THE PRECAMBRIAN ROCKS.
Study of a geological map of the world will shew that extensive
regions, such as parts of Scandinavia, many tracts of Central Europe,
a large area in Canada, and a considerable portion of Brazil and the
adjoining countries are occupied by crystalline schists, which
underlie the oldest known sedimentary strata in those places. These
crystalline schists form the floor upon which the sediments
constituting the bulk of the geological column rest, and it is
necessary that we should know something of the character of this
floor. Other rocks which can be definitely proved to be of Precambrian
age are often found associated with the crystalline schists, and these
associated rocks have often undergone more or less alteration
subsequently to their formation. The difference between the coarser
types of crystalline schists and these associated rocks is sometimes
so marked that geologists have necessarily paid attention to it, and
separated the two groups of rocks; the term Archæan has been used by
some geologists to include the crystalline schists, and Eparchæan for
the associated rocks of known Precambrian age, but though this
separation may sometimes be effected, there are cases when it is
impossible to draw any sharp line of demarcation between 'Archæan'
and 'Eparchæan' types.
In the present state of our knowledge, a chronological classification
of the Precambrian rocks when applied to wide and distant regions is
destined to break down, and it will be convenient if we consider at
some length the features of the Precambrian rocks of a particular
region, and apply the knowledge thus gained to a study of Precambrian
rocks of other areas, and to a consideration of our knowledge of the
Precambrian rocks as a whole. In doing so, the term 'crystalline
schists' will be used somewhat vaguely with reference to a complex of
schistose rocks of which the mode of origin cannot be fully
determined. We may take our own country as a region where a good
development of the Precambrian rocks occurs.
A few explanatory remarks concerning the mode of detection of
Precambrian rocks may not be amiss. If any true organisms have been
hitherto discovered amongst the rocks formed before Cambrian times
they are valueless as a means of correlating rocks, and accordingly
lithological characters only are available in attempting to correlate
the rocks of one area with those of another. Those who have read the
preceding chapters will have gathered that comparisons founded on
similarity of lithological character are not so valuable as those made
after careful scrutiny of the fossils of strata, but they are by no
means valueless, and when the rocks of two areas which are not far
distant from one another present close lithological resemblances,
their general contemporaneity may be inferred with some degree of
certainty.
It is only when we get the lowest Cambrian strata overlying earlier
rocks that we have absolute proof of the Precambrian age of the
latter, and it is necessary, therefore, that we should have some
definite lower limit to the rocks of the Cambrian system. It is now
generally agreed that that limit shall be drawn at the base of a group
of rocks containing what is known as the _Olenellus_-fauna, which will
be considered at greater length in the next chapter, and it will be
well, if the term Cambrian be not in future applied to any rocks
beneath the ones containing the relics of this fauna, for otherwise
there is danger of the indefinite downward extension of the Cambrian
system. We need not be surprised to find great thicknesses of rock
below the rocks containing the _Olenellus_-fauna, and passing upwards
with complete conformity into those rocks; nevertheless, if it can be
shewn that the _Olenellus_-fauna had not appeared during the
deposition of the underlying group, the rocks of that group should be
termed Precambrian. A case of this nature has not yet been detected in
our area, and all the rocks which have been proved to be Precambrian
in Britain are separated from the overlying Cambrian rocks by a
physical break, though that break is not necessarily very large, and
in some districts is probably of little importance. Hitherto the
_Olenellus_-fauna has been detected in Ross, Warwickshire, Shropshire,
Worcestershire and probably in Pembrokeshire, and the rocks underlying
the _Olenellus_-beds in those counties can be proved to be Precambrian
(i.e. if the _Olenellus_-age of the Pembrokeshire rocks be ultimately
established, and the researches of Dr Hicks tend to prove that it will
almost certainly be done). It will be convenient if we take the
instances where the age of the rocks can be proved with certainty or
with a considerable degree of probability first, and then consider the
examples of rocks which are found below Cambrian strata, though these
have not hitherto yielded the _Olenellus_-fauna, concluding with a
notice of rocks which have been claimed to be of Precambrian age on
account of their lithological characters, though they are not now seen
to be immediately succeeded by strata appertaining to the Cambrian
system.
Commencing with the region where we have the greatest development of
the known Precambrian rocks, namely Ross, Sutherland and the Hebrides,
we may explain the general relationship of the rocks by means of a
generalised section (fig. 15).
