BPSC 69th CCE Prelims General Studies · Question 91 of 144

Correct — A, Both A and R are true and R is the correct explanation of A. The stem is lifted almost word for word from NCERT Class XI, Fundamentals of Physical Geography, Chapter 4, where the placer-deposit evidence for continental drift reads in full: 'The occurrence of rich placer deposits of gold in the Ghana coast and the absolute absence of source rock in the region is an amazing fact. The gold bearing veins are in Brazil and it is obvious that the gold deposits of the Ghana are derived from the Brazil plateau when the two continents lay side by side.' The examiner has simply cut that sentence in two and labelled the halves A and R, so both halves of the assertion are true as the standard text states them. The reason is true as well: Africa and South America were both part of Gondwanaland, the southern component of the supercontinent Pangaea, which the same chapter says began to split about 200 million years ago. What decides between (a) and (b) is whether the reason does explanatory work, and here it does all of it. Placer gold is, by definition, gold that has been weathered out of a source rock and re-concentrated by moving water, so a rich placer with no source rock behind it is not a curiosity but a contradiction — the metal has to have come from somewhere. The chapter's answer is that the somewhere is the vein country of the Brazilian plateau, which lay against the Ghana coast before an ocean opened between them. Take the joined-continents fact away and the assertion has no explanation at all; put it back and the anomaly dissolves. That is exactly the relationship option (a) describes. The paragraph immediately before it in the same chapter supplies an independent check on the same pairing: 'The belt of ancient rocks of 2,000 million years from Brazil coast matches with those from western Africa. The earliest marine deposits along the coastline of South America and Africa are of the Jurassic age.' Those two-billion-year-old rocks are the same Palaeoproterozoic province that carries West Africa's gold, so the drift explanation for the gold is not an isolated assertion but a special case of a match already established on rock ages — which is why the textbook, and UPSC after it, treat it as evidence rather than coincidence.

• (b)Both A and R are true but R is not the correct explanation of A — The most tempting wrong option, and the one that catches candidates who read the assertion as two separate observations sitting side by side rather than as one puzzle. Choosing (b) means claiming that the joined continents are a true fact which happens to be irrelevant to Ghana's placer gold — but the assertion is not 'Ghana has gold and Brazil has gold'. It is that Ghana's gold arrived without a local source while the veins that could have supplied it lie across an ocean, and the only thing that closes that gap is the former adjacency. In an assertion–reason item, ask what work the reason is doing: if deleting R leaves A unexplained, R is the explanation. Here deleting R leaves A as an unexplained anomaly, which is precisely why the textbook lists it under evidence for continental drift.
• (c)A is false but R is true — Both halves of A are stated as fact in the standard text — rich placer gold on the Ghana coast, gold-bearing veins in Brazil — and both are independently true of the real world. The Gold Coast took its European name from the metal, is estimated to have supplied about two-thirds of Africa's gold production between the late fifteenth and the mid-nineteenth century, and has yielded on the order of 1,500 tonnes of gold across its history; Brazil's plateau gold sustained a colonial gold rush of its own. Nothing in A is denied by anyone. A candidate reaches this option only by assuming the observation must be wrong because the conclusion sounds improbable, which inverts the logic of an assertion–reason item — you test the statements first and the explanation second, never the other way round.
• (d)A is true but R is false — This is the trap for a half-remembered fact: that Wegener's theory was 'rejected'. What his contemporaries rejected was his proposed mechanism — the pole-fleeing force arising from the earth's equatorial bulge and the tidal force of the moon and sun — which NCERT records 'most of scholars considered … to be totally inadequate'. The drift itself was never falsified. Arthur Holmes offered mantle convection currents in the 1930s, post-war mapping of the ocean floor produced Hess's sea-floor spreading in 1961, and McKenzie and Parker, and independently Morgan, assembled plate tectonics in 1967, which absorbed drift as one of its consequences. So the mechanism changed and the claim survived: Africa and South America really were joined, and R is true.

Alfred Wegener, a German meteorologist, set out the continental drift theory in 1912, though Abraham Ortelius had noticed the Atlantic fit as early as 1596 and Antonio Pellegrini had drawn the three continents together. Wegener held that all land once formed a single mass, Pangaea — 'all earth' — surrounded by a single ocean, Panthalassa, and that about 200 million years ago Pangaea split into Laurasia in the north and Gondwanaland in the south, which broke up further into today's continents. He offered five main lines of evidence, and it is worth knowing them as a set because examiners draw from all five: the jig-saw fit of the Atlantic coasts, confirmed by Bullard's computer best-fit in 1964; rocks of the same age across the ocean; glacial tillite at the base of the Gondwana sequence; placer deposits; and the distribution of fossils. This question tests the fourth. Note what kind of evidence it is. It is not a resemblance between two shapes, which could be coincidence, but a mismatch between a mineral and its surroundings — an effect with no available cause — which only makes sense if the cause has since moved away. Evidence of that form is much harder to explain away than a coastline that happens to fit.

