quartzite; loose blocks of compact and granular
quartzite.
9.0 „ Summit of pass; blocks of white quartzite.
9.5 „ Muscovite schists, strike N.–S., foliation vertical.
9.20 „ Lukke.
9.30 „ Soft muscovite schists and quartzites.
9.40 „ Muscovite schists, strike N. 10° W., foliation vertical.
9.50 „ Broken fragments of quartzite, quartz-muscovite
pegmatite, and quartz-tourmaline pegmatite on road;
intrusive masses of graphic felspar and quartz with
crystals of muscovite, associated with exposures of
coarse muscovite granite.
10.10 „ Biotite gneisses much crumpled and folded, exposed in
bed of stream, strike N.W.
10.15 „ Quartzites and muscovite pegmatites.
10.20 „ Intrusive mass of muscovite granite with large crystals
of graphic quartz and felspar in a fine grained granitic
matrix; banded in places with coarse and fine layers.
10.40 „ Exposures of quartz-muscovite pegmatite.
11.0 „ Soft micaceous schist, strike N. 10° W., foliation
vertical.
11.5 „ In bed of river and for 500 yards beyond, garnetiferous
mica schists with quartz lenticles, strike N. 10° W.,
dip 15° W.
11.30 „ Schistose biotite granite, with schistose pegmatite
veins forming Sura hill.
11.40 „ Sura.
IV. Agwede to Kabba.
6.30 A.M.— Left Agwede hill. On top, white granite usually showing
faint foliation and frequently enclosing portions of
dark mica schist; on slope, whitish ribboned gneisses,
often folded and contorted, strike N.E. to E., foliation
vertical.
6.40 „ Dyke of graphic pegmatite in gneiss; sharp junction.
Frequent bands of schistose pegmatite in the pale
coloured banded gneisses; in places the gneisses are
hornblendic with knots of fine-grained hornblende gneiss
or schist.
7.0 „ Ascent to plateau over contorted gneisses.
7.20 „ Same contorted gneisses passing into and invaded by
white schistose granite; wooded plain, few exposures.
8.20 „ Occasional exposures of similar contorted and granitised
gneiss; strike E. 10° N. to E. 10° S.
9.5 „ Faintly banded and indistinctly foliated biotite gneiss.
9.30 „ Odouape.
10.30 „ Level surface of composite gneiss.
11.30 „ Much crumpled and folded biotite gneiss; strike N.–S.
Followed by garnetiferous biotite gneiss.
11.45 „ Composite gneiss; foliation vertical; strike N. 10° W.
12.0 „ Kabba.
V. Izon to Abuja (Nassarawa province).
6.25 A.M.— Left Izon. Open grassy or thinly-wooded plains for two
hours; no exposures.
8.20 „ Hills; ascent of 300 feet; biotite and hornblende gneiss
and amphibolite; epidote-hornblende gneisses; in places
twisted and contorted and extensively invaded and
injected by masses of biotite granite; strike varies
between N. 10° E. and N. 10° W.; foliation mostly
vertical.
9.0 „ Open plain; thick granite wash.
9.30 „ Hills; granite invading hornblende epidote gneisses.
9.42 „ Abuchi. Amphibolite veined with granite.
10.0 „ Narrow plain; thick drift.
10.30 „ Hills; soft micaceous gneisses, strike N. 10° W.,
foliation vertical or high to E.
10.35 „ Hard biotite gneisses, strike N.–S., foliation linear
and vertical.
10.45 „ Ribboned and banded biotite gneisses with quartz veins
parallel to or crossing the foliation; strike N.–S.;
foliation vertical.
11.0 „ Abuja granite with foliated margin; vertical crush lines
in granite striking N.–S.
11.15 „ Abuja Residency.
VI. Boi to Ronn (Southern Bauchi).
6.30 A.M.— Left Boi; hornblende epidote gneisses broken and invaded
by granite; in places much folded and crumpled.
6.40 „ Felspathic gneiss, strike E.–W., dip high to S.
7.20 „ Complex of quartz epidote rock, amphibolite, epidote
hornblende schist, and gneiss, in places much
granitised; general strike E. 10° N.
7.45 „ Hummocks of banded gneiss; open country; few exposures.
8.40 „ Hornblende epidote gneisses.
8.45 „ Goshin Duchi. Hills of biotite granite behind town.
9.3 „ Quartz schists, fine grained gneisses, epidote
hornblende and quartz hornblende rocks, much granitised
in places veins and dykes of pegmatite and basalt.
9.20 „ Blocks of basalt and porphyry.
9.35 „ Hills of fine grained biotite granite.
10.15 „ Granite rocks, in places schistose.
10.45 „ Rocky, hilly country; banded and contorted gneisses
micaceous and hornblendic; general strike E. 10° S.
11.10 „ Ronn.
It is unnecessary, however, to quote the details of particular traverses
at greater length, especially as such descriptions serve only to show
the character of the phenomena to be correlated. It is sufficiently
evident that there are within the Protectorate two fairly well-defined
series of crystalline rocks, a less metamorphosed series of quartzites,
schists and gneisses representing an original sedimentary succession,
and a more highly metamorphosed series of banded and contorted gneisses,
whose relation to the less metamorphosed group will be discussed in the
sequel. The less metamorphosed gneisses present in the field somewhat
granular and incoherent outcrops and may be conveniently termed the
“softer” gneisses, while the “harder” or more highly metamorphosed
gneisses are readily distinguishable by the smooth and compact character
of their exposed surfaces. The softer gneisses appear to disintegrate
readily into a gritty sand under present conditions, while the harder
gneisses exfoliate and pass into angular débris. Both series are
extensively invaded by granitic and other igneous intrusions.
_Borgu and the Niger Valley between Jebba and Yelwa._
In the province of Borgu the superficial accumulations are so extensive
and the exposures so few and so widely scattered that the general
character of the underlying rocks is somewhat difficult to ascertain. It
is probable, however, that, like the neighbouring region of Nikki,[18]
there is a floor of well foliated and granitoid gneisses of varied
origin, much pierced and invaded by veins of granite, pegmatite, quartz
porphyry and quartz. Tremolite schists are known from Mori, and sheared
porphyries from Kanikoko. The occasional inselberge are composed
invariably of grey porphyritic granite. At Jebba the gorge of the Niger
is carved out of a series of sheared quartzites and pebbly grits with
the shear planes at times well spaced and the deformed pebbles still
distinctly visible. Secondary mica has been sparingly formed along the
planes of movement, together with knots and streaks of tourmaline and
tourmaline quartz. The predominant strike is N.–S., and the prevailing
dip of the foliation 65° W. The well-jointed Juju Rock is composed of a
laminated quartzite schist, which splits readily into thin curving
slabs, whose surfaces are coated continuously with white mica and show a
well-defined strain-slip corrugation. Westward the Jebba quartzites pass
into highly sheared quartz schists, quartz mica schists and granulitic
gneisses, which are followed by crumpled and contorted muscovite biotite
schists and gneisses and by granite at the river Awo. The quartzites and
granulitic gneisses are produced along the strike to form the range of
low wooded hills which stretches southward from Jebba towards Ilorin. To
the east of Jebba the quartzites are abruptly limited by hard banded and
composite gneisses pierced by masses of porphyritic biotite granite.
To the north of Jebba the Kailema granite, over which the track leads
from Jebba to Bajibo, is a massive hornblende granite, porphyritic with
white and pink felspars and somewhat finer-grained on the margin. The
granite has evidently pierced a series of hornblende, epidote and talc
schists and micaceous gneisses which outcrop imperfectly to the east.
Still farther north, at Wuru, the channel of the Niger is occupied by a
hard and coarsely foliated epidote hornblende gneiss, much pierced by
irregular veins of schistose granite and pegmatite. The foliation planes
of the gneiss are vertical and strike approximately N. 20° E. On the
left bank the gneiss is succeeded by thinly bedded quartzites and quartz
schists with scattered knots, lenticles and veins of quartz and finely
fibrous tourmaline. Intercalations of much corrugated and wrinkled mica
schist bear witness to differential movements along certain lines. The
whole series, which strikes N.–S. and dips vertically or at high angles
to the east or west, has apparently suffered a rapid and repeated
folding. These rocks are followed by a series of fine-grained green and
yellow quartzites and well-cleaved slaty schists with numerous quartz
reefs, which pass eastward into a complex of hornblende gneisses,
epidote hornblende gneisses, amphibolites and fine grained schistose
granites which disappear at Kumboji under the sandstones of southern
Kontagora.
At Bussa on the right bank of the Niger, near the Residency, there are
exposures of much decomposed shaly phyllites, vertical or highly
inclined to the east and striking N. 10° E. These are followed on the
left bank by a thick series of little altered grey and greenish granular
quartzites, which in places become pebbly and conglomeratic and enclose
bands of greenish trachyte, also showing little or no sign of shearing.
Further exposures of black and coarsely-sheared trachytes are found in
the swamps between Bussa and Libata. Eastward the quartzites are much
invaded by masses of dark green diorite and by thick veins of tourmaline
quartz, the latter showing every variation between a banded tourmaline
quartz rock and a felted mass of finely fibrous tourmaline. Nearer Auna
these rocks are succeeded by well-foliated augen gneisses which are
overlaid by the sandstones of southern Kontagora.
To the north of Bussa phyllites are again exposed in the low rounded
hills to the north-west of Ngashki, where they strike N.–S., and possess
a high dip of 65° to the east. Locally, however, the strike may be
directed as much as 35° to the east of north, indicating that the rocks
do not pursue a constantly rectilineal direction from south to north,
but rather possess a zig-zag character with short and sharp deflections
towards the east. The phyllites, which are mostly grey in colour, but in
places mottled with large irregular whitish spots on a grey ground, pass
eastward into soft grey sericitic schists with crystals of magnetite and
pyrites. The whole series is pierced by quartz tourmaline veins, in
which all proportions are to be found, from almost pure quartz through
banded quartz tourmaline rocks to masses of pure tourmaline of the
finely fibrous variety. Sills of much decomposed vesicular dolerite are
common also amongst the phyllites. Farther east intercalations of
micaceous gneiss become abundant within the schists, and a red fine-
grained granulitic gneiss intervenes between them and the massive
biotite granite of Abara, with its large crystals of felspar. To the
north of Ngashki, phyllites and green slaty schists with lenticular
streaks and veins of quartz and tourmaline quartz occur near Lokofe. A
good exposure in the river Malenda shows pebbly lustrous schists
striking N.–S., and dipping steeply towards the east. At Yelwa a narrow
strip of phyllites and slaty schists on the left bank of the Niger,
passes westward into knotted mica schists and biotite gneisses in the
river bed and eastward into the granitic gneiss of Bin Yauri. In the
country which lies to the east of Lokofe and Yelwa, and especially in
the neighbourhood of Tabachi and Maibirro, green slaty schists of
varying hardness are frequently exposed, usually associated with harder
gritty bands and much invaded by veins of quartz and tourmaline quartz,
and irregular intrusions of granite. In places also there may be found
intercalations of hard, gritty, carbonaceous and little-metamorphosed
slaty beds. At Chuggu, moreover, between Tabachi and Maibirro, there is
a parallel belt of biotite gneisses, hornblende gneisses, and epidote
hornblende gneisses, while at Maibirro the slaty schists, here
associated with thinly-bedded quartzites, quartz schists and quartz
muscovite schists, striking N. 35° E., with a dip to the north-west of
65°, are succeeded in the river Malenda by well-banded biotite gneisses
much veined with felspathic and granitic material. The actual junction
of the banded gneisses with the quartz schists is, however, not exposed.
The rocks of the Niger valley, between Jebba and Yelwa, are thus of many
and varied types. The quartzites and phyllites, the slaty schists and
micaceous gneisses, and the green hornblendic and epidotic beds
represent the sandstones, shales, grits and arkoses of the sedimentary
series, while the sheared trachytes and porphyries, the schistose
granites and the coarse-grained amphibolites represent the original
igneous intrusions within the sedimentary succession. The whole series
strikes fairly constantly N.–S., and forms a practically continuous belt
between Jebba and Yelwa. In most places there are indications that the
rocks have suffered a repeated folding into fairly sharp anticlines and
synclines with vertical axial planes, the folding being apparently more
rapid in the phyllites and schists than in the quartzites and gneisses.