[Illustration: Fig. 15.]
The lowest rocks _a_ are crystalline schists, they are succeeded by a
set of arenaceous rocks _b_ known as the Torridonian beds, which rest
unconformably upon the upturned edges of the crystalline schists,
whilst the Cambrian rocks, _c_, rest with another unconformity
sometimes upon the partly denuded Torridonian beds, or where the
latter have been completely removed, as on the right side of the
figure, directly upon the crystalline schists, thus presenting an
example of unconformable overlap. The occurrence of the
_Olenellus_-fauna in the basement beds of the Cambrian system near
Loch Maree, proves the Precambrian age of the Torridonian strata,
whilst the unconformable junction between the latter and the
crystalline schists indicates that we are here dealing with two
distinct sets of Precambrian rocks, one of Eparchæan and the other of
Archæan type.
The crystalline schists consist of rocks of very varied lithological
characters, some with gneissose, and others with schistose structure,
and they vary in degree of acidity from ultrabasic rocks to those of
acid composition. Most of them exhibit parallel structures, which in
many cases can be shewn to have been impressed on the rocks
subsequently to their consolidation, though this need not have
occurred and probably did not occur with some of them, especially the
granitoid gneisses. The researches of the members of H. M. Geological
Survey have shewn that many of these rocks were originally intrusive
igneous rocks, though it is not yet known into what rocks those which
were first consolidated were injected, and the origin of the bulk of
the schists still remains to be elucidated. Subsequently to their
consolidation and before the deposition of the earliest Torridonian
rocks they were subjected to more than one set of earth-movements,
which folded them and impressed a series of parallel structures upon
many of them; and accordingly we find that the pebbles of the
crystalline schists which are found amongst the basal conglomerates of
the Torridonian rocks consist of fragments which had undergone the
alteration caused by these earth-movements before they were denuded
from their parent-rocks[48].
[Footnote 48: For an account of these rocks, their characters, and the
effects of earth movement upon them, the reader should consult a
"Report on the Recent Work of the Geological Survey in the North-West
Highlands of Scotland": _Quart. Journ. Geol. Soc._, vol. XLIV. p.
378.]
The Torridonian system is composed of rocks which are largely of
arenaceous character, the most prominent beds being formed of red
sandstones, and the bulk of the fragments in them have clearly been
derived by denudation from the crystalline schists, many of the beds
being composed of arkose, where the quartz is mixed with a large
proportion of felspar and often of ferro-magnesian minerals. The
deposits are clearly sedimentary, and are as little altered as many
strata of much more recent origin, only possessing structures produced
by metamorphic action under exceptional circumstances. The detailed
researches of the geological surveyors prove that the rocks of this
system have a much greater thickness and are of more varied
lithological characters than was previously supposed. The total
thickness of the strata is over 10,000 feet, and the sandstones are
associated with deposits of a muddy character, and with occasional
bands of limestone; in these circumstances the discovery of fossils
would excite no surprise, and in 1891 Sir A. Geikie announced the
detection of "traces of annelids and some more obscure remains of
other organisms in these strata," which have not yet been
described[49]. These Torridonian strata furnish us with the most
satisfactory group of Precambrian sediments yet detected in
Britain[50].
[Footnote 49: An account of the subdivisions and lithological
characters of the rocks of the Torridonian System will be found in the
_Annual Report of the Geological Survey of the United Kingdom_ for
1893.]
[Footnote 50: It has been recently maintained that some of the
Torridonian rocks are of Æolian origin.]
In the south-east Highlands is a great mass of crystalline schists of
a less gneissose character than that of the north-west, to which Sir
A. Geikie has applied the name Dalradian. Many of these schists will
be found by examination of the geological map of Scotland to be
separable into divisions, which by means of their lithological
characters can be traced long distances across the country, and they
present all the characters of sedimentary rocks, though they are
associated with intrusive igneous rocks, and have undergone great
metamorphic changes since their formation. Cambrian rocks have not yet
been discovered immediately above them, though they are clearly older
than Ordovician times, but the existence of rocks associated with them
along their north-west borders, which in lithological characters
closely resemble some of the rocks of the crystalline schists of the
north-west Highlands, indicates the probability of their general
Precambrian age. In some instances, the extreme types of metamorphism
which they exhibit are the result of the kind of action usually termed
pyrometamorphic as has been shewn by Mr G. Barrow[51].
