-basins of the earliest geological time as it has been of those of later periods.

The evidence of differentiation presented by superficial lavas, and by intrusive sills and bosses, acquires great interest when considered in connection with the changes which are seen to have occurred in the character of the materials erupted during the course of a definite volcanic period. An attentive examination of the volcanic products of the various ages, so fully recorded in the geological structure of the British Isles, shows that a recognizable sequence in the nature of the materials erupted during a single volcanic period can be traced from the earliest to the latest times, and that, in spite of occasional departures, the normal order remains broadly uniform.