There has unfortunately been no opportunity of tracing the belt in
detail along the strike, and little is known of the lateral variation or
repetition of the individual members or of the original sequence of the
sedimentary succession. There is no reason to believe that there has
been any increase or decrease in the intensity of the metamorphism
throughout the belt from south to north, and no indication has been
found of the occurrence of any thermal metamorphism in the neighbourhood
of the later granitic intrusions.
The most remarkable feature, however, about this belt of schistose rock
within the Niger valley is the manner in which the less metamorphosed
series of quartzites, phyllites and schists is bounded abruptly and
locally replaced by a more highly metamorphosed and much granitised
series of well-foliated and often banded gneisses. At Maibirro, for
example, the schists are abruptly succeeded by banded gneisses of a type
which is nowhere found between Maibirro and the Niger. Similarly at Wuru
and Jebba, the quartzites give place unexpectedly to composite
granitised gneisses, which must have originated under very different
conditions from those which brought about the metamorphism of the
phyllites and schists. This is a peculiarity of the fundamental
architecture which is repeated in many other parts of the Protectorate.
_Kabba and Ilorin._
Over the greater part of the province of Ilorin the surface
accumulations are so extensive that the nature of the underlying rocks
can be judged only from small and scattered exposures. So far as known
there are no belts of phyllites and quartzites upon the western or
central plains comparable with that of the Niger valley above Jebba. The
predominant rock in central Ilorin is a fairly coarse-grained, granular
and felspathic muscovite biotite gneiss, sometimes an augen-gneiss with
large eyes of felspar, which disintegrates readily into a felspathic
sand. With it are associated in places thin bands of quartzite, quartz
schist, quartz muscovite schist, mica schist and gneiss, often
garnetiferous, amphibolite and schistose granite, which, however, are
neither sufficiently extensive nor sufficiently resistant to give rise
to any prominent feature upon the surface. The whole series which
strikes approximately N.–S. is much veined by muscovite and tourmaline
pegmatites and invaded by masses of granite usually of a more or less
foliated character. The larger masses rise into turtlebacks or heaps of
boulders, and in their neighbourhood the gneisses have often been so
intimately veined and traversed by the granite as to produce a composite
or banded rock. At Share hard biotite gneisses with linear foliation are
extensively developed and associated with hornblendic gneisses and
amphibolites and cut by tourmaline pegmatites.
In the Orissa hills, which lie to the east of the great plain of Ilorin,
the softer gneisses enclose for the first time a prominent series of
quartzites, quartz schists, quartz mica schists and granulitic gneisses
which, with their general meridional strike and high dip to the west,
are very similar to those which form the low hilly ground to the south
and west of Jebba. The Shappa hills which succeed the Orissa hills to
the south are composed in great part of biotite and hornblende granites,
coarse-grained, fine-grained and porphyritic, which appear to be
intrusive into a series of hard biotite and hornblende gneisses, in
places well-banded and much veined with pegmatites, which occupies the
south-eastern corner of the province. Around Eri the softer gneisses are
again well developed and extensively exposed, the series here consisting
of thin quartzites, felspathic gneisses and much contorted and
occasionally garnetiferous micaceous gneisses and schists, with
interbedded hornblende schists and irregular masses of coarse-grained
amphibolite, probably of intrusive origin. The whole series is much
pierced by veins of quartz, tourmaline quartz and muscovite pegmatite,
the latter in places carrying tinstone.
PLATE VI
[Illustration: YENDAM HILL, YOLA PROVINCE.]
[Illustration: CRATERS AT MBOL.]
The softer gneisses and schists are found everywhere in northern Kabba
between Eri and the Ilorin border and the sandstone plateaux which
fringe the Niger. The usual succession of thinly bedded micaceous and
felspathic gneisses and hornblende schists is varied by the
intercalation of several belts of platy muscovite schists and quartz
muscovite schists, whose deeply decomposed surface usually supports a
luxuriant forest growth. Within the area mentioned there are at least
four of these belts, more or less well-defined and continuous along
their strike from north to south. The first runs southward from the
neighbourhood of Idofin by way of Kponyon, Onore, Ote, and Omwo into
southern Kabba: the second strikes southward from Okoro and Iye,
skirting Sanlu to the east and Moppa to the west: the third runs from
Shale and Oferre by way of Lukke, Olle, and Appe to the neighbourhood of
Odouape, and the fourth is exposed along the track leading from Wa to
Jakura on the north side of the Mimi valley. While the fissile quartz
muscovite schist may be taken as the index rock of the various belts,
each belt is in itself a complex of muscovite schists, quartz muscovite
schists, quartz schists, quartzites, biotitic and hornblendic schists
and other rocks and the proportions of these rocks vary not only in the
successive belts but at different points within the same belt. In the
third belt for example there is along with the muscovite schist a great
development of quartzites and quartz schists around Lukke, where they
form prominent rocky hills (_v._ Traverse No. III. p. 66). Farther north
several bands of chlorite and talc schist are found in the same belt in
the neighbourhood of Iye, while in the fourth belt between Wa and Jakura
there are intercalations of such other rocks as quartz magnetite schist,
magnetite hornblende schist, and crystalline limestone (_v._ Traverse
No. II. p. 65). The limestone is a white coarse-grained saccharoid
marble which may be traced for at least 1,000 feet across the strike
without affording any indication of repetition by folding. On the margin
the marble is distinctly schistose with the foliation planes striking N.
10° E. and dipping at a low angle towards the west. The rocks in the
neighbourhood are somewhat irregularly folded with the foliation planes
as a rule parallel with the bedding. Their junction with the marble is
not exposed, but if the marginal foliation of the latter be also
parallel with the bedding, the actual thickness of the original
limestone must have been very much less than 1000 feet. The marble is
very pure, consisting almost entirely of calcite with occasional streaks
and segregation bands of quartz-epidote-calcite-idocrase rock
representing the original impurities within the limestone. Minute spots
of graphite are abundant throughout and on the margin streaks and knots
of quartz and pyrites are common along the foliation planes. In places
the marble is irregularly crossed by later pegmatite veins.
The tracts which intervene between the various belts are occupied by a
variety of soft micaceous and hornblendic schists and differ from the
belts already described only in the comparative rarity of muscovite
schists and quartzites. The absence of sufficiently detailed
investigation, however, makes it impossible to say to what extent the
various belts described above represent repetitions of one and the same
belt, or to what extent they represent the original sequence of the
sedimentary succession. While the average strike of the whole series is
meridional, its undulating or zigzag character is evident from the
occurrence of individual exposures showing every angle between north and
east. The different members of the series, while frequently vertical,
may dip at various angles to the east or west or even in places approach
the horizontal. Individual bands are frequently puckered and contorted
and the whole series is invaded in places by extensive masses of biotite
granite which rise into turtlebacks and groups of rounded hills. The
granites are usually foliated on the margin and sometimes throughout and
frequently show at their junction with the gneisses and schists a narrow
zone of injected rock. Small detached exposures of the harder banded
gneisses, elongated upon a parallel strike, are distributed irregularly
throughout the softer series. The gneisses and schists are abundantly
pierced by muscovite and tourmaline pegmatites and veins of quartz and
tourmaline quartz, the latter in the neighbourhood of Iye being minutely
banded as in the case of some of the tourmaline quartz veins at Bussa
and Ngashki. With the pegmatites appear to be connected some small
intrusive masses of non-foliated granite which become more numerous in
the eastern portion of the area under consideration (_v._ Traverse No.
I. p. 64).
The striking feature of the architecture of the high plains of central
Kabba is the manner in which the softer gneisses of the north with their
intercalated bands of schists and quartzes are invaded, and in part
replaced, by masses of granite and of hard banded and composite
gneisses. The first belt of schists can be traced southward with
intervals from Omwo to the neighbourhood of Arigidi and Sequarre, where
the soft micaceous schists carry much staurolite. Harder gneisses
however appear to bound this softer belt irregularly to the west and
east. The prolongation of the second belt of schists in northern Kabba
is probably represented by two narrow strips of quartzite enclosed in
the harder gneisses between Kabba and Lupa. The third belt appears to be
somewhat abruptly replaced in the neighbourhood of Odouape by a complex
mass of granites and hard composite and banded gneisses which extends
westward across the high plains of Kabba from Agwede and Shokko Shokko
to Ikinre and Omwo (_v._ Traverse No. IV., p. 67), and is apparently
prolonged southward into the Egbira hills. The fourth belt appears to
suffer much lateral variation, and is represented on the main road
between Lokoja and Kabba only by some thin bands of muscovite schist
enclosed within a series of micaceous and felspathic gneisses.
In southern Kabba the few facts at our disposal seem to indicate the
almost complete absence of quartzites and muscovite schists and a
repeated alternation of the softer and harder gneisses, the whole series
being pierced by pegmatite veins and intrusive masses of granite.
Quartzites and quartz schists reappear, however, in the Kukuruku hills
on the borders of Northern and Southern Nigeria.[19]
The lower plains which stretch eastward to the Niger from the
escarpments of the Egbira and Agwede hills are occupied very largely by
gneisses of the softer series. Quartzites, quartz schists and quartz-
muscovite schists occur within the series, but their distribution over
the plains is so irregular, and their prolongation along the strike so
inconstant, that they cannot in any way be correlated with the
quartzites and schists of either the third or fourth belts in northern
Kabba. It would seem, indeed, that instead of the regular alternation to
be found in the north, there is over these eastern plains a greater
mixing of types and a greater variation in character of the individual
members when traced along the strike. Quartz-magnetite and quartz-
hæmatite schists are fairly common, typical occurrences being found in
the neighbourhood of Bangede and around Okuruku and Anamu near Okeli,
and on the plains to the west of the Againyi hills, as well as at the
foot of their western slope. Amongst the granites and granitoid gneisses
which form the escarpment of the Egbira hills, bands of felspathic
gneiss and micaceous schist are frequently enclosed, and on the plains
the gneisses and schists are crossed in the usual manner by pegmatite
veins and pierced by masses of more or less foliated granite, which form
the prominent isolated hills which diversify the surface of the plain.
Here and there also may be found a small exposure of composite or banded
gneiss, but it is only in the immediate neighbourhood of the Niger that
the harder gneisses attain any extensive development. There they form a
narrow belt, marked by prominent intrusions of granite, which stretches
southward from Lokoja along the right bank of the river, and adds
picturesqueness to the Niger valley between Igbo and Itobe (p. 6). At
Lokoja the harder gneisses are exposed in the river bed, and the softer
gneisses and schists at the foot of Mt. Patti. The average strike is
N.E. to S.W., and the whole series is much pierced by veins of granite
and graphic pegmatite, the latter containing frequent knots of
magnetite.
To the east of the Niger the softer gneisses reappear in the narrow
strip of crystalline rocks in Bassa province which fringes the left bank
of the river. Typical exposures are found in the neighbourhood of Igbo
and Itobe, where the gneisses are associated as usual with early
foliated or partially foliated granites and with later intrusions of
muscovite and tourmaline granites and pegmatites, the tourmaline
occurring as a rule in large and well-formed crystals. At Igbo the
micaceous gneisses enclose a number of narrow bands of fine-grained,
crystalline limestone, each 3 to 4 feet in thickness. The limestone
varies much in colour and hardness, the purest varieties being blue,
while weathering and impurities produce grey and brown tints. Much of
the limestone is gritty, but the commonest accessory is a pale-coloured
tremolite in rosettes and felted veins of fibrous crystals and in
scattered well-formed prisms.
The crystalline rocks of Kabba and Ilorin thus present considerable
resemblances in individual types to those of the Niger valley above
Jebba. The most striking differences are the occurrence of such new
types as crystalline limestone and magnetite and hæmatite schist in
eastern Kabba and Bassa, and the apparent absence in the southern
provinces of the characteristic phyllites and slaty schists of the Niger
valley to the north. The place of the phyllites, however, is probably
taken by the muscovite and quartz-muscovite schists, which form such
prominent members of the metamorphic series in northern Kabba. If so,
there would seem to have been a slight increase in the intensity of the
regional metamorphism in these southern provinces as compared with the
Niger valley above Jebba. Of thermal metamorphism, on the other hand,
there is little trace. The schists and gneisses in the neighbourhood of
Sequarre in western Kabba and in several localities in northern Kabba
and Ilorin contain a considerable quantity of staurolite and other
metamorphic minerals. Their distribution, however, appears to possess a
linear character which is somewhat inconsistent with a contact origin,
while sufficiently extensive igneous intrusions, to whose influence
their local development might be ascribed, have nowhere been definitely
recognised in the vicinity. The numerous intrusive masses of both
foliated and unfoliated granite throughout these provinces appear to
have induced little or no contact metamorphism in the surrounding
gneisses and schists. An exception to this rule may perhaps be made in
the case of the tourmaline granite behind Igbo on the Niger, to whose
influence may possibly be ascribed the extensive development of
tremolite in the neighbouring crystalline limestones. At the same time
it should be remembered that the occurrence of tremolite in metamorphic
limestones cannot be taken as an invariable indication of contact
action.