[Footnote 51: Barrow, G. "On an Intrusion of Muscovite-biotite gneiss
in the S.E. Highlands of Scotland, and its accompanying metamorphism."
_Quart. Journ. Geol. Soc._, vol. XLIX. p. 330.]
In England and Wales the rocks which have been shewn or inferred to be
Precambrian, when not intrusive, are largely of volcanic origin. The
most satisfactory example of the occurrence of the _Olenellus_-fauna
is that of the Cambrian Comley sandstone of Shropshire, which rests
unconformably upon a set of rocks termed by Dr Callaway the Uriconian
rocks; the latter are essentially volcanic, and strongly resemble
Precambrian rocks of other British areas. There is also strong reason
to suppose that the sediments to which the name Longmyndian has been
applied, which have been described by the Rev. J. F. Blake, are of
Precambrian age, for, as Professor Lapworth has pointed out, the three
great subdivisions of the Cambrian system are present in the area
under consideration, and the rocks of each are entirely different from
those of the adjoining Longmynd area. In Shropshire therefore we meet
with one set of volcanic rocks, and another set consisting of
sedimentary rocks, of which the former is certainly, the latter
almost certainly of Precambrian age, and as the Longmyndian rocks are
in a comparatively unaltered condition, consisting of normal
sediments, we may well expect the discovery of fossils in them
also[52]. The _Olenellus_-fauna has been found near Nuneaton in
Warwickshire in beds which unconformably succeed volcanic rocks, the
Caldecote series of Prof. Lapworth, and the latter are therefore of
Precambrian age[53]. A few fossils belonging to the _Olenellus_-fauna
have occurred in the oldest Cambrian rocks of the Malvern district,
and these rocks rest unconformably upon those of an old ridge which is
therefore composed of Precambrian rocks. The rocks of this ridge are
largely of intrusive igneous origin, though parallel structures have
been impressed upon them as the result of subsequent deformation, but
some of the rocks are almost certainly of contemporaneous volcanic
origin[54]. In the Wrekin ridge, igneous and pyroclastic rocks are
found succeeded unconformably by Cambrian rocks which resemble those
of the Malvern and Nuneaton districts, and probably belong to the
period of existence of the _Olenellus_-fauna, and these igneous and
pyroclastic rocks are presumably of Precambrian age, and the
contemporaneous rocks constitute Dr Callaway's typical Uriconian
group. Volcanic ashes and breccias are accompanied by devitrified
pitchstones and intruded granitic rocks, which may or may not be all
of the same general age[55]. The rocks which have been claimed as
Precambrian in Pembrokeshire and in Caernarvonshire have the same
general characters as those of the Wrekin ridge. Pyroclastic rocks
underlie the oldest Cambrian rocks, with discordance between the two,
and associated with these pyroclastic rocks are quartz felsites which
according to some are of contemporaneous nature whilst others maintain
their intrusive origin. In each county granites are found which are
now generally recognised to be intrusive, though there seems to be no
doubt as to their being of the same general age as the rocks with
which they are associated, and therefore presumably Precambrian. The
Pembrokeshire rocks are marked by the occurrence of a certain amount
of metamorphism, probably of more than one kind, which has converted
pyroclastic volcanic rocks into sericitic-schists and quartz-felsites
into hälleflintas[56]. The term Pebidian given by Dr Hicks to the
contemporaneous volcanic fragmental rocks should be retained, and if
these rocks be eventually shewn to be contemporaneous with similar
volcanic rocks of other districts, may be applied generally, as it has
priority over other terms as Uriconian and Caldecote series. The term
Dimetian was applied to rocks known to be intrusive, and must be
dropped as a chronological term, whilst the existence of an Arvonian
system separate from the Pebidian system is not fully proved.
[Footnote 52: The reader may consult a paper by Prof. Lapworth "On
_Olenellus Callavei_ and its geological relationships," _Geol. Mag._
Dec III. vol. VIII. p. 529, for information concerning the
relationship of the _Olenellus_ beds of Shropshire to the more ancient
rocks; the Uriconian rocks are described by Dr Callaway in a series of
papers, especially in the _Quarterly Journal of the Geological
Society_, vol. XXXV. p. 643, vol. XXXVIII. p. 119, vol. XLII. p. 481
and vol. XLVII. p. 109, whilst the lithological characters of the
Longmyndian rocks are described by the Rev. J. F. Blake (_Quart.
