PLUTONIC ROCKS--GRANITE.
General aspect of granite--Decomposing into spherical masses--Rude
columnar structure--Analogy and difference of volcanic and plutonic
formations--Minerals in granite, and their arrangement--Graphic and
porphyritic granite--Mutual penetration of crystals of quartz and
felspar--Occasional minerals--Syenite--Syenitic, talcose, and schorly
granites--Eurite--Passage of granite into trap--Examples near
Christiania and in Aberdeenshire--Analogy in composition of trachyte
and granite--Granite veins in Glen Tilt, Cornwall, the Valorsine, and
other countries--Different composition of veins from main body of
granite--Metalliferous veins in strata near their junction with
granite--Apparent isolation of nodules of granite--Quartz
veins--Whether plutonic rocks are ever overlying--Their exposure at
the surface due to denudation.
The plutonic rocks may be treated of next in order, as they are most nearly
allied to the volcanic class already considered. I have described, in the
first chapter, these plutonic rocks as the unstratified division of the
crystalline or hypogene formations, and have stated that they differ from
the volcanic rocks, not only by their more crystalline texture, but also by
the absence of tuffs and breccias, which are the products of eruptions at
the earth's surface, or beneath seas of inconsiderable depth. They differ
also by the absence of pores or cellular cavities, to which the expansion
of the entangled gases gives rise in ordinary lava. From these and other
peculiarities it has been inferred, that the granites have been formed at
considerable depths in the earth, and have cooled and crystallized slowly
under great pressure, where the contained gases could not expand. The
volcanic rocks, on the contrary, although they also have risen up from
below, have cooled from a melted state more rapidly upon or near the
surface. From this hypothesis of the great depth at which the granites
originated, has been derived the name of "Plutonic rocks." The beginner
will easily conceive that the influence of subterranean heat may extend
downwards from the crater of every active volcano to a great depth below,
perhaps several miles or leagues, and the effects which are produced deep
in the bowels of the earth may, or rather must be, distinct; so that
volcanic and plutonic rocks, each different in texture, and sometimes even
in composition, may originate simultaneously, the one at the surface, the
other far beneath it.
By some writers, all the rocks now under consideration have been
comprehended under the name of granite, which is, then, understood to
embrace a large family of crystalline and compound rocks, usually found
underlying all other formations; whereas we have seen that trap very
commonly overlies strata of different ages. Granite often preserves a very
uniform character throughout a wide range of territory, forming hills of a
peculiar rounded form, usually clad with a scanty vegetation. The surface
of the rock is for the most part in a crumbling state, and the hills are
often surmounted by piles of stones like the remains of a stratified mass,
as in the annexed figure, and sometimes like heaps of boulders, for which
they have been mistaken. The exterior of these stones, originally
quadrangular, acquires a rounded form by the action of air and water, for
the edges and angles waste away more rapidly than the sides. A similar
spherical structure has already been described as characteristic of basalt
and other volcanic formations, and it must be referred to analogous causes,
as yet but imperfectly understood.
[Illustration: Fig. 484. Mass of granite near the Sharp Tor, Cornwall.]
Although it is the general peculiarity of granite to assume no definite
shapes, it is nevertheless occasionally subdivided by fissures, so as to
assume a cuboidal, and even a columnar, structure. Examples of these
appearances may be seen near the Land's End, in Cornwall. (See figure.)
[Illustration: Fig. 485. Granite having a cuboidal and rude columnar
structure, Land's End, Cornwall.]
The plutonic formations also agree with the volcanic, in having veins or
ramifications proceeding from central masses into the adjoining rocks, and
causing alterations in these last, which will be presently described. They
also resemble trap in containing no organic remains; but they differ in
being more uniform in texture, whole mountain masses of indefinite extent
appearing to have originated under conditions precisely similar. They also
differ in never being scoriaceous or amygdaloidal, and never forming a
porphyry with an uncrystalline base, or alternating with tuffs. Nor do they
form conglomerates, although there is sometimes an insensible passage from
a fine to a coarse-grained granite, and occasionally patches of a fine
texture are imbedded in a coarser variety.
[Illustration: Fig. 486. Gneiss. (See description, p. 464.)]