The irregular association of a less metamorphosed series of quartzites,
schists, and gneisses with a more highly metamorphosed series of well-
foliated, banded and contorted gneisses, exhibited by the crystalline
rocks of the Niger valley above Jebba, forms likewise a striking feature
of the crystalline rocks of Kabba and Ilorin. The banded gneisses may be
either biotitic or hornblendic and the lighter and darker bands may
alternate so regularly and rapidly as to give rise to a striped or
ribboned appearance even in the hand specimen. They are for the most
part of a medium and even grain and never porphyritic, although
sometimes they enclose lenticular masses of finer grained biotite gneiss
or of amphibolite. As a rule they are much invaded by granite and
frequently rendered composite by thin granitic intrusions. The
foliation, which may be parallel or oblique to the striation, is their
least conspicuous feature. Pegmatite veins are usually abundant and
frequently foliated. As already described (p. 74) these hard banded and
contorted gneisses are found irregularly replacing the softer gneisses
both along and across the strike.
So far as known there are no schistose rocks in Kabba and Ilorin
comparable to the sheared trachytes of the Niger valley and the sheared
porphyries of Borgu. Rare occurrences of thin basaltic veins have been
observed, but as a rule the later intrusions in Kabba and Ilorin take
the form of small masses of red or white aplitic granite or of dykes and
veins of muscovite and tourmaline pegmatite or of graphic granite. These
later granites have usually a peculiar granular and incoherent character
and never possess chilled margins. Frequent occurrences have been noted
of the granular granite acting as a kind of ground mass to large
scattered porphyritic masses of graphic quartz and felspar (_v._
Traverses Nos. I, II, III, pp. 64–66). Frequently also the granites show
an ill-defined banding caused by an alternation of coarser and finer
grained layers. It is possible that the muscovite and tourmaline
pegmatites, the veins of graphic granite, the quartz reefs, the banded
tourmaline quartz rocks, and the small intrusive masses of acid granite
may all possess a certain genetic connection. Only at Igbo, however,
does there occur an extensive mass of reddish muscovite tourmaline
granite which to some extent unites the characters of all the dyke rocks
and represents in a general way the original magma from which the
various dykes and veins and intrusive masses have been differentiated.
The abundance of tourmaline throughout Kabba and Ilorin led to the hope
that tinstone would in places be associated with it, but only at Eri in
eastern Ilorin have the pegmatites proved stanniferous. The quartz reefs
also have nowhere been found to yield more than the minutest trace of
gold.
4. _The Central Crystalline Area._
The peculiarities of structure which have been remarked in the western
provinces are continued throughout the larger crystalline area in the
centre of the Protectorate. To the east as to the west of the Niger
there has been an irregular and local replacement of a series of softer
gneisses, quartzites, and schists by a complex of harder composite and
often well-banded gneisses. There is also in the western part of the
central area within the softer gneissic series a well-defined grouping
of the quartzites and phyllites or micaceous schists into regular and
definite belts which preserve their identity for considerable distances
along the strike. In the east, however, this regular grouping appears to
be absent, the quartzites and mica schists being distributed irregularly
and in short and narrow bands amongst the felspathic and micaceous
gneisses.
In the east of Sokoto province there are at least two broad and well-
defined belts of phyllites and indications of a third. The first is
found between Bukwium and Anka near the Kontagora border, and is well
exposed in the Anka river. The strike of the phyllites varies from N.–S.
to N. 20° E. Surface creep here and there makes the dips uncertain, but
the inclination is usually high, and is sometimes to the west, sometimes
to the east. This fact probably accounts for the great width of the
belt. It is not known whether the phyllites extend to the east of Anka,
but after crossing them at right angles to the strike for thirty miles
to Bukwium they are still in full force, and the western limit is
unknown. There are numerous quartzite bands, especially about Anka and
to the south of it. The phyllites are soft, fine-grained and dark-
coloured, and are traversed by abundant quartz veins, some of which
carry pyrites. It is interesting to note that the phyllites are only
exposed in the neighbourhood of the river. Away from the river, on the
level plain, there is no indication of them, and this fact is of
significance when considering the possible extension of the phyllite
belts. There is a numerous and interesting series of igneous rocks
within the phyllite area, including such types as hornblende granite and
syenite, pegmatite and quartz porphyry. These rocks, as a rule, show no
sign of crushing, and are probably later intrusions. Some of the
porphyries, however, show an imperfect foliation, and appear to have
been exposed to some of the later crustal movements. Bands of
amphibolite and veins of tourmaline quartz also occur within the
phyllitic series. (A. L.)
The second group of phyllites and quartzites forms a belt of low hilly
country striking N.N.E.–S.S.W. across the Sokoto-Kano caravan route
between the villages of Maradu and Karakai, and continues northward at
least as far as Bazai. The series attains across the strike a breadth of
about thirteen miles, and consists of a succession of slaty schists and
knotted phyllites and quartzites, much pierced by quartz veins and thin
bands of schistose granite, and dipping fairly constantly in the west at
70° E.S.E., but gradually heeling over in the east to the W.N.W. Towards
Karakai the quartzites predominate and contain much tourmaline, and many
samples may be found showing quartz and tourmaline or quartz, slate and
tourmaline in alternating ribbon-like bands. The third belt is very
imperfectly exposed on the borders of the province between Woneka and
Duru, and is represented only by some scattered outcrops of slaty
phyllite and mica schist.
Between Maradu and the margin of the Sokoto sandstones to the west, the
softer gneisses are exposed upon the sandy plains in a very fragmentary
manner, and appear to consist of micaceous and hornblendic gneisses and
schists, and of hornblende epidote gneisses and amphibolites with
numerous small granitic intrusions. The rocky knobs and kopjes two miles
west of Maradu are formed of a coarsely banded volcanic agglomerate,
dipping eastward at angles of 30°–40°. The agglomerate may be traced to
within a mile of the village, where it gives place to a thinly bedded
series of red ashy sandstones and ash beds. East of the rest camp, these
beds strike N.N.E. and dip at 40° E.S.E. while west of the camp they
strike E.N.E. and dip at 30° S.S.E. The ash is exposed in, and to the
west of, the village, but to the east its place is taken by a much
broken and shattered granitic rock. The actual junction, however, was
not observed. Such evidence as there is points to the former existence
of a volcanic neck or vent in this locality.
Between Karakai and Woneka the surface accumulations obscure the rock
for long distances, especially to the west and east of the granite hills
of Kotorkoshi. Around Bungudu, however, there are numerous exposures of
a coarse-grained composite gneiss which probably extends eastward to the
boundary of the province. Beyond Duru a grey biotite granite with a
distinctly foliated margin forms the broken hilly country between Runka
and Kurefi, but between Kurefi and Sani the granite gives place to a
group of dark-coloured biotite gneisses before the crystalline rocks
disappear under the sandstones of Katsina. Knobs and hummocks of
granite, however, protrude through the sedimentary rocks round the walls
of the town, and Barth[20] has reported the occurrence of similar mounds
and ridges immediately to the north. To the south-east of Katsina the
surface rock appears to be everywhere a reddish granite as far as Duru,
but beyond Duru the superficial accumulations so effectually cover up
all but the occasional kopjes and boulders of granite that the character
of the underlying rocks is difficult to ascertain. From the scattered
fragments of quartzite, gneiss and amphibolite, and occasional granite
boulders in the surface drift, it is probable, however, that for a
considerable distance to the west of Kazaure the floor is formed of the
sedimentary gneisses with the usual granitic intrusions. The hills
around Kazaure are composed of hard blue quartzite much pierced by later
veins of white quartz, and striking N.N.E.–S.S.W. with a dip of 30° to
the west. This belt can be traced southward to the neighbourhood of
Kasuan Kuka, where the quartzites become more micaceous and where some
rocky exposures of granular amethystine quartz probably represent the
intrusive quartz veins of Kazaure. From Kasuan Kuka to Kano, granites
and an occasional quartz vein are the only rocks exposed, while the deep
drift effectually prevents the discovery of the relationship between the
surrounding rocks and the decomposed diorites of the two flat-topped
hills within the walls of Kano. A few miles east of Kano the crystalline
rocks disappear under the sands and clays of Katagum and Bornu. There is
every reason, however, to believe that they are continuous to the north-
east with the granites, gneisses and quartzites of Machina, Zinder and
Mounio.
The two detached hills, Dala and Kogon Dutsi, the landmarks of Kano,
with their precipitous slopes and tabular summits, 150 feet above the
level of the plain, present from a distance a decidedly sedimentary
appearance. Clapperton,[21] indeed, described them as consisting of
“argillaceous ironstone mixed with pebbles and a rather soft kind of
marl,” while Dala, the eastern mount, “is of clay-ironstone and
conglomerate lying upon a bed of soft light clay apparently mixed with
vegetable remains.” Closer investigation has shown, however, that both
hills are of crystalline origin, Dala being composed entirely of earthy
decomposition products with a gritty ferruginous cap, while Kogon Dutsi
still retains cores or kernels of undecomposed diorite in the midst of
the friable earthy material which forms the greater part of the hill.
The felspar and hornblende of the original diorite have decomposed in
the lower part of Dala into a whitish clay streaked and spotted with
dark green chloritic material. In the upper part the colouration is more
uniformly white or yellow, the bulk of the iron having been leached out
to form the red ferruginous cap. That the latter is itself composite,
however, is evident from the occurrence of gritty grains, subangular
fragments of quartz and rolled pebbles of ironstone in the vacuolar and
concretionary crust. Kogon Dutsi presents a similar composition, with
the exception that the material becomes coarser and more granular in the
neighbourhood of the kernels of undecomposed rock. The extensive
decomposition of the crystalline rocks to which the earthy hills of Dala
and Kogon Dutsi bear witness will be found in the sequel (p. 201) to be
an important factor in the evolution of the characteristic scenery of
the high plains of Hausaland.
PLATE VII
[Illustration: CRATERS IN THE MBOI HILLS.]
[Illustration: A CRATER IN THE MBOI HILLS.]
Around Kontagora and beyond the northern limit of the sandstones which
occupy the southern portion of the province of the same name, there are
found hard biotite gneisses in places well striated with pink felspathic
and dark biotitic bands and crossed by numerous pegmatite veins. To the
east the crystalline floor is deeply covered with drift, and outcrops
are comparatively rare. In the Beri river, however, there is a good
exposure of soft felspathic biotite gneisses, with knots and bands of
pegmatite, while at Beri a pink granite forms a compact group of rocky
hills. The granite possesses a distinctly foliated margin, and is
followed to the east by hard hornblende and biotite gneisses much
pierced by granite veins, which may be traced to within six miles of
Bobi before they are replaced by the softer micaceous series. On account
of the extent of the superficial accumulations, however, these softer
gneisses are well exposed for the first time only in the rocky bed of
the Kara river which forms the boundary between the provinces of
Kontagora and Zaria.
Though the structure of the north-eastern part of Kontagora province is
complicated in detail, it may be described in general terms as a
succession of gneiss and quartzite bands alternating with one another.
They strike a few degrees east of north, and probably pass to the east
of the Bukwium-Anka phyllites. The quartzites are frequently tourmaline-
bearing, and one band west of Kotonkoro carries scales of graphite. This
fact is interesting in connection with the graphitic schists reported
from a neighbouring part of Zaria.[22] The gneisses are usually biotite
gneisses, sometimes markedly banded, at other times indistinctly so.
Schistose rocks are seen in places, quartz schists being prominent to
the south of Mahorro, where talc schists are also seen. The gneiss and
quartzite country is traversed by pegmatite and quartz veins, but
intrusions of unfoliated granite are rather uncommon. Most of the
turtle-backs are foliated throughout, differing in this respect from
those of many other parts of the Protectorate. Where the Sokoto road
leaves the province, the boundary is marked by a high sharp ridge of
amphibolite. (A. L.)
