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271
MADAGASCAR


I. The Archean Region)-This region, nearly coincident with the mountainous upper portion of the island, is chiefly composed of the following crystalline rocks: gneiss, which is the most common of them all, quartzite and quartz-schist, with occasional beds of crystalline limestone and mica-schist, although this latter rock is very rare. The gneiss is mostly grey, but occasionally pinkish, its essential constituents (felspar and quartz) being almost always associated with dark mica (biotite) and hornblende in variable quantity. The to S.S.W.), but in its western portion the strike is frequently from N.N.W. to S.S.E. In both cases the strike of the rocks is coincident with the direction of several large valleys, which mark huge faults in the crystalline rocks. Almost the whole of this region is covered by a red soil, often of great thickness, which resembles and is often described as “ clay, ” but is really decomposed rock, chiefly gneiss, reddened with oxidized magnetite.

II. The Sedimentary Region.-The sedimentary rocks extend conrock is therefore a hornblende-granitite-gneiss. Granittfmore tmuously along the western side of Madagascar, following the coast- line; in the north these series of strata are only

A 44° B 48° C from 20 to 30 m. across, but farther south they We mate I- ' reach 'a breadth of nearly 100 m., while opposite Md b the Betsileo province they extend nearly half across fiery the island. A narrow band, of Cretaceous age, occurs also on the east coast, for about 120 m., between Vatomandry and Mananjary. Thefollow-1 I ing formations arelrepresented:-

1 Rai/way# Primary. It is thought 'that certain beds of - slaty rocks, véhich have been rvfccfqgnized at different T laces, ma e on to some o the rimar strata.

M-rsmilisi V G, -gat Camaro Il" G (G'°'f§ '°) gome siliceous schgsts of the Permian age iii/ere dis-M'f°“i n4“ 500% c Amb, ° covered in 1908 in the valley of the Sakameira, 12 Pom e°;(§ >¢3 '*~Y.s, ;“'»é'j-6, , S Lu '=° south of the Onilahy, or Augustine river. (S V M0h;';7'¢ b ', , f, § iA/ljuanf./¢>1w¢»fm) "°'0;, . '§ ?>;l§ i$J¢"§§ f coast). These contain reptilian remains, and also (lohil/a) £2320 8 Znmbum /. 3a>£'f


imprints of leaves of the Glossoptefis indica,

as well as other indications of an ancient vegetation. In the same region conglomerates have been

and retinite) appear at various points of the two

t, .2, = 4

~ll';l:é' rf "°"“°“ found containing enormous blocks, apparently j, ', I, s““<.; @~7§ ?§ ;i;. brought b lacial action, and said to be identical A . Y g . . . . .

2 "° °““""“°" “am;, .:~%grms=p§ ~-Q§ t*, . °'““"5“'° 2 in character with those described as existing 1n the 1-»»¢/:mm Q . .

4f., ,” 4, ,, , ., ., »=f, g¢er~, €, s-~, . ~s=~ .».., ,, ,, ., ., ., . Transvaal. True coal has also been obtained in gf <- “(1, }[* a;» Donny;, . . .

Wax? , ,, ,, ,, , m, the same district, the deposits varying from a
 11.4% a, ,, °    third to half a metre in thickness.
'
>°;fr'; "'2%;SB Q: 0 "'4~°." s- .;;§ ., Q“'“""“ 2. Secondary. The lowest members of these rest

EAST AFRICA Q ” cr "'f¢, ,%', ; U ;'1f;C°”' directly upon the central mass of crystalline rocks, ¢»»;f», ¢€ »ff¢, , o ¢, , M§ ; I) »;:a '>fg;§ § ;~;»>i§§ ~ °m "ww and consist of sandstones, conglomerates and Q vu., , '?, ,“~2 3:, j-~»r ;, tfM mf§ we shales, which have been supposed by some to 6 l”'i°- °?1° u -nate €f 9'53 x= r='?».¢-Mme/H belon to the Trias, without, however, the dis-0..§ t.Andre L;(, ,, ,W .U 1;, = . . DL . 3 ~, » 9 Q', ,, L, ,, ,, ,3 ';, ; ¢~ ~ ~§ , @ " '), ¢i i§ mwg21;>-2: =mv%9 covery of any fossil necessary to confirm this M m m 5 5531 ' supposition, except some silxcihed trunks of trees. Q i, =v§§ 'f1=-1~°= if “if /~S¢°~”“'f@ These beds are most probably lower members of J nn 11 Q . .., ;, ,, s °, ¢ Q -., ~"%».§ .¢., t, x, — Ambnaifsunrné . . a). M Aw %, ,§ ”~~§§ ; 1 =, f. ., :' A2;~'°5; ;;' <'ff=pz;4§ , ,.m9.'g . the Jurassic series. Westward of and above these Pi '==.' . 2” ' 1 ., *'“ '~ w F“"°°"'° strata the Middle and U er urassic formations ll /».~, , q , nt, /iff it F 1 mu » P