Felspar, quartz, and mica are usually considered as the minerals essential
to granite, the felspar being most abundant in quantity, and the proportion
of quartz exceeding that of mica. These minerals are united in what is
termed a confused crystallization; that is to say, there is no regular
arrangement of the crystals in granite, as in gneiss (see fig. 486.),
except in the variety termed graphic granite, which occurs mostly in
granitic veins. This variety is a compound of felspar and quartz, so
arranged as to produce an imperfect laminar structure. The crystals of
felspar appear to have been first formed, leaving between them the space
now occupied by the darker-coloured quartz. This mineral, when a section is
made at right angles to the alternate plates of felspar and quartz,
presents broken lines, which have been compared to Hebrew characters.
[2 Illustrations: Graphic granite.
Fig. 487. Section parallel to the laminae.
Fig. 488. Section transverse to the laminae.]
As a general rule, quartz, in a compact or amorphous state, forms a
vitreous mass, serving as the base in which felspar and mica have
crystallized; for although these minerals are much more fusible than silex,
they have often imprinted their shapes upon the quartz. This fact,
apparently so paradoxical, has given rise to much ingenious speculation. We
should naturally have anticipated that, during the cooling of the mass, the
flinty portion would be the first to consolidate; and that the different
varieties of felspar, as well as garnets and tourmalines, being more easily
liquefied by heat, would be the last. Precisely the reverse has taken place
in the passage of most granitic aggregates from a fluid to a solid state,
crystals of the more fusible minerals being found enveloped in hard,
transparent, glassy quartz, which has often taken very faithful casts of
each, so as to preserve even the microscopically minute striations on the
surface of prisms of tourmaline. Various explanations of this phenomenon
have been proposed by MM. de Beaumont, Fournet, and Durocher. They refer to
M. Gaudin's experiments on the fusion of quartz, which show that silex, as
it cools, has the property of remaining in a viscous state, whereas alumina
never does. This "gelatinous flint" is supposed to retain a considerable
degree of plasticity long after the granitic mixture has acquired a low
temperature; and M. E. de Beaumont suggests, that electric action may
prolong the duration of the viscosity of silex. Occasionally, however, we
find the quartz and felspar mutually imprinting their forms on each other,
affording evidence of the simultaneous crystallization of both.[439-A]
[Illustration: Fig. 489. Porphyritic granite. Land's End, Cornwall.]
_Porphyritic granite._--This name has been sometimes given to that variety
in which large crystals of felspar, sometimes more than 3 inches in length,
are scattered through an ordinary base of granite. An example of this
texture may be seen in the granite of the Land's End, in Cornwall (fig.
489.). The two larger prismatic crystals in this drawing represent felspar,
smaller crystals of which are also seen, similar in form, scattered through
the base. In this base also appear black specks of mica, the crystals of
which have a more or less perfect hexagonal outline. The remainder of the
mass is quartz, the translucency of which is strongly contrasted to the
opaqueness of the white felspar and black mica. But neither the
transparency of the quartz, nor the silvery lustre of the mica, can be
expressed in the engraving.
The uniform mineral character of large masses of granite seems to indicate
that large quantities of the component elements were thoroughly mixed up
together, and then crystallized under precisely similar conditions. There
are, however, many accidental, or "occasional," minerals, as they are
termed, which belong to granite. Among these black schorl or tourmaline,
actinolite, zircon, garnet, and fluor spar, are not uncommon; but they are
too sparingly dispersed to modify the general aspect of the rock. They
show, nevertheless, that the ingredients were not everywhere exactly the
same; and a still greater variation may be traced in the ever-varying
proportions of the felspar, quartz, and mica.
_Syenite._--When hornblende is the substitute for mica, which is very
commonly the case, the rock becomes Syenite: so called from the celebrated
ancient quarries of Syene in Egypt. It has all the appearance of ordinary
granite, except when mineralogically examined in hand specimens, and is
fully entitled to rank as a geological member of the same plutonic family
as granite. Syenite, however, after maintaining the granitic character
throughout extensive regions, is not uncommonly found to lose its quartz,
and to pass insensibly into syenitic greenstone, a rock of the trap family.
Werner considered syenite as a binary compound of felspar and hornblende,
and regarded quartz as merely one of its occasional minerals.
_Syenitic-granite._--The quadruple compound of quartz, felspar, mica, and
hornblende, may be so termed. This rock occurs in Scotland and in Guernsey.