A tract of broken hilly country, almost entirely carved out of the
softer gneissic series, occupies the south-western corner of Zaria
province, and a diagonal traverse across the hills from Moriga on the
Kara river by way of Garun Gabbas and Zungeru to Kato and Mikonkeli
presents a particularly instructive section. The hills which begin about
ten miles south-east of Moriga consist between Inga and Garun Gabbas of
a succession of quartz schists, quartz muscovite schists, micaceous and
hornblendic schists, felspathic gneisses and amphibolites, pierced by
numerous quartz reefs and veins and by a few dykes of basalt and
porphyry, the whole series possessing an average strike of N. 10° E.
Between Garun Gabbas and the Kaduna river ridges of quartz and quartzite
rise conspicuously at intervals out of the pebbly drift which covers the
surface. In the railway cutting on the right bank of the Kaduna near
Zungeru, quartzites and quartz schists are associated with bands of a
hard and minutely foliated granulitic gneiss containing knots and
lenticles of quartzo-felspathic and chloritic material. In the river bed
below the railway bridge the most conspicuous rock is a well-bedded
white quartzite striking N. 30° E. and dipping at an angle of 64° to the
south-east (Plate VIII, Fig. 2). The quartzite varies much from a
massive variety with knots of pyrites and scattered flakes of white mica
through a streaky lustrous micaceous quartzite with linear foliation
planes, into a well-foliated quartz muscovite biotite schist. The
different varieties are often arranged in alternating parallel bands and
the whole series is pierced by occasional narrow veins of quartz. The
trench which at the Kaduna bridge forms the low water channel of the
river has been carved out of a band of hornblende schist intercalated
along the strike of the quartzites. The schist, which in places is
composed almost entirely of hornblende, but in others interfoliated and
streaked with felspar, has been more easily worn away than the
quartzite, and grooved and pot-holed by the action of the river. The
quartzites and quartz schists, with intercalated sills of hornblende
schist, continue eastward to the adjoining channel of the Dago, where
they are replaced by finely striated biotite gneisses. At Zungeru
micaceous and felspathic gneisses are mingled with soft, thinly bedded
and repeatedly folded hornblende mica epidote gneisses or schists. The
whole series is pierced in the usual manner by veins of quartz,
pegmatite and granite and strikes in a general way N. 10° E. with a dip
of 60° to the east. Immediately east of the railway station, however,
these rocks give place to well-foliated granulitic gneisses with knots
of felspar, striking N. 20° E. and dipping at 45° E. These gneisses,
which probably represent more completely altered portions of the
sedimentary series, are in places well striated, with the bands wavy and
contorted and crossed irregularly by veins of quartz. They are followed
in the vicinity of Kwarra-town by coarsely schistose grits which are
among the least altered rocks within the Protectorate. Between Zungeru
and Gidan Motu there are numerous exposures of green hornblendic beds
similar to those at Zungeru. They become, however, more fissile and
contorted towards the east. The strike oscillates between N.N.E. and
N.N.W., and the dip varies between vertical and high angles to the east
or west, while at Gidan Motu there is an intercalated belt of platy
biotite gneisses which strike N. 60° W. and dip at 60° S.S.W. Between
Gidan Motu and Kato green slaty schists and phyllites are extensively
developed and steeply folded at Kato into sharp anticlines and synclines
which possess an average strike of N. 20° W. The phyllites enclose much
quartz in lenticles and veins and contain abundant knots and spots of
pyrites. Beyond Kato the phyllites are succeeded by soft mica schists,
which in turn give place to micaceous and felspathic gneisses. The
latter around Mikonkeli, Minna and Paiko are replaced by hard composite
and in places banded rocks and invaded by extensive masses of granite
which usually show a distinct marginal foliation.
Southward the Zungeru belt of quartzites and phyllites disappears under
the sandstones of Nupe, but any correlation with the quartzites and
schists of Kabba cannot yet be attempted. Northward the belt has been
followed as far as Birnin Gwari and from its pronounced development in
the neighbourhood of that place, it is probable that it extends
considerably farther north again. For twenty or thirty miles north of
Tegina there are a number of granite hills strung together on a roughly
north and south line, the granite having apparently been intruded along
the strike of the schistose rocks. The granites become foliated locally.
Between Bugai and Koriga there is a twenty mile belt of phyllites and
subordinate mica schists which runs northward to Birnin Gwari, where the
phyllites form a number of sharp north and south ridges and are
excellently exposed, both in the sides of the ridges and in the
intervening streams. The phyllites are traversed by many quartz veins
and gold is widely disseminated in the streams. At Koriga there is an
extensive development of hornblende gneiss. To the east are mica
schists, and graphitic schists are reported from the neighbourhood.[23]
The rocks which rise above the plain in the vicinity of Zaria are all of
granite. (A. L.)
So far as known, the Zungeru belt is the last prominent group of
phyllites which can be traced within the Protectorate. To the east their
place is taken by muscovite and biotite schists, which are to be
regarded as more completely reconstructed representatives of the
original shales, the metamorphic action having apparently been somewhat
more intense in eastern and southern Zaria and Nassarawa. Several belts
of quartzites and schists are enclosed in the usual manner within a
complex of softer and harder gneisses to the east of Minna and
Mikonkeli. At Kuta, for example, there is a belt of quartz muscovite
schist passing on the one hand into knotted micaceous schists and on the
other into quartzites and carrying much sillimanite and magnetite and
large lenticles and knots of quartz and tourmaline quartz. The whole
series strikes approximately N. 30° E. and dips at 45° N.W. Again
between Chiri and Guni there is much soft micaceous schist rising into
low rounded and well-wooded hills, striking generally N.–S., well
frilled in places, with a high content in ilmenite and numerous knots
and veins of tourmaline quartz. Farther east the hills which run from
the neighbourhood of Dorka to Kaderri and Kaserimi are composed very
largely of quartzites, quartz schists, quartz muscovite schists and
micaceous gneisses, striking N. 10° W., dipping at an angle of 60° W.,
and pierced by quartz veins, muscovite pegmatites and granites. Between
these more prominent belts occur numerous smaller exposures of schists
and quartzites and of both the softer and the harder gneisses. To the
east of Kaserimi and Duchi and in the valley of the Kaduna the surface
is so deeply and extensively covered with drift that little rock beyond
the marginally foliated biotite granites of the isolated hills is
available for investigation. There are, however, indications from
occasional fragments in the drift and rare exposures in the stream
courses that a considerable extent of the buried surface is composed of
a softer gneissic series. To the east of the Kaduna, the Ruruma and
Kerko hills which rise so prominently from the south-eastern plains are
composed throughout of a coarse-grained biotite granite, locally
carrying hornblende in addition and in places distinctly foliated on the
margin. The junction with the gneisses is only rarely seen on account of
the thick masses of drift which envelop the base of the hills. At Guri
on the eastern slope of the Ruruma hills there are exposures of finely
laminated hornblende epidote gneisses, while the granites and the
marginal rocks are pierced by numerous dykes of basalt, dolerite and
quartz porphyry.
The high plains of south-eastern Zaria are occupied by mica schists and
gneisses, with granite hills in one or two localities. Along the eastern
border of the province the Rukuba plateau ends in a ragged edge of
biotite granite which veins the gneisses at its foot. The country around
Jengre and Limoro is interesting on account of the occurrence of masses
of soda granite and especially on account of numerous intrusions of dyke
rocks, felsites often with flow structure, porphyries and quartz
porphyries. Tinstone is frequently met with in the streams of the
district. Similar dyke rocks are exposed around the base of the Kudaru
hills to the north-west of Leri and there, in addition, dykes and
veinlets of basalt penetrate the granite. These hills are made up of
hornblende granites with syenitic modifications, and locally, towards
the boundary, the granite takes on a distinctly foliated structure, with
rounding of the porphyritic felspars as in augen-gneiss. (A. L.)
In Nupe, between the Gurara river and the sandstones of the Niger
valley, the crystalline series is introduced by muscovite biotite
gneisses, quartzites, hornblende and talc schists and amphibolites,
pierced by extensive masses of granite around Gaung, Lefu and Paiko. In
the bed of the Gurara, between Gaung and Izon, soft gneisses and
hornblende epidote rocks alternate with hard and well-foliated gneisses
striking a little west of north, and varying in dip from vertical to 40°
or 50° E. Five miles beyond Izon, the Abuja road ascends over a crumpled
and contorted series of biotite, hornblende, and epidote hornblende
gneisses much injected by masses of biotite granite. Beyond Abuchi a
second ascent over soft micaceous gneisses, striking N. 10° W. and
vertically foliated, leads up to the striated, ribboned and banded
gneisses which have been invaded by the biotite granite of Abuja. The
banded gneisses strike N.–S., and are much granitised in the immediate
neighbourhood of the intrusive rock, while the granite itself is
marginally foliated and crossed by crush lines at the Izon gate (_v._
Traverse No. V, p. 68). Partially foliated granites and banded and
composite gneisses continue to within five miles of Kao, whence a belt
of soft micaceous gneisses and quartz muscovite schists, striking N. 20°
E., stretches eastward to the neighbourhood of Kari. At Kari the
escarpment of the northern tableland is formed of soft biotite gneisses,
much pierced by veins of quartz and tourmaline quartz. Eastward by way
of Kukwi, Anta, Tatara and Jaginde the escarpment shows much granite,
associated both with banded gneisses and with softer micaceous and
hornblendic rocks. The gneisses and granites are extensively invaded by
muscovite and tourmaline pegmatites, and by irregular intrusions of
basalt and dolerite. Between Jaginde and Darroro quartzites, quartz
schists and mica schists outcrop along the escarpment, and at Darroro a
similar series of thin quartzites and gneisses, striking generally
N.–S., is pierced by extensive masses of quartz-muscovite-tourmaline
pegmatite, and capped by the basalts of the Morrua plateau. The
pegmatite is rudely banded parallel with the strike of the gneisses, the
various bands being distinguished by their richness in tourmaline. All
proportions may be found between quartz muscovite and quartz tourmaline
rock, the tourmaline in the latter being either in large well-formed
prisms, or in thin alternating bands of felted fibrous crystals. North-
east of Darroro the vertical granite cliffs of the Kagoro massif form
the margin of the Bauchi plateau, while in the neighbourhood of Assab
the later basalts much obscure the crystalline rocks of the lower
plains. At Assab the escarpment is largely composed of biotite granite,
in places porphyritic, and apparently intrusive into a series of
micaceous gneisses which strike N. 10° E. and dip at an angle of 40° to
the west.
The northern tableland and the central plains of Nassarawa are composed
very largely of the softer gneisses with scattered outcrops of harder
composite rocks. Quartz muscovite schists occur in a broad belt on the
tableland in the vicinity of Aribi, and calc-silicate rocks between Kwai
and Jaginde. The composite and banded gneisses reach their greatest
development in the Karshi hills between Abuja and Keffi. The Mada hills
and the greater part of eastern Nassarawa are still unexplored. In
southern Nassarawa a broad belt of quartz muscovite schists, mica
schists, hornblende schists and chlorite schists extends between Gwombe
and Buga, the mica schists in the neighbourhood of Zagabutu being
particularly rich in staurolite, garnet and imperfectly crystallised
knots of andalusite, presumably of contact origin. The whole series has
been steeply folded on a meridional axis and then coarsely corrugated
transversely, the result being that individual exposures may strike at
any angle between E.–W. and N.–S. The average strike is inclined around
Gwombe to the north-west, and around Buga to the north-east. To the east
of Buga the schists give place to the usual varied succession of softer
gneisses, pierced in the usual manner by veins of quartz and tourmaline
quartz, and associated from time to time with banded and composite rocks
and detached masses of granite. Numerous dykes of basalt and quartz
porphyry appear to be associated with the granites of the Kurudu and
Anagoda hills, which lie respectively west and south-east of the town of
Nassarawa.
The crystalline rocks of Bauchi province present similar characters to
those of other parts of the Protectorate. The broad belts of schists and
quartzites found in other provinces are here, however, conspicuously
absent, such types being represented only by thin bands intercalated
within the softer gneisses. Banded and composite rocks, on the other
hand, appear to be present in somewhat greater proportion than
elsewhere, while domes and turtlebacks of foliated or partially foliated
granite (Plate XII, Figs. 1, 2) are in places particularly abundant. The
softer and harder rocks are, however, so intimately mingled that a
detailed description of the succession is impossible. Excellent
exposures of the harder series are to be found in the Kogin Delimi below
Naraguta in the west of the province, and of the softer gneisses in the
Jarawa hills to the south of Bauchi, and of both in the broken rocky
country between Kanna and Angass in the southern portion of the province
(_v._ Traverse No. VI, p. 68). The strike of the gneisses, while very
variable, appears to have a greater tendency to assume an E.–W.
direction in Bauchi than in other provinces, a tendency which is
probably due to the greater intensity of the transverse forces which
have elsewhere corrugated the prevailing meridional strike of the
gneisses and schists. This feature is particularly marked in the
neighbourhood of the Ningi massif, which is elongated approximately
along the strike of the surrounding softer and harder gneisses.