cwmn it;—is. wail?-2 . al "nf, °"”° are found (Lias, Lower Ool1te, 'Oxford1an, &c.), 3 unn¢1fm@f% Q; 3 with well-marked and numerous fossils (Ammo-Bd » '§ .§ .=', iz5 -, ~@§ , ;§ >;;§ ,5, ;s., ;;;;, i.7“§ v§ “'°“'”°“' nztes, Nennaea, Natzca, Astarte, Rhynchonella, rr¢nls.~ 5 ”~>>'»f e, = ~ e J ' at—' ' "~=.

>. .1 < . f 'gé § Q>»¢ /=a»~g.=/am Echznodermata, &c.); then the Cretaceous rocks, Q) smunim -, ' lL~f*'f', €- 1' . mm: - -¢,

m m'° A, ;w J @¢r3§ .§ % »j.{pt;, »vq#; . M d vo ° both these and the ]urass1c series being largely a M#-">4f"~°, ,&¢j;, § &<Y'°§ ,5p, , u'»§ f Lq¢.2;$§ 1, .. ~A"'4'f, ;§§ g;5 V mmm developed, the Cretaceous fossils including Nau°/»}»'£ef Y tzlus, Belemnfttes, Ostrea, Grypltaea, &c., and some Q. Tsirginu kilns .. jf very large Amrnomtes (Pachydzscus). The Second"""'“°'“°“ M...;|, ;..§ if-?, ., ., ¢*, ,§ , :, ,§ '3a@fg§ °';%;, §§ &qM.h, ,, ., .° Q ., ary strata show generally a very slight dip west° °' M, ,, ,, ., , f$'2 . "'It,5;¢ T '2° Wards and are consequently almost horizontal.

=¥fZf'°e£» ' <.“'H., ..;.1, = i- 'ii ' § "§ . They do not seem to have been greatly disturbed,

° new '% V F<f1°»f|v° although faults occur here and there. E } ~¢, Y¢f§ ';i, g4-3 4-b¢£ii ”', ,~.., ' Q/1 { if wma E 3. Tertzary. A small strip of coast of Eocene " '1' > ~ -, "T-"' ' '

is known near Tullear S.W. coast and rocks

Nammm of the same period occur in Nossi-bé, at Mahajamba Bay, and at Diego-Suarez, with Nummulites- and other foramimfera. Near the latter locality, beds of Oligocene age have been noticed, consisting of coarse limestones. 4. Quaternary and Recent. A narrow band of these deposits extends along the west coast, from north of Cape St Andrew nearly to the extreme southern point of the island. But the most notice-Z; able of these are those in the ancient bed of the Alaotra Lake, which formerly extended far south-Lfslmammpctsolsa

Wards along the valley of the Mangoro; also those in the marshes of Antsirabe and of Ifanja, in the Ikopa valley (the great rice plain west of the Mauritius °u, § /':Zg§ h=»- capital), and also in the plain of Tsiémmparihy in Bétsiléo, and especially the recent deposits of

wig" Ampasambazimba, north-west of Lake Itasy, discs

Mmimrg d . Th b d . h . .

covere in 1902. ese e s, ric in sub-fossil

remains, have yielded important additions to our ical

knowledge of the extinct fauna of the island.

A Long East of 44 Greenwich 5 9.° euman fr') ° -. . .

f 5 L1te, diorxte, gabbro, porphyry, porphyrlte, norite Emnrvwnlkzv

frequently granitite-occurs in several places, as well as pyroxenegranulite, serpentine, argillate, &c.; and gold is found widely disseminated, as well as other metals, but these latter, as far as at present known, except iron, are not abundant. The general strike of the rocks is the same as that of the trend of the island itself (N.N.E. 1 In the apparent absence of any Cambrian formation above them, there is little doubt that these rocks are Archean, although this cannot be absolutely proved.

previously described regions. In the Archean region the gneiss is very often found passing into granite, but certain granitic masses have a sufficiently distinct character. In the midst of the sedimentary region are two well-recognized masses of plutonic rocks, belonging to the syenites, sometimes quartziferous in structure. (2) Volcanic rocks.-Recent volcanic eruptive rocks (including rhyolite, trachyte, phonolite, andesite and basalt) have been examined at a number of points throughout both the geological regions of the island. In