_Talcose granite_, or Protogine of the French, is a mixture of felspar,
quartz, and talc. It abounds in the Alps, and in some parts of Cornwall,
producing by its decomposition the china clay, more than 12,000 tons of
which are annually exported from that country for the potteries.[440-A]
_Schorl rock, and schorly granite._--The former of these is an aggregate
of schorl, or tourmaline, and quartz. When felspar and mica are also
present, it may be called schorly granite. This kind of granite is
comparatively rare.
_Eurite._--A rock in which all the ingredients of granite are blended into
a finely granular mass. Crystals of quartz and mica are sometimes scattered
through the base of Eurite.
_Pegmatite._--A name given by French writers to a variety of granite; a
granular mixture of quartz and felspar; frequent in granite veins; passes
into graphic granite.
All these granites pass into certain kinds of trap, a circumstance which
affords one of many arguments in favour of what is now the prevailing
opinion, that the granites are also of igneous origin. The contrast of the
most crystalline form of granite, to that of the most common and earthy
trap, is undoubtedly great; but each member of the volcanic class is
capable of becoming porphyritic, and the base of the porphyry may be more
and more crystalline, until the mass passes to the kind of granite most
nearly allied in mineral composition.
The minerals which constitute alike the granitic and volcanic rocks
consist, almost exclusively, of seven elements, namely, silica, alumina,
magnesia, lime, soda, potash, and iron; and these may sometimes exist in
about the same proportions in a porous lava, a compact trap, or a
crystalline granite. It may perhaps be found, on farther examination--for
on this subject we have yet much to learn--that the presence of these
elements in certain proportions is more favourable than in others to their
assuming a crystalline or true granitic structure; but it is also
ascertained by experiment, that the same materials may, under different
circumstances, form very different rocks. The same lava, for example, may
be glassy, or scoriaceous, or stony, or porphyritic, according to the more
or less rapid rate at which it cools; and some trachytes and
syenitic-greenstones may doubtless form granite and syenite, if the
crystallization take place slowly.
It has also been suggested that the peculiar nature and structure of
granite may be due to its retaining in it that water which is seen to
escape from lavas when they cool slowly, and consolidate in the atmosphere.
Boutigny's experiments have shown that melted matter, at a white heat,
requires to have its temperature lowered before it can vapourize water; and
such discoveries, if they fail to explain the manner in which granites have
been formed, serve at least to remind us of the entire distinctness of the
conditions under which plutonic and volcanic rocks must be produced.[441-A]
It would be easy to multiply examples and authorities to prove the
gradation of the granitic into the trap rocks. On the western side of the
fiord of Christiania, in Norway, there is a large district of trap, chiefly
greenstone-porphyry, and syenitic-greenstone, resting on fossiliferous
strata. To this, on its southern limit, succeeds a region equally extensive
of syenite, the passage from the volcanic to the plutonic rock being so
gradual that it is impossible to draw a line of demarcation between them.
"The ordinary granite of Aberdeenshire," says Dr. MacCulloch, "is the usual
ternary compound of quartz, felspar, and mica; but sometimes hornblende is
substituted for the mica. But in many places a variety occurs which is
composed simply of felspar and hornblende; and in examining more minutely
this duplicate compound, it is observed in some places to assume a fine
grain, and at length to become undistinguishable from the greenstones of
the trap family. It also passes in the same uninterrupted manner into a
basalt, and at length into a soft claystone, with a schistose tendency on
exposure, in no respect differing from those of the trap islands of the
western coast."[441-B] The same author mentions, that in Shetland, a
granite composed of hornblende, mica, felspar, and quartz, graduates in an
equally perfect manner into basalt.[441-C]
In Hungary there are varieties of trachyte, which, geologically speaking,
are of modern origin, in which crystals, not only of mica, but of quartz,
are common, together with felspar and hornblende. It is easy to conceive
how such volcanic masses may, at a certain depth from the surface, pass
downwards into granite.
[2 Illustrations: Fig. 490. Fig. 491.
Junction of granite and argillaceous schist in Glen Tilt.