Another notable feature of the province of Bauchi is the number and
variety of the later igneous intrusions. Pegmatites, quartz reefs and
veins of tourmaline quartz are as abundant in Bauchi as elsewhere, but
the attention is somewhat diverted from these commoner intrusive types
by the presence of an abundant series of dyke rocks, and of extensive
masses of non-foliated granite and syenite. The pegmatites are in places
particularly rich in ilmenite, rutile and chalcedonic quartz; the dyke
rocks present almost every variety between basalt and quartz porphyry;
and many of the granites are especially rich in soda, and in places show
chilled margins at their junction with the gneisses. The summit of the
plateau around Bukuru is occupied by one of these extensive granitic
intrusions, which is of peculiar interest from the fact that in places
it is veined and impregnated with tinstone and other minerals. A similar
granite, also carrying tinstone, forms the greater part of the
Kwandokaya hills to the north of Toro. Many intrusions of syenite are
found around and to the east of the town of Bauchi. The Ningi massif is
largely composed of a soda granite, with a well-chilled margin,
accompanied by numerous dykes of granite and porphyry, and in places
carrying tinstone. Similar granites are found at Kila and Fagam on the
borders of Bauchi and Kano, at Shira in Katagum to the north, and at
Gurkawa on the Muri border to the south. The Murchison Hills are
believed to be largely granitic, but they and the southern margin of the
plateau are still practically unexplored. A small detached area of
granite and gneiss, pierced by numerous pipes of basalt and trachyte, is
found around Tangalto and Chongwom in the south-east of Bauchi province.
_Southern Muri and Yola._ (A. L.)
The strip of crystalline rocks which runs along the southern boundary of
Muri and Yola has an average width of forty miles and is separated from
the central area by the sedimentary rocks of the Benue plain. The
crystalline floor appears at the surface, however, in a number of small
inliers around Arofu in western Muri, and thus quite near to the river.
Five miles east of Arofu a small area of fresh biotite-granite about
fifty yards in diameter peeps from beneath the limestone. Immediately
north of the town itself there is a knoll of granitoid gneisses striking
north and south. On proceeding southward along this line of strike
limestone is passed over for three miles, but then the gneiss appears at
the surface again, and this time forms a ridge running southward for at
least some miles. It may well be that other inliers of crystalline rocks
occur in the unexplored Munchi country.
Farther south the transition from sedimentary to crystalline rocks is
not accompanied by any feature in the drift-covered plain. Though the
exposures are not good, it appears that biotite-gneisses and schists
with the customary northerly strike are penetrated by granite veins. The
rocky masses which lie south and south-west of Takum are probably
largely or entirely of granite. The bulk of the material which forms the
scattered hills around and to the north of Takum is also granite, but
here it is inextricably mixed up with gneiss which is met with in patchy
fashion from top to bottom of the hills. The foothills of Shebshi are
schistose grits and gneisses invaded by granites in the usual
manner.[24]
PLATE VIII
[Illustration: EXFOLIATION IN THE KILBA HILLS.]
[Illustration: THE GORGE OF THE KADUNA AT ZUNGERU.]
Only the northern part of the Mumuye-Manna plateau has been traversed.
The rock there is almost always biotite-granite, but it is by no means
always of the same variety. Occasional quartz veins are seen on the
plateau and also on the lower ground between Numnai and Panteti, where
the quartz carries pyrites. Intrusions of quartz-porphyry are often seen
cutting through the granites, _e.g._, east of Numnai, around Shega Mundi
hill, around Panti Baturi and at Yendam Hill. The granites only
occasionally carry included blocks and fragments of gneiss, but on
descending to the plain biotite-gneiss is met with striking N. 10° E.,
and the plain from Chukol eastward to the Vere Hills is occupied by
biotite-gneiss, with some intrusions of granite. The Vere Hills
themselves are made up almost entirely of granite, though to a small
extent of gneiss also. Biotite-gneiss is frequently seen round the base
of the plateau, generally much penetrated by granite or quartz-porphyry.
Thus, speaking generally, the plains of southern Yola are mainly of
gneiss and the plateau areas mainly of granite.
To the north of the Benue plain again, the country around Song, Goila
and Korubi is a biotite-granite area with some conspicuous dykes of
quartz-porphyry running for miles across the country. Near Zummu
porphyry rises just above the soil of the plain and exhibits flow-
structure, while between Zummu and Holma a felsite dyke is seen cutting
through the granite. The hills around Holma are built of a well-jointed
biotite-granite, and the sides of the hills are concealed by the loose
blocks split off along the joints. The granite of the Kilba Hills, near
Pella, is a very fresh, coarsely-crystalline biotite-granite, and
locally, where the Kilba Hills rise out of the plain to the west, the
granite takes on an augenstructure. In consequence of exfoliation (Plate
VIII, Fig. 1) most of these hills rise up as smooth bare rocks, but some
are made up of heaps of blocks, and it is not possible to explain the
contrast as due to a difference in material. Between the hills around
Holma and those around Pella there is a plain of gneiss about fifteen
miles broad. The gneiss has been formed by the injection of a micaceous
gneiss or schist with innumerable strings and veins of granite. The
exposures are excellent throughout, but the gneiss never forms hills.
The country north of Pella is comparatively uninteresting. It consists
of an extensive plain with isolated hills and rocks of biotite-granite.
The nature of the rocks underlying the plain is practically unknown. The
small area of the Kwaba Hills is composed of biotite-granite with some
quartz-porphyry. (A. L.)
Crystalline rocks, largely of a gneissic character, are exposed also in
southern Bornu around Kwoia to the east of the Gongola and along the
base of the escarpment on the north-western margin of the Burra volcanic
plateau. In eastern Bornu the granite rocks of Chibuk and Kobshi are the
first prominent outcrops of crystalline rocks to the south of Maidugari.
Barth,[25] however, mentions an isolated granitic cone, Mt. Deladeba, to
the N.N.E. of Isge, while Courtet,[26] Gentil and Freydenberg[27] note
the occurrence of knobs of crystalline rock between Lake Chad and Lake
Fittri. It is highly probable indeed that the thick alluvial
accumulations to the east and south-east of Chad and in the lower Shari
valley rest for the most part upon a crystalline floor, provided with
scattered relics of Tertiary sandstone. (J. D. F.)
[Footnote 18: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 325.]
[Footnote 19: Parkinson, _Q. J. G. S._ (63), 1907, p. 317.]
[Footnote 20: Barth, _Central Africa_, 1857, Vol. II, pp. 41, 42.]
[Footnote 21: Denham and Clapperton’s _Narrative_, 1826, pp. 50, 57.]
[Footnote 22: Dunstan, _Colonial Reports—Miscellaneous_, No. 26, p. 15.]
[Footnote 23: Dunstan, _Colonial Reports—Miscellaneous_, No. 26, p. 15.]
[Footnote 24: See also Downes, _Yola and Cross River Boundary
Delimitation_, 1910, p. 35.]
[Footnote 25: Barth, _Central Africa_, 1857, Vol. II, p. 369.]
[Footnote 26: Courtet, _C. Rd. Ac. Sc._, 140, p. 160.]
[Footnote 27: Gentil and Freydenberg, _Bull. Soc. Geol. Fr._ (4), VIII,
1908, p. 44.]
CHAPTER III
PETROGRAPHY OF THE GNEISSES AND SCHISTS AND ASSOCIATED IGNEOUS
INTRUSIONS.
Subdivision of the gneisses and schists; predominant meridional strike.
Gneisses and schists of sedimentary origin; variation in degree of
metamorphism; phyllites and slaty schists; green schists; quartzites;
quartz schists and granulitic gneisses; crystalline limestones;
micaceous and felspathic gneisses; calc silicate rocks; epidote
hornblende rocks; magnetite and hæmatite schists; sheared trachytes of
Bussa; altered dolerites of Ngashki; amphibolites and talc schists;
granulites. Petrography of the banded Archæan gneisses: relationship of
the two groups of gneisses. The gneisses and schists of the Sahara and
of West Africa. Age of the sedimentary gneisses. Intrusive rocks: the
older granites; variation in degree of reconstruction; brecciation and
injection structures; the younger granites; aplitic and tourmaline-
bearing granites and pegmatites; tourmaline quartz veins; stanniferous
pegmatites; schorl rocks; soda granites; biotite and riebeckite
granites; tinstone associated with soda granites; biotite and augite
syenites; augite diorites; riebeckite quartz porphyries; ægirine
granophyre; augite syenite porphyries; alkali syenite porphyries;
diorite porphyrites; mica porphyrites; hornblende porphyrites; diabases
and basalts. Granites and dyke rocks of the Sahara. Age of the soda
granites; successive periods of intrusion.
THE GNEISSES AND SCHISTS.
For descriptive purposes it has been found convenient to separate the
crystalline rocks of the Protectorate into two groups, (1) a group of
quartzites, phyllites, schists, and soft felspathic and micaceous
gneisses, and (2) a group of hard well-foliated, much granitised and
frequently banded and contorted micaceous and hornblendic gneisses. The
first group occupies by far the greater area within the Protectorate,
and presents in the west a well-marked alternation of belts in which
quartzites, phyllites and schists predominate, with belts of which the
prevailing members are felspathic and micaceous gneisses. The second
group has no definite distribution within the Protectorate, but appears
to replace locally and irregularly the members of the first group
throughout the whole crystalline area. The members of both groups appear
to be irregularly folded along axial planes, which are usually vertical
or highly inclined, but it is probable that detailed investigation would
reveal a regular system of major folds, accompanied by much local and
minor crumpling. The folding, which is typically meridional, has been
affected by equatorial stresses in such a way as to produce a transverse
twisting or corrugation of the predominant strike. The result is that
individual exposures may vary in direction of strike from N.–S. to
E.–W., while in places, as in Bauchi province, the strike may become
regionally equatorial. A similar equatorial divergence of the prevalent
meridional strike has been noted by M. Hubert[28] in eastern Dahomey,
but his suggestion that the equatorial direction becomes predominant in
Borgu and the Niger valley has been found to be unsupported by local
evidence. It is impossible to believe, moreover, that there can be any
connection, as suggested by M. Chudeau,[29] between this locally
divergent strike of the gneisses and the distribution of the overlying
Cretaceous and Eocene rocks.
The sedimentary origin of the first group is fairly certain. The pebbly
quartzites, quartz schists, schistose grits, mica schists, slaty schists
and phyllites represent corresponding varieties of sandstones, grits and
shales, and frequently exhibit traces of their sedimentary origin in the
deformed pebbles and clastic grains which they contain. The granular
felspathic and micaceous gneisses on the other hand are more highly
reconstructed, and afford in themselves little or no evidence of a
clastic origin. Their constant association and alternation with the
quartzites and schists make it probable, however, that they represent
the arkoses and greywackes of the original sedimentary series. Many of
the green hornblendic and epidotic beds probably also represent original
gritty accumulations of chloritic and epidotic character, while the
quartz magnetite schists of eastern Kabba correspond to ferruginous
grits or gritty ironstones of sedimentary origin. The fine-grained
crystalline limestones of Igbo, and the course granular marble between
Jakura and Wa, similarly represent original clastic calcareous rocks
within the sedimentary succession. To what extent the recurrence of
belts of quartzites, phyllites and schists in the western part of the
Protectorate represents the repetition of one or more belts is quite
unknown. The phyllites and schists are moreover everywhere
unfossiliferous and the age of the original sedimentary rocks is
consequently undetermined. The foliation of the reconstructed rocks is,
as a rule, parallel with the bedding, but the more micaceous schists
frequently show a more or less well-defined and oblique strain-slip
corrugation.
The alternation of belts of less and more highly metamorphosed rocks in
the west of the Protectorate coincides to some extent with an
alternation of belts in which shear structures predominate, with belts
in which reconstruction is prevalent. The various belts appear to pass
insensibly into each other, but it has been found impossible in the
course of a rapid survey to decide either to what extent shearing may
have everywhere preceded reconstruction, or to what extent the varying
degree of reconstruction in alternate belts may represent a
corresponding variation in the intensity of the accompanying folding.
