The external and internal transforming agents are therefore of
the deepest interest and of the highest economic importance,
and the further study of the deep-seated changes leads directly
to a consideration of the formation and sustained activity of the
molten materials which find their main present-day expression in the
phenomena of volcanoes. Their extension in the past is also revealed
by the wearing hand of time in the wide distribution of coarsely
crystalline granites and other igneous rocks, once deep-seated,
but now exposed in regions which are either the cores of ancient
continents or centres of exceptional deformation.
Igneous action and movements of the earth’s crust stand in intimate
relation to one another, a point which has been clearly stated by
Dr. Harker[9] as follows:—“Setting aside operations conducted in
hypothetical intercrustal magma-basins, or generally in the unknown
depths of the earth’s crust, we recognize the actual manifestations
of igneous action chiefly in the forcing outward of molten magmas from
a lower to a higher level within the crust or through the crust to the
surface.” This study has led directly to the recognition of three
phases of igneous action, of which two are intrusive, being phases in
which the molten materials are raised from a lower to a higher level
within the earth’s crust, and an extrusive phase, in which these
materials are raised to the surface and poured out there as lavas.
Study in all parts of the world has further shown that these events
follow a definite sequence or cycle of igneous activity, volcanic
phenomena marking its commencement, followed by the movement of
deep-seated igneous molten magmas (representing the plutonic phase),
and closing with a number of minor intrusions, which may seam both the
volcanic, plutonic, and sedimentary strata. Egypt clearly illustrates
this remarkable succession, and thus gives additional grounds for
believing that it is of fundamental importance. Volcanic activity
was developed on a gigantic scale when the most ancient sediments
of Egypt, now forming the folded and altered slates and schists
of the Red Sea hills, were being laid down. To this part of the
cycle belong some of the most interesting rocks of the country,
the imperial porphyry of Dokhan, the dark andesites that crown
the highest summits in Sinai, and the country-rock in which some
of the most ancient of Egypt’s gold-mines are situated. Here,
too, belong the banded and columnar lavas of the Sixth Cataract,
and the fragments of volcanic rocks which play an important part in
the characteristic conglomerates of the Eastern Desert.
Still more conspicuous is the phase of plutonic activity, vast
masses of granite, diorite, and other highly crystalline rocks
as molten magmas having been in contact with or intruded into the
overlying sediments and volcanic materials. From Sinai to the Sudan
there is a geographical complex due to the intermingling of these
deep-seated igneous rocks with the older and metamorphosed volcanic
and sedimentary members. Desolate volcanic hills of dull-green shade,
whose sides are covered with weathered debris of irregular outline,
alternate with broad plain, out of which rise rounded masses
of granite, or with mountain ranges, whose precipitous sides and
serrated outlines constitute some of the most striking features of
the Red Sea hill scenery.
The final phase, that of the minor intrusions, is admirably
illustrated in Egypt, large areas of the desert consisting of low
ridges formed of parallel bands or dykes of hard rocks, which have
seamed the softer granite, the latter having subsequently been worn
down by erosion. In other cases the dyke is of more basic composition
and more easily denuded than the adjacent rocks.
When these evidences of past igneous activity are examined, they
reveal the interplay of such varied and complicated chemical and
physical effects that a large volume would be required to set forth
the great body of facts observed. Of these only a bare statement of
the most incomplete order must suffice. Igneous rocks are of very
variable chemical composition, and for the purposes of discussion of
their complex relationships have been subdivided into three series
according to the amount of silica they contain, viz.:—The Acid
Rocks with 50 to 70 per cent silica; The Intermediate Rocks with 50
to 60 per cent silica; The Basic Rocks under 50 per cent silica. As a
whole, the igneous rocks are compounds of the silicates of alumina,
magnesia, lime, potash and soda, with oxide of iron; the silicates
of alumina, soda and potash are most abundant at the acid end of
the scale, and those of lime, magnesia, with the oxides of iron
and phosphates at the basic extreme. In consequence, the basic
rocks are usually darker in colour and heavier than the more acid
varieties. Not only have the igneous rocks a varying composition,
but also a varying structure depending on the differences of
origin and position. We may note their history as involving several
stages, viz.:—(1) Solidification from the initial molten magma;
(2) Deformation when under the influence of great earth pressures;
(3) Transformation under the meteorological influences, at or near
the earth’s surface. In addition there are the contact alterations
produced near their junctions with the overlying strata, both within
themselves and in the beds affected by the molten material.
A molten magma varies greatly in its behaviour according to
its position, the solidification of that portion which remained
deep-seated in the hidden reservoir beneath the earth’s crust
taking place slowly and imperceptibly. A rock such as granite is
not formed suddenly by instantaneous cooling, but step by step
individual minerals are crystallized from the complex solution, and
tend to develop as regular and often most beautiful crystals. It
is a source of surprise to most to learn that the greater number
of mineral substances, if free to develop from solution without
external hindrances, tend to form solid bodies which are fashioned on
geometrical principles, constituting one of the most interesting and
mysterious problems with which scientific thought has to deal. The
minerals, as a rule, tend to crystallize out directly in the order
of their basicity, the oxides of iron with their eight-sided crystals
being among the first, so that their octahedral outlines are usually
well preserved. The other minerals of the rocks follow in succession;
sometimes complicated intergrowths occur owing to two crystals of
different composition starting their formation at the same time in the
same portion of the solution. Finally, the last-formed minerals have
to occupy the interspaces left by their consolidated neighbours, and
in the more acid rocks this unenviable position is usually reserved
for the free silica, forming _quartz_. These deep-seated masses,
slowly solidified and entirely crystalline in their structure,
have been called the _Plutonic_ rocks, the example familiar to all
dwellers in Egypt being the red Aswan, or monumental granite. In it,
the felspars and micas or hornblendes present tend towards their
true crystal form, while the quartz occupies the interspaces.
Intermediate between the extruded volcanic rocks, and the highly
crystalline varieties of the internal reservoirs of molten material,
are the dyke or hypabyssal rocks, which as narrow vertical or nearly
vertical intrusions pass in all probability from the deep-seated
source towards the surface of the earth, when they reach it giving
rise to volcanic effects. The dykes have a tendency to be parallel to
one another, and in some regions are the most conspicuous features,
if harder than the surrounding rock giving rise to marked ridges
separated in many cases by comparatively shallow valleys. Not a few
of the main summits in the Red Sea hills and Sinai have assumed their
present form and outline owing to the unequal denudation of one of
these hard bands, while many an ascent, which otherwise would have
been by no means free from danger, is simplified by the formation of
gullies due to the wearing away of the softer and more basic members
of this series.
In the hypabyssal varieties, as well as in surface flows, an
examination of hand-specimens often reveals the presence of
larger crystals (usually termed porphyritic crystals) scattered
in a ground-mass in which no definite structure is visible except
under the microscope. These may in part have been formed before
the molten magma of the rock had begun its upward ascent, but in
general they and the finer-grained base, have crystallized under
similar conditions. Examinations of the ground-mass microscopically
also shows it to be entirely crystalline in its nature, but the
individual components are of very small size.
In the extruded volcanic rocks, on the other hand, the porphyritic
crystals are probably of much earlier date than the ground-mass in
which they are present, the latter being solidified above-ground,
where cooling was rapid, pressure suddenly reduced, and many of
the gaseous constituents were free to escape readily. As crystals
require a certain temperature and a slow liberation of the heat for
their formation, in many volcanic rocks the rapid cooling results
in the production of a glass instead of a crystalline aggregate,
so that the presence of a glassy matrix is in itself evidence of a
quick loss of heat.