(MacCulloch.)[442-A]]
I have already hinted at the close analogy in the forms of certain granitic
and trappean veins; and it will be found that strata penetrated by plutonic
rocks have suffered changes very similar to those exhibited near the
contact of volcanic dikes. Thus, in Glen Tilt, in Scotland, alternating
strata of limestone and argillaceous schist come in contact with a mass of
granite. The contact does not take place as might have been looked for, if
the granite had been formed there before the strata were deposited, in
which case the section would have appeared as in fig. 490.; but the union
is as represented in fig. 491., the undulating outline of the granite
intersecting different strata, and occasionally intruding itself in
tortuous veins into the beds of clay-slate and limestone, from which it
differs so remarkably in composition. The limestone is sometimes changed in
character by the proximity of the granitic mass or its veins, and acquires
a more compact texture, like that of hornstone or chert, with a splintery
fracture, effervescing feebly with acids.
The annexed diagram (fig. 492.) represents another junction, in the same
district, where the granite sends forth so many veins as to reticulate the
limestone and schist, the veins diminishing towards their termination to
the thickness of a leaf of paper or a thread. In some places fragments of
granite appear entangled, as it were, in the limestone, and are not visibly
connected with any larger mass; while sometimes, on the other hand, a lump
of the limestone is found in the midst of the granite. The ordinary colour
of the limestone of Glen Tilt is lead blue, and its texture large-grained
and highly crystalline; but where it approximates to the granite,
particularly where it is penetrated by the smaller veins, the crystalline
texture disappears, and it assumes an appearance exactly resembling that of
hornstone. The associated argillaceous schist often passes into hornblende
slate, where it approaches very near to the granite.[442-B]
[Illustration: Fig. 492. Junction of granite and limestone in Glen
Tilt. (MacCulloch.)
_a._ Granite. _b._ Limestone.
_c._ Blue argillaceous schist.]
The conversion of the limestone in these and many other instances into a
siliceous rock, effervescing slowly with acids, would be difficult of
explanation, were it not ascertained that such limestones are always
impure, containing grains of quartz, mica, or felspar disseminated
through them. The elements of these minerals, when the rock has been
subjected to great heat, may have been fused, and so spread more
uniformly through the whole mass.
[Illustration: Fig. 493. Granite veins traversing clay slate. Table
Mountain, Cape of Good Hope.[443-A]]
In the plutonic, as in the volcanic rocks, there is every gradation from
a tortuous vein to the most regular form of a dike, such as intersect
the tuffs and lavas of Vesuvius and Etna. Dikes of granite may be seen,
among other places, on the southern flank of Mount Battock, one of the
Grampians, the opposite walls sometimes preserving an exact parallelism
for a considerable distance.
As a general rule, however, granite veins in all quarters of the globe are
more sinuous in their course than those of trap. They present similar
shapes at the most northern point of Scotland, and the southernmost
extremity of Africa, as the annexed drawings will show.
It is not uncommon for one set of granite veins to intersect another; and
sometimes there are three sets, as in the environs of Heidelberg, where the
granite on the banks of the river Necker is seen to consist of three
varieties, differing in colour, grain, and various peculiarities of mineral
composition. One of these, which is evidently the second in age, is seen to
cut through an older granite; and another, still newer, traverses both the
second and the first.
In Shetland there are two kinds of granite. One of them, composed of
hornblende, mica, felspar, and quartz, is of a dark colour, and is seen
underlying gneiss. The other is a red granite, which penetrates the dark
variety everywhere in veins.[444-A]
[Illustration: Fig. 494. Granite veins traversing gneiss, Cape Wrath.
(MacCulloch.)[444-B]]
[Illustration: Fig. 495. Granite veins traversing gneiss at Cape Wrath, in
Scotland. (MacCulloch.)]
The accompanying sketches will explain the manner in which granite veins
often ramify and cut each other (figs. 494. and 495.). They represent
the manner in which the gneiss at Cape Wrath, in Sutherlandshire, is
intersected by veins. Their light colour, strongly contrasted with that
of the hornblende-schist, here associated with the gneiss, renders them
very conspicuous.
Granite very generally assumes a finer grain, and undergoes a change in
mineral composition, in the veins which it sends into contiguous rocks.
Thus, according to Professor Sedgwick, the main body of the Cornish granite
is an aggregate of mica, quartz, and felspar; but the veins are sometimes
without mica, being a granular aggregate of quartz and felspar. In other
varieties quartz prevails to the almost entire exclusion both of felspar
and mica; in others, the mica and quartz both disappear, and the vein is
simply composed of white granular felspar.[444-C]
Fig. 496. is a sketch of a group of granite veins in Cornwall, given by
Messrs. Von Oeynhausen and Von Dechen.[445-A] The main body of the granite
here is of a porphyritic appearance, with large crystals of felspar; but in
the veins it is fine-grained, and without these large crystals. The general
height of the veins is from 16 to 20 feet, but some are much higher.