All that can be said with certainty is that the sedimentary series as a
whole has been subjected to a highly complex folding accompanied by much
shearing and reconstruction, and that while over the greater part of the
Protectorate the stresses have been for the most part relieved by, or
translated into, a molecular and mineralogical rearrangement, there are
certain belts in which relief has been primarily obtained by mechanical
readjustment or shearing. Moreover, as in the reconstructed areas there
are certain beds which appear to be more coarsely crystalline and less
distinctly foliated than others, so in the shear belts there are certain
beds which have assumed the character of granulitic gneisses, while
others in the immediate neighbourhood have retained to a very large
extent their original clastic and gritty character. Most of the
reconstructed gneisses exhibit under the microscope faint strain
phenomena which, with the incipient strain-slip foliation of the more
micaceous schists, are probably to be ascribed to regional stresses
affecting the whole series after the period of maximum reconstruction.
Grey and purple sericitic phyllites with knots of magnetite and pyrites
are extensively developed in the neighbourhood of Ngashki in the Niger
valley, around Anka and Bukwium on the borders of Sokoto and Kontagora,
and in the Zungeru belt in eastern Zaria from Kato to Birnin Gwari and
Koriga. In Kabba and Nassarawa and southern Zaria their place is taken
by fine-grained quartz muscovite schists with abundant magnetite. With
the phyllites are associated in the Niger valley and Sokoto province
green quartzose slaty schists (1195, 1325),[30] whose colouration is due
to recrystallised chloritic and hornblendic material. Mr. Parkinson[31]
has described similar slaty schists from Southern Nigeria. In the
neighbourhood of Zungeru these green schists become softer and show
under the microscope (1563–4) ragged porous or fibrous crystals and
crystal aggregates of reconstructed hornblende in a matrix of granulitic
quartz, felspar and magnetite, which encloses also scattered grains of
hornblende, biotite, chlorite and epidote, as well as relics of original
clastic grains of quartz and felspar. These rocks present a considerable
resemblance to the “green beds” of the Scottish Highlands.[32] True
quartzites are comparatively rare and only at Kazaure do pure siliceous
rocks attain any extensive development. Elsewhere the rocks which
present the characteristic appearance of quartzite in the field are
usually more or less felspathic and micaceous, and include schistose
grits, micaceous quartzites, quartz muscovite schists and granulitic
gneisses which show all the familiar stages in the deformation of
felspathic grits and sandstones. Some of the white and pink granulitic
gneisses, such as those of Zungeru, may, however, be ultimately proved
to have been originally of an eruptive character. A characteristic
feature of these rocks is the development of tourmaline and sometimes of
pyrites along the shear and foliation planes as the result of later
pneumatolytic action. At Kuta a belt of coarse quartz muscovite schist
contains abundant sillimanite in sheaf-like aggregates.
Mica schists in which biotite predominates are common throughout and
frequently carry garnet, magnetite and staurolite in conspicuous
crystals. In the Igbo limestones the original impurities have
recrystallised as tremolite, diopside, scapolite (122), sphene, biotite,
felspar, quartz and pyrites, while the streaks of silicates in the
limestone between Jakura and Wa contain diopside, epidote, idocrase,
felspar, and quartz. In the latter limestone also, scales of graphite
are common throughout and pyrites on the margin. Amongst the
reconstructed gneisses, which show a remarkable and rapid variation in
texture, the paler-coloured felspathic rocks are usually rich in
orthoclase and microcline, and occasionally muscovite, while the darker-
coloured varieties, in which biotite predominates, frequently carry
garnet, sillimanite, staurolite and in places also sphene, tremolite and
carbonates. Andalusite-bearing rocks are known from certain localities,
but kyanite gneisses and schists have not been definitely recognised in
place. The local occurrence of kyanite on the river sands, however,
points to its presence within the neighbouring rocks. Calc silicate
types are represented by the tremolite schists of the Nassarawa
tableland (668, 689), by a diopside tremolite schist from Borgu (1221),
a diopside and wollastonite-bearing rock with sphene from Maibirro in
the Niger valley (1192), a biotite tremolite schist with calcite from
Olle in northern Kabba (879), and a diopside and scapolite-bearing rock
with sphene and green spinel from Atuka, near Okuruku, in eastern Kabba
(843). The epidote hornblende gneisses of Boi in southern Bauchi (976)
contain much diopside and sphene, and are probably of sedimentary
origin, while relics of clastic felspar are common in an epidote rock
from Goshin Duchi a few miles east of Boi (983). A similar clastic
origin is presumed for many of the epidosites and epidote hornblende
rocks associated with the sedimentary gneisses in other parts of the
Protectorate. The quartz magnetite and quartz haematite schists include
coarse granular and fine-grained granulitic types (844, 1370), the
latter presenting considerable resemblance to the itabirites of the
United States. Their sedimentary origin is presumed throughout and only
in the case of the massive quartz magnetite rock of Okuruku (148), which
is associated with a magnetite-bearing epidiorite, is there any
probability of an igneous origin.
Two stages of alteration can be recognised among the sheared trachytes
of Bussa (p. 72). The least altered types (1164, 1167) show fracturing
of the phenocrysts of felspar and the replacement of the original ferro-
magnesian minerals by fibrous uralite, chlorite, epidote and calcite.
The more altered types (1166) are characterised by the production of
abundant white mica in the ground-mass and the development of biotite,
magnetite and carbonates from the original ferro-magnesian material.
Original vesicles of quartz and calcite in these rocks have been
deformed and the quartz replaced by a mosaic of angular grains. A
sheared quartz-porphyry from Kanikoko (1206) exhibits deformation and
granulation of the phenocrysts of quartz and felspar and the abundant
development of white mica in the ground-mass. In the altered dolerites
of Ngashki (1175, 1176) the original structures are almost entirely
obscured by the extensive production of granular epidote, calcite,
uralite, chlorite, magnetite, felspar and quartz. Knots of tourmaline
are common in these altered rocks. The types grouped as amphibolites
range from epidiorites to hornblende schists and amongst them may be
recognised all the familiar stages of saussuritisation and
recrystallisation of original tonalitic, dioritic and diabasic rocks.
It is probable, however, that amongst the amphibolites are included
rocks which were intruded at many different periods during the
metamorphism of the gneisses and that to difference of age as well as to
local intensification of the metamorphism is to be ascribed the variable
degree of construction which the rocks have severally undergone. The
chlorite and talc schists are to be regarded as basic intrusions which
had suffered considerable decomposition before being subjected to
metamorphism.
Amongst highly reconstructed rocks of doubtful derivation, associated
with the sedimentary gneisses, are a garnetiferous quartz enstatite
biotite granulite from Atuka near Okuruku in southern Kabba (841) and a
garnet granulite from the adjoining village of Kuturu (168) composed of
labradorite, quartz and garnet with a distinct centric structure. The
garnet is in spongy masses associated with a small quantity of green
pyroxene, sphene, apatite and iron ore. A pyroxene granulite, from
Againyi (823), consists of a porous intergrowth of pale green pyroxene
and green hornblende, the former predominating, in a matrix of clear
labradorite. Sphene and ilmenite are also present in irregular
intergrowths as well as a little accessory epidote and apatite. Another
variety (832) contains no amphibole and consists of pale green pyroxene
and garnet in a matrix of labradorite and quartz. A third type from the
same locality (821) is an epidote granulite consisting of predominant
epidote and garnet with subordinate green pyroxene, sphene and ilmenite
in a matrix of labradorite. A pyroxene granulite from Nassarawa province
(854) shows ragged crystals of diallage with quartz inclusions and
rounded garnets with inclusions of both quartz and diallage in a matrix
of coarsely granular quartz. A hornblende granulite from the same
province (859) consists of rounded spongy garnets and radiate groups of
actinolite fibres with a little accessory ilmenite in a typical fine-
grained quartzo-felspathic mosaic. A remarkable biotite granulite from
Eri in eastern Ilorin (1056) contains in addition to biotite and
granular garnet as the predominate ingredients much sillimanite,
staurolite and andalusite in a matrix of untwinned felspar and quartz.
The harder gneisses of the second group are subordinate in amount and
irregularly distributed throughout the micaceous and felspathic gneisses
of the first group. They consist of a series of much granitised medium
and even-grained biotitic and hornblendic gneisses which usually show a
lenticular or linear foliation but frequently also a ribboned or banded
and contorted character. In some cases the banding is broad and only to
be distinguished in the field, while hand specimens present quite a
homogeneous character. All variations may be found between acid aplitic
and basic hornblendic bands, and knots and lenticular masses of more
basic material are common within the more acid types. As a rule the
banding is much interrupted by pale coloured granitic material into
which the lighter coloured bands seem to pass imperceptibly. A striking
feature is the presence of a series of early pegmatite veins folded and
foliated with the gneisses. There is in fact every indication that these
banded rocks are of a primarily composite character and represent either
an originally sedimentary or a differentiated igneous succession invaded
and injected by acid granitic material and pegmatite veins in the deeper
portions of the crust and subsequently subjected to a later folding,
foliation and recrystallisation. The foliation is in general vertical
and parallel with the banding. Under the microscope the striated and
banded gneisses exhibit all the characteristic features of typical
archæan rocks (801, 846). They are composed of a granular aggregate of
quartz, felspar, hornblende or biotite with sphene, apatite and iron
ores as accessories. The felspars are usually allotriomorphic and vary
from labradorite in the more basic to orthoclase and microcline in the
more acid rocks. The biotite occurs in tabular flakes singly or in
aggregates with the basal plane well-developed but with ragged margins
in the prism zone, while the hornblende forms spongy growths or
irregular plates with embayed margins and inclusions of quartz. Strain
shadows are common in the quartz and felspar but only traces of
cataclastic granulisation or of secondary micropegmatite are as a rule
recognisable. Van Hise[33] is of opinion that the strain phenomena and
the vermicular quartz to be observed in these thoroughly reconstituted
gneisses are due to the partial lagging of reconstruction behind
deformation. It is equally probable, however, that the strain effects
and micropegmatitic structures have been induced after the period of
maximum reconstruction by later strains of lesser magnitude.
The mutual relations of the two groups of gneisses are difficult to
determine. The actual contact between the two has nowhere been found
clearly exposed and its character must therefore be to a certain extent
conjectural. The apparent irregular replacement of the softer gneisses
by members of the harder group when traversed across the strike and the
apparent extension into the softer gneisses of tongue-like masses from
the larger areas of harder rock is certainly suggestive of an intrusive
character as advocated by Mr. Parkinson[34] in Southern Nigeria and
Liberia. Upon this hypothesis the harder gneisses would appear to
correspond to an intimate mixture of granitic and dioritic material,
whose intrusion took place during the folding and reconstruction of the
sedimentary gneisses. It is difficult, however, to understand why these
rocks should present in their even-grained and closely foliated and
banded character such an entirely different aspect in the field from
that of even the earliest recognisable granitic intrusions within the
sedimentary gneisses and why they alone should possess an early set of
folded and foliated pegmatite veins in addition to the later pegmatites
which are common to both groups of gneisses. All the phenomena indeed
are more readily explicable on the assumption that the two groups differ
in age, that the sedimentary gneisses were originally deposited
unconformably upon an earlier and probably archæan gneissic or igneous
complex and that the latter series was refoliated and recrystallised
during the reconstruction of the former. The peculiar distribution of
the harder gneisses and their intimate alternation with the softer
series would then be due in part to the original unconformability
between the two groups and in part to the irregular character and the
varying amplitude of the later folding to which both were subjected. The
abrupt replacement of the sheared but little altered quartzites of the
Niger valley by the harder gneisses at Jebba, Wuru and Maibirro probably
indicates also a certain amount of faulting and thrusting accompanying
the folding. On account of the subordinate and ill-defined character of
the distribution of the harder rocks no attempt has been made to
distinguish upon the map between the archæan and the later sedimentary
gneisses.
PLATE IX
[Illustration: ON THE SUMMIT OF THE BAUCHI PLATEAU.
The Parade Ground at Bukuru.]
[Illustration: ON THE PLATEAU LOOKING WESTWARD FROM BUKURU.]