Journ. Geol. Soc._, vol. XLVI. p. 386).]
[Footnote 53: See Lapworth, C., "On the sequence and systematic
position of the Cambrian rocks of Nuneaton," _Geol. Mag._ Dec III.
vol. III. p. 319; and Waller, T. H., "Preliminary Note on the Volcanic
and Associated Rocks of the neighbourhood of Nuneaton," _ibid._ p.
322.]
[Footnote 54: For details concerning the rocks of the Malvern Hills
see papers by Callaway in the _Quarterly Journal of the Geological
Society_, vol. XXXVI. p. 536, XLIII. p. 525, XLV. p. 475, and XLIX. p.
398, and a paper by Prof. A. H. Green, _ibid._ vol. LVI. p. 1.]
[Footnote 55: Callaway, C., _Quart. Journ. Geol. Soc._, vol. XXXV. p.
643.]
[Footnote 56: The Pembrokeshire area is of interest as the probable
existence of Precambrian rocks in Britain was first indicated on good
evidence in this county. The general structure of the district is
fairly simple, consisting of Cambrian rocks beneath which Precambrian
rocks are exposed in at least two ridges of which the northerly and
more important one runs through St Davids. The rocks of the St Davids
ridge consist of a binary granite (granitoidite), felsites, and
volcanic ashes and breccias of intermediate composition. Much
diversity of opinion has existed, and to some extent still exists as
to questions of detail, and a very extensive literature has been
devoted to these rocks. Amongst the numerous papers which treat of
them, the student may consult the following:--Hicks, H., _Quart.
Journ. Geol. Soc._, vol. XXXIII. p. 229, XXXIV. p. 147, XXXV. p. 285,
XL. p. 507, XLII. p. 351, Geikie, A., _ibid._ vol. XXXIV. p. 261,
Blake, J. F., _ibid._ vol. XL. p. 294, and Morgan, C. Ll., _ibid._
vol. XLVI. p. 241. Much of the matter contained in these papers is
controversial, and need not be fully read by those who merely wish to
obtain a general account of the rocks of the district.]
In Caernarvonshire two ridges are found, the one running from Bangor
to Caernarvon, and the other through Llanberis lake. The rocks of
these are generally similar to those of St Davids, and as the lowest
Cambrian rocks of the area closely resemble those of St Davids, the
Precambrian age of the rocks of these ridges is rendered highly
probable, though until the discovery of the _Olenellus_-fauna in the
area, it cannot be regarded as proved[57].
[Footnote 57: These rocks are described by T. M^{c}K. Hughes, _Quart.
Journ. Geol. Soc._, vol. XXXIV. p. 137, and XXXV. p. 682; by Prof. T.
G. Bonney, _ibid._ vol. XXXIV. p. 144; and by Dr Hicks, _ibid._ vol.
XXXV. p. 295.]
The actual position of the similar rocks of Anglesey has not been so
clearly fixed, as the rocks associated with them are of Ordovician
age, but their resemblance to the rocks of the adjoining regions
renders their Precambrian age highly probable. It is interesting to
find in association with the rocks which resemble those of
Caernarvonshire, others which Sir A. Geikie recognises as quite
similar to some existing amongst the crystalline schists of the
north-west Highlands of Scotland, and when these ancient rocks of
Anglesey have been mapped in detail, they will probably be found to
present greater variety than is afforded by any Precambrian rocks of
Great Britain occurring S. of the Scotch border[58].
[Footnote 58: Papers upon the old rocks of Anglesey will be found in
many volumes of the _Quarterly Journal of the Geological Society_; see
especially Hicks, vol. XXXV. p. 295, Callaway, vol. XXXVI. p. 536,
XXXVII. p. 210, and Blake, XLIV. p. 463.]
Of rocks whose age is more uncertain, but which are probably of
Precambrian age, those of Charnwood Forest in Leicestershire may first
be noticed. They are largely of pyroclastic origin, and from their
likeness to similar rocks of proved Precambrian age, they are very
probably of this age, as suggested by Messrs Hill and Bonney[59]. A
group of crystalline schists is found in the south of Cornwall,
especially near the Lizard, and similar rocks are found in the Channel
Isles. As their relationship to newer rocks is not clear, little can
be said about them, which has not already been noticed in mentioning
the crystalline schists of other regions[60].