[Illustration: Fig. 496. Granite veins passing through hornblende slate,
Carnsilver Cove, Cornwall.]
In the Valorsine, a valley not far from Mont Blanc in Switzerland, an
ordinary granite, consisting of felspar, quartz, and mica, sends forth
veins into a talcose gneiss (or stratified protogine), and in some places
lateral ramifications are thrown off from the principal veins at right
angles (see fig. 497.), the veins, especially the minute ones, being finer
grained than the granite in mass.
[Illustration: Fig. 497. Veins of granite in talcose gneiss.
(L. A. Necker.)]
It is here remarked, that the schist and granite, as they approach, seem to
exercise a reciprocal influence on each other, for both undergo a
modification of mineral character. The granite, still remaining
unstratified, becomes charged with green particles; and the talcose gneiss
assumes a granitiform structure without losing its stratification.[445-B]
Professor Keilhau drew my attention to several localities in the country
near Christiania, where the mineral character of gneiss appears to have
been affected by a granite of much newer origin, for some distance from the
point of contact. The gneiss, without losing its laminated structure, seems
to have become charged with a larger quantity of felspar, and that of a
redder colour, than the felspar usually belonging to the gneiss of Norway.
Granite, syenite, and those porphyries which have a granitiform
structure, in short all plutonic rocks, are frequently observed to
contain metals, at or near their junction with stratified formations. On
the other hand, the veins which traverse stratified rocks are, as a
general law, more metalliferous near such junctions than in other
positions. Hence it has been inferred that these metals may have been
spread in a gaseous form through the fused mass, and that the contact of
another rock, in a different state of temperature, or sometimes the
existence of rents in other rocks in the vicinity, may have caused the
sublimation of the metals.[446-A]
There are many instances, as at Markerud, near Christiania, in Norway,
where the strike of the beds has not been deranged throughout a large area
by the intrusion of granite, both in large masses and in veins. This fact
is considered by some geologists to militate against the theory of the
forcible injection of granite in a fluid state. But it may be stated in
reply, that ramifying dikes of trap, which almost all now admit to have
been once fluid, pass through the same fossiliferous strata, near
Christiania, without deranging their strike or dip.[446-B]
[Illustration: Fig. 498. General view of junction of granite and schist of
the Valorsine. (L. A. Necker.)]
The real or apparent isolation of large or small masses of granite detached
from the main body, as at _a b_, fig. 498., and above, fig. 492., and _a_,
fig. 497., has been thought by some writers to be irreconcilable with the
doctrine usually taught respecting veins; but many of them may, in fact, be
sections of root-shaped prolongations of granite; while, in other cases,
they may in reality be detached portions of rock having the plutonic
structure. For there may have been spots in the midst of the invaded
strata, in which there was an assemblage of materials more fusible than the
rest, or more fitted to combine readily into some form of granite.
Veins of pure quartz are often found in granite, as in many stratified
rocks, but they are not traceable, like veins of granite or trap, to
large bodies of rock of similar composition. They appear to have been
cracks, into which siliceous matter was infiltered. Such segregation, as
it is called, can sometimes be shown to have clearly taken place long
subsequently to the original consolidation of the containing rock. Thus,
for example, in the gneiss of Tronstad Strand, near Drammen, in Norway,
the annexed section is seen on the beach. It appears that the
alternating strata of whitish granitiform gneiss, and black
hornblende-schist, were first cut through by a greenstone dike, about
2-1/2 feet wide; then the crack _a b_ passed through all these rocks,
and was filled up with quartz. The opposite walls of the vein are in
some parts incrusted with transparent crystals of quartz, the middle of
the vein being filled up with common opaque white quartz.
[Illustration: Fig. 499. _a, b._ Quartz vein passing through gneiss and
greenstone, Tronstad Strand, near Christiania.]
[Illustration: Fig. 500. Euritic porphyry alternating with primary
fossiliferous strata, near Christiania.]