In the Sahara to the north, M. Chudeau[35] has described the occurrence
of archæan and sedimentary gneisses whose mutual relations are similar
to those which prevail in Northern Nigeria. The archæan areas are of
comparatively small extent, and are primarily distinguished, according
to M. Chudeau, by the presence of numerous domes and turtlebacks of
granite. This method of distinguishing between the two groups, however,
cannot be applied in Nigeria, where intrusive granites are equally
common amongst both the earlier and the later gneisses. The metamorphic
rocks of sedimentary origin attain a great development in the Sahara[36]
and include quartzites, phyllites, schists, gneisses and crystalline
limestone similar in type to those of Nigeria, and folded in a similar
complex manner along axes whose direction is predominantly meridional.
The decrease northward of the intensity of the metamorphism, as
advocated by M. Chudeau, requires much further investigation in view of
the local occurrence of little altered rocks within the Protectorate. M.
Chudeau believes in the original unconformability of the later gneisses
upon the archæan and points out, with M. Haug,[37] the analogy between
the system of folding of the gneisses of the Sahara and the Sudan, and
the Caledonian folding of the crystalline rocks of Scotland and
Scandinavia. He demonstrates the pre-Devonian age of the sedimentary
gneisses, and assigns them provisionally to the Silurian on the strength
of their supposed continuity with the graptolite-bearing shales of
Tindesset[38] and Hassi el Kheneg.[39] The stratigraphical evidence
available, however, seems rather to point to an unconformability between
the shales and the schists, and M. Flamand[40] prefers to assign a pre-
Cambrian age to the whole series of crystalline schists and gneisses.
In Dahomey, M. Hubert[41] has described a similar complex of archæan and
sedimentary gneisses in which the latter clearly predominate. His
separation as Silurian of the quartzites and schists of Atacora from the
other metamorphic rocks appears, however, to be unjustifiable.
Quartzites, gneisses and schists of sedimentary origin associated with
granites and archæan rocks, are found also in West Africa, in French
Guinea and the Fouta Djalon,[42] in Liberia,[43] the Ivory Coast,[44]
the Gold Coast,[45] Togoland,[46] and Southern Nigeria,[47] and in
Central Africa to the south and south-east of Chad,[48] in the
Kameruns,[49] and in the French Congo.[50] Similar rocks attain also a
remarkable development to the south of the equator.[51]
The comparison which M. Chudeau has drawn between the strike of the
gneisses and schists of the Sudan on the one hand, and of the Scottish
Highlands on the other, may be further extended not only to the
petrographical character of the constituent groups of the gneissic
complex in West Africa, but also to the respective relations of the
various groups to each other, and to the intrusive rocks which they
enclose.[52] Dr. Voit[53] has, moreover, arrived at similar conclusions
in the course of his investigation into the structure of the crystalline
complex of the northern Transvaal, the constitution and mutual
relationships of the Swaziland system and the older granites and
gneisses being closely paralleled in the rocks of Nigeria.
THE INTRUSIVE ROCKS.
Many of the thin bands of well-foliated augen and granitoid gneiss
enclosed within the metamorphic series undoubtedly represent original
granitic intrusions, but these are so intimately bound up with the
adjoining gneisses and schists, that no discrimination can yet be
attempted. Attention has been more particularly directed in the field to
those granitoid rocks which form the prominent inselberge and isolated
groups of hills, and amongst them it has been found possible to
distinguish an older from a younger series of intrusions. The older
intrusive rocks consist very largely of granite, while the younger
intrusions include granitic, syenitic and diabasic types with numerous
associated dyke rocks. To what extent the epidiorites, amphibolites, and
granulites of the gneissic series represent more basic intrusions
associated with the older granites is quite unknown.
(_a._) _The older Granites._
The older granites are never sharply marked off from the adjoining
gneisses, and invariably show a certain amount of foliation in the
field, and of cataclastic deformation and reconstruction under the
microscope. They appear to have been intruded at different times within
the period of the evolution of the gneisses, and the amount of
reconstruction which they themselves have undergone may be taken as a
general indication of their age. Vermicular micropegmatite is most
extensively developed in those schistose granites which have stopped
short of complete reconstruction and which still afford abundant
evidence of cataclastic deformation. It may also occur, however, in
small amount in thoroughly foliated and reconstructed granites, in which
it is assumed to have been developed, as in the case of the banded and
striated gneisses (p. 118), by minor crustal strains, after the period
of maximum reconstruction.
The most highly altered types form extensive lenticular or phacolitic
masses of well-foliated and fairly coarse-grained biotitic and
hornblendic gneisses (849, 481), containing streaks and patches of
darker-coloured and finer-grained basic material. Under the microscope
they possess a coarsely granular and thoroughly reconstructed character,
and in the mutual relations of their constituents present a considerable
resemblance to the banded gneisses of similar composition. The quartz
and felspar show much later straining with incipient granulisation and
traces of micropegmatite. These rocks have participated in all the
changes which the adjoining gneisses have undergone, and were apparently
intruded before the period of maximum reconstruction of the latter.
The least altered of the older granites (806, 1560) are typically
distinguished by the irregular batholitic character of the intrusions
and the partial and frequently only marginal character of the foliation.
They sometimes have a tendency to become finer-grained towards their
junction with the gneisses, and they usually enclose portions of the
adjoining rocks and vein and inject with granitic material the marginal
gneisses and schists. The inclusions may be of either the harder or
softer gneisses according to the character of the rocks which the
granite has invaded. In places a parallel arrangement of the felspars
appears to have been induced through streaming during crystallisation.
The partial foliation is parallel in a general way with that of the
surrounding gneisses and schists. The granites vary much in character
from biotitic and hornblendic rocks with syenitic modifications to
muscovite and microcline granites and aplites. The quartz and felspar
are strained throughout and cataclastic granulitisation is common on the
margins of the crystals. Vermicular quartz is most abundant in the
marginal and more distinctly foliated portions. These rocks would appear
to have been intruded after the period of maximum reconstruction of the
gneisses, but before the final cessation of stress and strain. In
places, as at Abuja, crush lines cross the granites, and in their
immediate neighbourhood the rocks show a higher degree of reconstruction
and a greater development of micropegmatite in the matrix.
These later members of the older granites appear to have produced little
or no contact metamorphism in the surrounding rocks. Brecciation and
injection structures on the other hand are very frequent. The hard
banded gneisses on the margin of the Abuja granite, for example, are
abundantly injected with quartz and quartzo-felspathic material.
Similarly at Zwoll, in Bauchi province, a porphyritic biotite granite
has brecciated a series of banded and contorted biotite and hornblende
gneisses, and many of the enclosed fragments are partially absorbed or
minutely injected with granitic material. The Okuruku granite in
southern Kabba, on the other hand, has invaded and brecciated members of
both the harder gneisses and the softer series of biotite schists,
magnetite schists and amphibolites. In the neighbourhood of the
inclusions the granite becomes knotted and richer in biotite,
hornblende, sphene and other minerals, while the inclusions themselves
are irregularly veined and minutely penetrated by quartzo-felspathic
material. In every case it would appear that the rocks, which the
granites have brecciated and invaded, possessed their gneissic or
schistose character before the actual intrusion of the granites, and
that they, as well as the larger inclusions, suffered little further
change as the result of the stresses and strains which brought about the
partial foliation of the granites. The gneisses as a whole show strain
phenomena, but only traces of granulitisation and of secondary formation
of micropegmatite, while the granites show extensive marginal formation
of micropegmatite, together with intense cataclastic deformation. It
would seem indeed that these granites were intruded after the period of
maximum reconstruction of the gneisses, but before the final cessation
of stress and strain, and that while the gneisses which had originated
under conditions of maximum strain were comparatively stable under
similar strains of diminishing intensity, the granites which had
crystallised from igneous fusion were in a condition of unstable
equilibrium with respect to the stresses and strains to which they were
subjected after crystallisation. The gneisses, as it were, possessed a
certain amount of elasticity and transmitted the later strains with
little resultant deformation, while the rigid masses of granite arrested
those strains and suffered in consequence a partial deformation. The
production of a strain-slip foliation in some of the more micaceous beds
is probably to be referred to the same period of differential
deformation of the granites. These later strains became locally
intensified along crush lines, and in the case of the Abuja granite
produced granulitisation both of the granite and of the felspathised
gneisses in the neighbourhood of the crush line.
In addition to the two extreme types of the older granites there are
many elliptical masses of similar material, elongated in a direction
parallel with the strike of the gneisses, which show a decided foliation
throughout, sometimes more and sometimes less intense (48, 1223). As a
rule they are indistinctly marked off from the adjoining rocks and
preserve their normal texture up to the margin of the intrusion. The
granites are predominantly biotite granites, more rarely muscovite
biotite granites, and the felspars are frequently porphyritic. Under the
microscope they show a higher degree of reconstruction than the least
altered granites, but a less complete reconstitution than the most
highly altered types. The felspars vary from oligoclase to microcline,
and the plagioclases often show basic interiors and zoned margins. Both
the felspar and the quartz show abundant strain phenomena, and the
felspars are usually partially surrounded by a secondary fringe of
vermiculate micropegmatite in optically discontinuous grains. Frequently
also the felspars are corroded or irregularly penetrated by quartz or
micropegmatite, while the matrix consists largely of granular and
partially recrystallised felspar, muscovite, microcline and quartz. The
darker patches within these granites may be taken to represent portions
of the adjoining rocks caught up during intrusion and partially
absorbed, recrystallised and foliated with the granites. These rocks may
possibly be correlated in the period of their intrusion with the least
altered granites and regarded as the smaller and therefore more readily
reconstructed representatives of the larger partially foliated masses.
On the other hand, in the absence of the inclusions of adjoining
gneisses which form the characteristic feature of the least altered
granites, it is possible to regard them as of somewhat earlier origin
and as having participated to a considerable extent in the general
reconstruction of the gneisses themselves.
It seems probable indeed that the crustal movements which brought about
the metamorphism of the gneisses were of a somewhat spasmodic character
and spread over a very considerable interval of time and that the
intrusion of granitic and other igneous material took place at intervals
throughout the whole period. No hard and fast line can therefore be
drawn between the highly reconstructed and gneissose granites and the
imperfectly or only partially foliated types, a complete series of
intermediate varieties being apparently possible between the two extreme
members of the series.
(_b_) _The Younger Intrusions._
1. Granites and pegmatites.
Amongst the later granites, which present for the most part a batholitic
habit, two types may be distinguished, differing in mineralogical
composition and probably also to some extent in age. The rocks of the
first type never show chilled margins, and were evidently intruded while
the adjoining gneisses still possessed a comparatively high temperature.
They include non-foliated aplitic granites and pegmatites usually rich
in muscovite and tourmaline. The second type includes riebeckite and
biotite granites which are sharply marked off from the adjoining rocks
by felsitic and fine-grained margins. The rocks of the first type
usually exhibit under the microscope a faint straining of the quartz and
felspar and were probably intruded before the final cessation of the
crustal movements to which the reconstruction of the gneisses and the
foliation of the earlier granites were due. The members of the second
type, on the other hand, are practically free from cataclastic phenomena
and evidently assumed their present position after the cessation of
movement within the crust. There is no reason, however, to assume any
great difference in age between the two types or any marked
discontinuity in magmatic conditions. The same magma, of a somewhat acid
and alkaline character, was probably present in the depths throughout
the whole period of intrusion, and the varying character of the products
may be taken as affording some clue to the nature of the differentiation
within the magma itself.
The alkaline granites of the first type are pale-coloured granular rocks
composed almost entirely of quartz, microcline, orthoclase, perthite,
and acid plagioclase. Biotite and other ferro-magnesian minerals are
comparatively rare, while muscovite and tourmaline are frequently
abundant and in large primary crystals. The quartz and felspar have a
tendency to pegmatitic intergrowth, and sometimes masses of graphic
pegmatite occur porphyritically in a normal quartzo-felspathic matrix.
The tourmaline and muscovite may occur also as at Igbo in two
generations, in large crystals and in the ground mass. Frequently also
these granites possess a tendency to a coarsely banded structure, the
banding being due either to variation in size of grain of the quartz and
felspar or to variation in the abundance of tourmaline and muscovite in
successive bands. These granites rarely form intrusions of any size, the
largest known being the tourmaline granites of Igbo and Darroro, but
even they are insignificant in comparison with the alkaline granites of
the second type.