[Footnote 59: Hill and Bonney, _Quart. Journ. Geol. Soc._, vol.
XXXIII. p. 754, XXXIV. p. 199 and XLVII. p. 78; see also Watts, W. W.,
_Rep. Brit. Assoc._ for 1896, p. 795.]
[Footnote 60: For an account of the Volcanic History of Britain in
Precambrian times, see Sir A. Geikie, Presidential Address to the
Geological Society, _Quart. Journ. Geol. Soc._, vol. XLVII. p. 63.]
The Precambrian rocks of the European continent consist largely of
crystalline schists which in their general aspects recall those of the
north-west Highlands of Scotland. Important masses are found in
Bavaria, Bohemia, France, Spain, Scandinavia and Russia. The
Scandinavian and Russian rocks of Archæan type are in places succeeded
by the _Olenellus_-bearing beds of the Cambrian rocks, and rocks of
Eparchæan character are not extensively developed, though certain
Norwegian rocks may be the equivalents of the Torridonian rocks of
Scotland, and other rocks of this type are found in places in Sweden.
In Bohemia and in Brittany Precambrian strata of Eparchæan type have
been discovered, and this type probably occurs elsewhere in Europe.
The North American rocks require some notice, for it was in Canada
that the existence of Precambrian rocks was first recognised, and the
term Laurentian, originally applied to an Archæan type of Precambrian
rocks in Canada, was subsequently adopted in speaking of many
Precambrian rocks elsewhere, though it is now wisely restricted to the
type of rock in the original area to which the name was first given.
These Laurentian rocks acquired a special, interest on account of the
occurrence in their limestones of a supposed reef-building
foraminifer, _Eozoon canadense_, but detailed study of its structure
and mode of occurrence has convinced most geologists that the
structure is inorganic.
The Laurentian rocks of the typical Laurentide region are largely
crystalline schists associated with massive crystalline rocks. The
attempt to separate them chronologically into a Lower and Upper
division was premature, as shewn by the fact that many of them, upon
detailed study, prove to be intrusive igneous rocks. In the
neighbourhood of Lake Huron, a set of sedimentary rocks overlying the
Archæan rocks is of Eparchæan type, consisting to a great extent of
volcanic rocks, clay-slates and schists with intrusive igneous rocks;
it has been termed the Huronian System, and this term has also been
extensively applied to other Eparchæan types found elsewhere, but
should be restricted to the rocks of the Huron district. A number of
other rocks of Eparchæan type have been discovered in various parts of
North America, and have been grouped together under the title of
Algonkian, a name proposed for them by Dr C. D. Walcott, and an
attempt has been made to arrange them in chronological order, though
in the absence of fossils, the rocks of different districts can only
be so arranged by reference to lithological characters; nevertheless a
detailed study of the Eparchæan and some of the more finely
crystalline schistose rocks points to the existence of a number of
divisions of sedimentary rocks of Precambrian age, some of which may
attain to the dignity of forming separate systems[61]. By far the most
instructive development of American Precambrian rocks has been found
in the Rainy Lake region of Canada, and it is the subject of a special
memoir by Dr A. C. Lawson[62]. The Archæan rocks of the region are
divided into a lower Laurentian and an upper division, which is
further subdivided into the Coutchiching series below and the Keewatin
series above, though the rocks of the Keewatin series are largely of
Eparchæan character. The Laurentian rocks of this region resemble
those of the Laurentide area, and consist of highly crystalline
schistose and gneissose rocks associated with compact rocks. The
Coutchiching series consists of mica schists and grey laminated
gneisses, which appear to have been of sedimentary origin, altered by
subsequent metamorphic action, while the Keewatin series, which
reposes sometimes upon the rocks of the Coutchiching series (when the
junction is an unconformable one), sometimes upon the Laurentian
rocks, is formed of pyroclastic rocks and lava flows with intercalated
sedimentary rocks; some of the Keewatin rocks are highly metamorphosed
but others have undergone little or no metamorphic change. The most
important point in connexion with these rocks of the Rainy Lake Region
has reference to the relationship between the Laurentian rocks and
those of the Coutchiching and Keewatin series. Lawson demonstrates the
igneous nature of the Laurentian rocks, and brings forward evidence of
various kinds that they were formed "by the fusion of the basement or
floor upon which the formations of the upper division of the Archæan
were originally deposited. With the fusion of this floor it seems
probable that portions of the superincumbent strata, which once formed
integral parts of either the Coutchiching series or the Keewatin, have
also been absorbed into the general magma, and reappeared on
crystallization as Laurentian gneiss. This fusion, however, only
extended up to a certain uneven surface, which surface constitutes the
demarcation between the present upper and lower Archæan. Above this
surface, or upper limit of fusion, the formation of the Coutchiching
and Keewatin series retained their stratiform or bedded disposition,
and rested as a crust of hard and brittle rocks upon the magma,
subject to its metamorphosing influences[63]."