We have seen that the volcanic formations have been called overlying,
because they not only penetrate others, but spread over them. Mr. Necker
has proposed to call the granites the underlying igneous rocks, and the
distinction here indicated is highly characteristic. It was indeed
supposed by some of the earlier observers, that the granite of
Christiania, in Norway, was intercalated in mountain masses between the
primary or paleozoic strata of that country, so as to overlie
fossiliferous shale and limestone. But although the granite sends veins
into these fossiliferous rocks, and is decidedly posterior in origin,
its actual superposition in mass has been disproved by Professor
Keilhau, whose observations on this controverted point I had
opportunities in 1837 of verifying. There are, however, on a smaller
scale, certain beds of euritic porphyry, some a few feet, others many
yards in thickness, which pass into granite, and deserve perhaps to be
classed as plutonic rather than trappean rocks, which may truly be
described as interposed conformably between fossiliferous strata, as the
porphyries (_a c_, fig. 500.), which divide the bituminous shales and
argillaceous limestones, _f f_. But some of these same porphyries are
partially unconformable, as _b_, and may lead us to suspect that the
others also, notwithstanding their appearance of interstratification,
have been forcibly injected. Some of the porphyritic rocks above
mentioned are highly quartzose, others very felspathic. In proportion as
the masses are more voluminous, they become more granitic in their
texture, less conformable, and even begin to send forth veins into
contiguous strata. In a word, we have here a beautiful illustration of
the intermediate gradations between volcanic and plutonic rocks, not
only in their mineralogical composition and structure, but also in their
relations of position to associated formations. If the term overlying
can in this instance be applied to a plutonic rock, it is only in
proportion as that rock begins to acquire a trappean aspect.
It has been already hinted that the heat, which in every active volcano
extends downwards to indefinite depths, must produce simultaneously very
different effects near the surface, and far below it; and we cannot suppose
that rocks resulting from the crystallizing of fused matter under a
pressure of several thousand feet, much less miles, of the earth's crust
can resemble those formed at or near the surface. Hence the production at
great depths of a class of rocks analogous to the volcanic, and yet
differing in many particulars, might almost have been predicted, even had
we no plutonic formations to account for. How well these agree, both in
their positive and negative characters, with the theory of their deep
subterranean origin, the student will be able to judge by considering the
descriptions already given.
It has, however, been objected, that if the granitic and volcanic rocks
were simply different parts of one great series, we ought to find in
mountain chains volcanic dikes passing upwards into lava, and downwards
into granite. But we may answer, that our vertical sections are usually of
small extent; and if we find in certain places a transition from trap to
porous lava, and in others a passage from granite to trap, it is as much as
could be expected of this evidence.
The prodigious extent of denudation which has been already demonstrated to
have occurred at former periods, will reconcile the student to the belief
that crystalline rocks of high antiquity, although deep in the earth's
crust when originally formed, may have become uncovered and exposed at the
surface. Their actual elevation above the sea may be referred to the same
causes to which we have attributed the upheaval of marine strata, even to
the summits of some mountain chains. But to these and other topics, I shall
revert when speaking, in the next chapter, of the relative ages of
different masses of granite.
FOOTNOTES:
[439-A] Bulletin, 2d serie, iv. 1304.; and Archiac, Hist. des Progres
de Geol., i. 38.
[440-A] Boase on Primary Geology, p. 16.
[441-A] Bulletin, vol. iv., 2d ser., pp. 1318. and 1320.
[441-B] Syst. of Geol., vol. i. p. 157.
[441-C] Ibid., p. 158.
[442-A] Geol. Trans., 1st series, vol. iii. pl. 21.
[442-B] MacCulloch, Geol. Trans., vol. iii. p. 259.
[443-A] Capt. B. Hall, Trans. Roy. Soc. Edin., vol. vii.
[444-A] MacCulloch, Syst. of Geol., vol. i. p. 58.
[444-B] Western Islands, pl. 31.
[444-C] On Geol. of Cornwall, Camb. Trans. vol. i. p. 124.
[445-A] Phil. Mag. and Annals, No. 27. new series, March, 1829.
[445-B] Necker, sur la Val. de Valorsine, Mem. de la Soc. de Phys. de
Geneve, 1828. I visited, in 1832, the spot referred to in fig. 497.
[446-A] Necker, Proceedings of Geol. Soc., No. 26. p. 392.
[446-B] See Keilhau's Gaea Norvegica; Christiania, 1838.