The pegmatite dykes and veins which are so abundant throughout the
gneissic series evidently belong to the same period of intrusion. Like
the aplitic and tourmaline-bearing granites they never exhibit fine-
grained margins at their junction with the gneisses. They vary much in
composition, and include graphic granites, quartz-felspar pegmatites
with biotite, magnetite or ilmenite, quartz-felspar pegmatites with
muscovite or tourmaline or both, and quartz-muscovite pegmatites with or
without tourmaline. At Eri there are stanniferous pegmatites carrying
quartz, muscovite and cassiterite, but no tourmaline. The massive
quartz-reefs which frequently enclose portions of the surrounding
schists and carry tourmaline, pyrites and sometimes gold, are also to be
referred to the same period of intrusion. Inseparable from them and from
the pegmatites are the smaller veins of quartz and tourmaline quartz and
the schorl rocks of a foliated or striated character which occur as a
rule in association with the veins of tourmaline quartz.[54] The schorl
rocks with a gneissic habit in which the quartz is minutely granulitic
and the tourmaline is in small crystals evidently recrystallised and
full of quartz inclusions, are probably altered gneisses and schistose
granites in which the felspars have been replaced by quartz and
tourmaline. Traces of decomposed felspathic material are to be found in
every specimen, but no pseudomorphs in tourmaline of felspar or of other
minerals have been definitely recognised. The striated schorl rocks in
which the darker bands are formed of finely fibrous tourmaline, appear
to arise also as modifications of the adjoining rocks in the vicinity of
the tourmaline quartz veins. Sometimes a foliated rock may be traced
through a striated type into a massive tourmaline rock on the margin of
the vein in which the original structures have entirely disappeared, and
in which the original minerals, including the quartz, have been entirely
replaced by finely fibrous tourmaline. In the quartz veins, pegmatites
and granites, on the other hand, the tourmaline is in well-formed
crystals of primary origin. It would seem, therefore, that the
pneumatolytic or metasomatic formation of the schorl rocks was brought
about by emanations from the quartz veins and pegmatites before the
final consolidation of the latter. The extensive development of
tourmaline and pyrites in the quartzites and schists of certain areas is
also to be ascribed to the circulation of mineralising vapours and
solutions throughout the adjoining rocks during the same period. The
remarkable absence of tinstone in the tourmaline pegmatites, together
with its appearance in pegmatites at Eri which do not carry tourmaline,
seems to point to the stanniferous pegmatites having originated at a
somewhat later date than the normal tourmaline-bearing pegmatites. This
suggestion is supported by the occurrence of tinstone in association
with the soda granites of Bauchi province.
The alkaline granites of the second type are typically distinguished by
possessing fine-grained porphyritic and felsitic margins which indicate
a considerable difference in temperature between the granites and the
surrounding gneisses at the period of intrusion. To this group are
referred the biotite granite of the Anagoda hills in S. Nassarawa, the
riebeckite granite of the Gurkawa Hills in northern Muri, the biotite
granite of Bukuru and Ngell, and the riebeckite granites of the
Kwandokaya Hills and of the Ningi-Burra massif in Bauchi province, the
riebeckite granite of Fagam, Gadama and Galambi on the borders of Bauchi
and Kano, the riebeckite granite of Shira in the sub-province of
Katagum, and probably also the Kailema granite near Jebba and the
biotite granite of Abara near Ngashki in western Kontagora. The alkaline
character of the original magma is reflected in the nature both of the
felspars and of the ferro-magnesians. The felspars are much kaolinised
and consist chiefly of tabular crystals of untwinned orthoclase probably
rich in soda, and of streaky perthitic intergrowths of orthoclase and
albite. Where enclosed in quartz, the terminations of the crystals are
frequently idiomorphic. Microlites of riebeckite are common inclusions
and small plagioclases of an earlier growth are frequently enclosed in
the later felspars and in the quartz. The quartz forms aggregates of
irregular grains showing faint strain shadows and has a great tendency
to form coarsely pegmatitic intergrowths with the felspar. Where biotite
alone is present, it is of a normal greenish-brown colour. Riebeckite,
however, is the most characteristic ferro-magnesian, and occurs for the
most part in irregular plates moulded on the felspars and sometimes
enclosing small crystals of felspar in poikilitic fashion. Occasionally,
as in the Shira granite, the riebeckite builds elongated crystals more
or less idiomorphic in the prism zone and frequently intergrown with
ægirine. Crystals of riebeckite over an inch in length are common in
knots in the Kila granite, while on the other hand the margin of the
Gurkawa granite is stippled with minute porous crystals of the same
mineral. Associated with the riebeckite in places are other blue and
brown soda amphiboles[55] as well as green ægirine and a golden-coloured
mica, while zircon, apatite and iron ores are constant but scanty
accessories. Pyrites is sometimes abundant in the porphyritic and
felsitic marginal rocks.
A remarkable feature of some of these later granites is the occurrence
in them of tinstone and numerous sulphide ores. It would appear that in
places these later granites have been broken and fissured, and that
along these fissures vapours and solutions have risen which have
chloritised and mineralised the adjoining rock and in part refilled the
fissures with ore-bearing pegmatite and vein stuff. In the Bukuru
granite the cassiterite is associated in the neighbourhood of Ngell with
pyrites, chalcopyrite, tetrahedrite, blende and galena. Topaz and brown
zircon are found accompanying the cassiterite in the tin-bearing
alluvium at Naraguta and elsewhere, and are probably also of
pneumatolytic origin. The granites of the Kwandokaya hills, of the
Jengre and Limoro hills, of the Ningi-Burra massif, and of the Gadama
and Fagam hills also yield in places a tin-bearing drift, and have
probably suffered, like the granite of Bukuru, a local fissuring and
mineralisation. Tourmaline has not been definitely recognised as a
product of these later emanations.
Similar soda granites with microgranitic and felsitic margins have been
described by MM. Foureau, Gentil and Lacroix[56] from Zinder, by M.
Chudeau[57] from Mounio and Goure, by M. Garde[58] from Machina and by
M. Hubert[59] from Dahomey, while Captain Freydenberg[60] has noted the
occurrence of a soda syenite with granitic modifications at Melfi to the
south-east of Chad.
PLATE X
[Illustration: A WORN VOLCANIC HILL ON THE BAUCHI PLATEAU NEAR ZAMAGAN.]
[Illustration: THE TWIN CONES OF KERENG FROM PANYAM]
2. _Syenites and Diorites_
These rocks, which are by no means abundant, occur as small bosses or
intrusive masses within the gneisses and schists. As a rule their
junction with the surrounding rocks is not exposed, and their
relationship to the later granites has not been definitely ascertained.
They should probably be regarded, however, as somewhat basic
modifications or differentiation products of the later granitic magma.
Slight strain effects are usually apparent in the quartz and felspar.
The low hills around the town of Bauchi are composed of a coarse-grained
augite syenite, in which orthoclase felspar predominates in large
twinned crystals with a peculiar brownish tint and a resinous lustre.
Under the microscope (49) subordinate oligoclase is present in smaller
crystals, while the knots of darker minerals are composed of
intergrowths of violet-coloured augite, green hornblende, brown biotite
and iron ores. Quartz is interstitial and occurs also in secondary
vermicular growths on the margins of the felspars. The accessories are
zircon and apatite, and the whole rock is much stained with secondary
limonite. A rock of similar appearance from Kende to the east of Bauchi
(69) is somewhat more basic and may be termed a quartz augite-diorite.
The predominant felspar is a basic andesine, while the ferro-magnesians
are pale-coloured augite and green hornblende in irregular plates and
intergrowths. The quartz is interstitial or intergrown with the felspar
in pegmatitic fashion. A similar rock from Kanna (1677) contains much
biotite in addition to the augite and hornblende. The rock of Kogon
Dutsi (1338) within the walls of Kano is a basic augite diorite without
quartz, consisting predominantly of acid labradorite and pale-coloured
augite. Both the augite and felspar have a tendency to schillerisation
and the augite is accompanied by, and in part intergrown with, green
hornblende, brown biotite and iron ores, while hypersthene is a scanty
accessory. An augite-diorite from eastern Kontagora (1302) contains a
little interstitial quartz and felspars varying from oligoclase to acid
labradorite. The ferro-magnesian minerals are much intergrown and
include pale-coloured augite, green hornblende and brown biotite.
3. _Porphyries, Porphyrites, Diabases and Basalts._
The later sills and dyke rocks are most abundant in the central
provinces and especially in Bauchi, eastern Zaria and northern
Nassarawa, but are somewhat sparingly distributed throughout the
remainder of the Protectorate. They include quartz porphyries,
orthoclase porphyries, mica and hornblende porphyrites, diabases and
basalts. In the central area the high content in soda which
characterises the more acid types undoubtedly indicates a genetic
connection between the later soda granites and the dyke rocks, although
investigation has not yet proceeded far enough to allow of the
recognition of the major foci to which the dyke rocks may be referred.
Amongst the quartz porphyries micro-granitic, granophyric and
felsophyric types are equally common. In the central provinces
riebeckite is frequent both among the phenocrysts and in the ground
mass, while elsewhere biotite is the commonest coloured constituent. An
ægirine granophyre occurs at Chikobo (1417) on the borders of Bauchi and
Zaria. Orthoclase porphyries are commonest in the central area and
possess typically a trachytic ground mass, of which riebeckite and
ægirine are frequent constituents. Augite syenite porphyries are known
from the Kudaru Hills in eastern Zaria (1397) and from the vicinity of
Limoro on the borders of Bauchi and Zaria (1413). The latter occurrence
contains hornblende and biotite in addition to the augite, as well as a
small amount of quartz in the ground mass. An extensive mass of
granophyric diorite porphyrite occurs in western Kontagora between Auna
and Bussa (1162). The felspars are turbid and much epidotised and
surrounded by a fringe of more acid material which passes outwards into
the granophyric matrix, while smaller irregular crystals of green augite
and ilmenite and abundant minute plates of green mica are also enclosed
within the pegmatitic material. A peculiar rock from Toro in Bauchi
province (958) may be termed a quartz diorite porphyrite, and presents a
certain affinity with the basic members of the charnockitic series of
the Ivory Coast.[61] The phenocrysts have a tendency to aggregate in
groups and include rounded quartz grains, tabular crystals of andesine
and labradorite with inclusions of granular augite, elongated crystals
of hypersthene and irregular plates of biotite much corroded by the
ground mass and enclosing grains of augite and apatite. The ground mass
is composed of granular green augite, felspar and quartz, with much
magnetite and apatite. The felspar of the ground mass is frequently
untwinned and more acid than the phenocrysts, while the quartz forms in
places micropoikilitic plates enclosing the other ingredients of the
ground mass. As alkali-syenite porphyries may be grouped the bluish-grey
trachytoid rocks of Kanikoko (1217–1219). The phenocrysts include
tabular plates of sanidine, microcline, microperthite and microcline-
microperthite and fairly well-formed crystals of diopside and enstatite,
much intergrown with brown mica and bluish amphibole. The ground mass
consists of microlites of alkali felspar and granules of the ferro-
magnesian minerals. Typical mica porphyrites with phenocrysts of biotite
and zoned andesines occur at Barazara (1274), and between Dan Gerumfa
and Gwashi (1276) in eastern Kontagora. A hornblende porphyrite between
Naraguta and Tilde (956) is inconspicuously porphyritic and composed of
allotriomorphic green hornblende with subordinate biotite and augite in
a matrix of acid plagioclase, orthoclase and quartz. A hornblende
porphyrite from Borgu (1220) contains large poikilitic crystals of
augite and hornblende in a granular matrix of labradorite, augite,
hypersthene, hornblende and iron ore. A somewhat similar rock from
Bauchi province (1602) contains large poikilitic hornblende crystals in
a matrix of granular augite, magnetite and lath-shaped crystals of
labradorite. A labradorite porphyrite from Toro in Bauchi province
contains large tabular phenocrysts of acid labradorite in a ground mass
of green augite, brown biotite, secondary green hornblende and lath-
shaped crystals of acid plagioclase. The diabases and basalts of the
central area are particularly numerous round the margin of the Bauchi
plateau and the Nassarawa tableland, and probably include intrusions of
many different ages. Some of the coarser-grained diabases present
affinities with gabbros in the idomorphism of their augite crystals (47,
1695). Others exhibit a typical ophitic structure (962, 985), while
others are predominantly granulitic (690). A hornblende diabase with
pale green augite, greenish-brown hornblende and deep red biotite occurs
in the Ruruma hills (1466) in eastern Zaria. Biotite also accompanies
the augite in an ophitic diabase from the Kudaru hills (1390) in north-
east Zaria. Uralite diabases are fairly common, but olivine diabases
(747) are conspicuously rare. The basalts present few features of
interest and are typically compact, non-porphyritic rocks with a
holocrystalline, ophitic or granulitic structure and free from olivine.