[Footnote 61: A large number of classifications have been proposed for
the Archæan rocks of America; the most plausible one is given in Sir
A. Geikie's _Text Book of Geology_, Third Edition, p. 716.]
[Footnote 62: Lawson, A. C., _Report on the Geology of the Rainy Lake
Region_. Montreal, 1888.]
[Footnote 63: Lawson, _op. cit._ p. 139.]
We may now pass briefly in review the evidence which has been so far
obtained as to the mode of formation of the various Precambrian rocks.
The existence of a very varied fauna amongst the earliest Cambrian
strata has been commented upon by many geologists, and according to
accepted explanations of the origin of that fauna, an enormous period
of time elapsed before the deposition of the earliest Cambrian strata.
During portions of that long period, the undoubtedly clastic rocks of
Eparchæan type were deposited, and probably many others which are now
so altered by metamorphism, like some of the Coutchiching rocks of
Canada, that their original clastic origin can only be inferred and
not directly proved. Volcanic activity was very rife during the
deposition of some of these Eparchæan rocks, though perhaps not more
so than during the formation of some of the Lower Palæozoic Rocks. All
attempts to prove the occurrence of organisms in Precambrian strata
have hitherto failed, for no undoubted fossil has been described which
is unhesitatingly accepted as of Precambrian age, notwithstanding the
many asserted occurrences of such fossils. That fossils will
eventually be discovered is more than probable, and their
non-detection at the present time is in no way very surprising, when
we remember the long time that elapsed after the existence of
stratified rocks below the Upper Palæozoic rocks had been recognised,
before definite faunas were discovered in them. The determination of
the Precambrian age of stratified rocks is recent, and now that this
determination has been made, the search for fossils will be more
eager, and is likely to be rewarded by their discovery. Furthermore,
experience shows that when fossils are discovered in rocks of unknown
age, there is a tendency to refer those rocks to some known period,
and consequently we may actually possess Precambrian fossils, out of
beds which have been erroneously referred to the Cambrian or a later
period.
Another important question is that of the metamorphism of a large
number of Precambrian rocks, and here again recent research tends to
show that the metamorphism is not of a kind different from that which
occurred after the end of Precambrian times; the discovery of
crystalline schists in Norway, Kirkcudbrightshire and Westmorland
amongst Lower Palæozoic rocks, which resemble those of Archæan masses
in all respects except in the extent of area which they cover, shows
that similar processes to those which occurred in Precambrian times
went on during later periods, though perhaps not on so large a scale.
The great extent of these metamorphic rocks of Precambrian age can
hardly be due in any great degree to the longer time during which they
have been subjected to metamorphic influence, for there is evidence
that much of the change took place in Precambrian times, far more than
has occurred since, and it is a significant fact that these old rocks
are more extensively penetrated by intrusive igneous masses than those
of later periods; here again we find that much of the intrusion
actually occurred in Precambrian times. The greater extent of
intrusion and metamorphism amongst these Precambrian rocks than
amongst later sediments indicates some differences of conditions in
the case of Precambrian and later times. If besides intrusion, actual
fusion of floors of Precambrian rocks occurred, we may well suppose
that the earlier records of the rocks are for ever lost to us, the
earliest sediments having been fused, but that the history of life
upon our earth is to be revealed to us first in so late a stage as
that of Cambrian times is highly improbable, and we may look forward
with confidence to laying bare the records of the rocks composing the
geological column some way below the Cambrian portion of the column.
Upon this foundation of igneous rock, sediment and volcanic material,
formed in Precambrian times, whose history we have only begun to
study, was laid down the great mass of sediment which the geologist
has more completely studied, where abundant traces of life are
preserved, and concerning whose history we can gain a greater insight
than is permitted us in the case of the old Foundation Stones.