An assertion–reason item is decided in three steps, and the third is where the marks are lost. Is A true? Yes — it is the textbook's own sentence. Is R true? Yes — Africa and South America were adjacent within Gondwanaland. Does R explain A? Here you must ask what the assertion is actually asserting. It is not 'Ghana has gold'; it is that Ghana's gold is placer gold with no local source while the veins that could have produced it lie across an ocean in Brazil. That gap between the placer and its source is the anomaly, and only the former adjacency of the two coasts closes it. So R is the explanation and the answer is (a), not (b). The single discriminating idea is the difference between placer and lode: placer gold is transported and re-concentrated, so it must have come from somewhere, and if it did not come from nearby, it came from somewhere that is no longer nearby. One honest caveat, for the candidate who knows more geology than the chapter does. The weak clause in the textbook sentence is 'the absolute absence of source rock'. Ghana in fact has abundant source rock of its own: its major primary gold lodes sit in the shear zone between the Lower Birimian phyllites and the Upper Birimian greenstones as quartz veins and lenticular reefs, and the quartz-pebble conglomerates of the Tarkwaian Group are themselves ancient placers reworked from that belt. What is not in dispute — and what R actually claims — is that the gold-bearing Palaeoproterozoic rocks of West Africa and their counterparts in eastern South America were one province before the Atlantic opened. That is the stronger form of the same evidence, it is what the chapter's own 'rocks of 2,000 million years' paragraph records, and it leaves (a) correct. Answer this item on the textbook's formulation; the deeper geology reinforces the answer rather than upsetting it.

• NCERT Class XI, Fundamentals of Physical Geography, Ch. 4, verbatim: 'The gold bearing veins are in Brazil and it is obvious that the gold deposits of the Ghana are derived from the Brazil plateau when the two continents lay side by side' — note the text reads 'the Ghana', not 'the Ghana coast', in the second sentence
• Wegener published the continental drift theory in 1912; Pangaea ('all earth') was surrounded by Panthalassa ('all water') and began to split about 200 million years ago into Laurasia and Gondwanaland
• The independent match across the same ocean, from the paragraph before: 'The belt of ancient rocks of 2,000 million years from Brazil coast matches with those from western Africa,' and the earliest marine deposits on both coastlines are of Jurassic age — dating the ocean between them
• The jig-saw fit was confirmed by Bullard's computer best-fit of the Atlantic margin in 1964, tried at the 1,000-fathom line rather than at the present shoreline
• Fossil evidence for the same joining: Mesosaurus, a small reptile of shallow brackish water, is found only in the Southern Cape province of South Africa and the Iraver formations of Brazil — localities now 4,800 km apart across an ocean
• Tillite evidence: the Gondwana system of sediments from India has counterparts in six Southern Hemisphere landmasses — Africa, the Falkland Islands, Madagascar, Antarctica and Australia — with thick glacial tillite at the base
• Wegener's proposed forces, the pole-fleeing force from the earth's rotation and the tidal force of the moon and sun, were judged 'totally inadequate'; Arthur Holmes offered mantle convection in the 1930s, Hess proposed sea-floor spreading in 1961, and McKenzie and Parker, and separately Morgan, framed plate tectonics in 1967
• The real geology behind the assertion: West Africa's gold sits in the Palaeoproterozoic Birimian Supergroup of the West African Craton's Man Shield, folded during the Eburnean orogeny 2.2–2.0 billion years ago; Ghana's major primary lodes are quartz veins and lenticular reefs in the shear zone between Lower Birimian phyllites and Upper Birimian greenstones, and the Tarkwaian Group's quartz-pebble conglomerates are reworked ancient placers from the same belt
• Scale of the deposit the assertion is about: roughly two-thirds of Africa's gold production between the late fifteenth and the mid-nineteenth century came from the Gold Coast, and Ghana has produced on the order of 1,500 tonnes of gold in its history

• Marking (b) because both statements read as independently true — an assertion–reason item asks whether R accounts for A, and here the whole force of A is that its gold has no local source
• Marking (d) on a memory that 'continental drift was rejected' — what was rejected was Wegener's mechanism, not the drift
• Confusing placer gold, which is transported and re-deposited, with lode or vein gold, which sits where it formed — the distinction is the entire logic of this evidence
• Over-correcting from real geology. Ghana does have its own Birimian source rock, so the textbook's 'absolute absence' is loose; but the correlation of West Africa's gold-bearing Palaeoproterozoic belt with South America's is exactly what the Reason claims, so the answer stays (a)

BPSC prefers the assertion–reason shell, which is why the '(A)' inside this stem is a label and not an option, and it draws the content straight from the NCERT sentence without paraphrasing — so the candidate who has read the chapter recognises the wording before finishing the stem. UPSC uses the same material but as a multi-statement item: in 2025 it set the Brazil–West Africa rock belt, the Ghana gold and the Gondwana tillite together and asked which of the three are evidence of continental drift, with all three correct. The practical consequence is the same for both papers — learn the five NCERT evidences as a numbered list with their named details (Bullard 1964, Mesosaurus, the 2,000-million-year belt, the six tillite landmasses), because between them the two Commissions have now used four of the five.

• practice — not a real PYQ

  Which of the following was NOT among the forces suggested by Alfred Wegener for the drifting of the continents?

  • (a)Pole-fleeing force arising from the rotation of the earth
  • (b)Tidal force due to the attraction of the moon and the sun
  • (c)Convection currents in the mantle
  • (d)Both the pole-fleeing force and the tidal force

  Answer(c) Convection currents in the mantle — that explanation was offered by Arthur Holmes in the 1930s. Wegener proposed only the pole-fleeing and tidal forces, which most scholars found totally inadequate.

• practice — not a real PYQ

  The fossil of Mesosaurus, a small reptile adapted to shallow brackish water, is cited as evidence of continental drift because its skeletons are found only in

  • (a)India and Madagascar
  • (b)the Southern Cape province of South Africa and the Iraver formations of Brazil
  • (c)Australia and Antarctica
  • (d)the Falkland Islands and western Africa

  Answer(b) the Southern Cape province of South Africa and the Iraver formations of Brazil — two localities now about 4,800 km apart with an ocean between them, which a brackish-water reptile could not have crossed.