PrepTest 127, Section 2, Question 8
This boulder is volcanic in origin and yet(39 more words not shown)
Which one of the following,(12 more words not shown)
- AMost boulders that(13 more words not shown)
- BThe closest geological(11 more words not shown)
- CThe closest geological(11 more words not shown)
- DThere are no(9 more words not shown)
- ENo other boulders(10 more words not shown)
Reading the argument
A volcanic boulder surrounded by sedimentary rock seems out of place. We don't need a geology lesson to see the puzzle: this rock differs from the rocks around it. How did it get there?
A glacier could have picked it up elsewhere and left it behind when the ice melted. That's a possible explanation. But does the passage establish that explanation rather than another one? Perhaps an eruption threw the rock out, it rolled from a different location, or people transported it. Those are possibilities to investigate, not facts we know about this boulder.
What we’re looking for
The question asks us to undermine the glacier explanation. My first thought is evidence for another way the boulder arrived.
Working the answers
- A
Most boulders traveling no more than 100 miles leaves room for some traveling farther. A general pattern doesn't settle the history of this particular boulder. Just as most people not attending law school doesn't establish that a particular student won't attend, most shorter trips don't exclude this longer one. A provides less direct grounds for doubt than D.
- B
The closest volcanic source being south of the boulder doesn't rule out another source farther north. The proposed glacier could have picked the boulder up from that more distant northern source and carried it south. We aren't told it had to come from the closest source.
- C
A source 50 miles north is in the direction from which a southward-moving glacier would come. Its being the closest source also doesn't prove it was this boulder's birthplace. There could be another source hundreds of miles farther north, so C doesn't rule out the stated journey.
- DCorrect
This attacks the proposed route rather than supplying the alternative origin I first expected. To carry the boulder south to its current location, the glacier would need to pick it up somewhere to the north. If there is no geological source of volcanic rock there, the account is missing the source from which this volcanic boulder supposedly came. That directly undermines the explanation.
- E
This tells us the boulder is unusual in its immediate surroundings, which we already had reason to think. It doesn't explain how this particular boulder arrived. A glacier could have left one volcanic boulder without leaving others within 50 miles.
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