LSAT 6 | Section 3 | Logical Reasoning: Q17

LR Question 17 Explanation

QUESTION TEXT: Samples from the floor of a rock shelter in Pennsylvania…

QUESTION TYPE: Weaken

CONCLUSION: The skeptics conclude that the samples could have been contaminated by dissolved old carbon carried by percolating groundwater from nearby coal deposits.

REASONING: The date of the samples were too early. Most timelines of human migration to North America only had humans arriving later than 19,650 years ago.

ANALYSIS: This is a hard question, if only because there are many technical details.

Researchers found many different human samples. They were able to date them, in order. The order they found matched the depth at which the samples were found. Older samples were deeper.

The oldest and deepest sample is 19,650 years old (+/- 2400.) That is before current theories claim humans first migrated to North America. So either the theory is wrong or the dating of the sample is wrong.

That’s why critics don’t believe the results. They propose one alternative explanation: the samples were contaminated by coal.

We need to weaken the conclusion that this explanation is correct. We could do it by showing any of the following: humans were in North America earlier than we thought, the samples were not contaminated but the test malfunctioned, the samples were contaminated by a different method or the samples could not have been contaminated by coal.

___________

1. CORRECT. The samples began with the present. If the method of contamination had affected all samples, it would have been obvious that the present day sample was dated incorrectly. Yet the present day sample was correctly dated to the present. So there couldn’t have been contamination.
2. We don’t know why those other applications were rejected by scientists. We also don’t know if the scientists were correct to reject them. It is true that knowing that the test malfunctioned would weaken the critics’ argument, but this is too vague.
3. Coal existed before we decided to use it for fuel. It could still have contaminated the samples, so this doesn’t weaken their argument.
4. This shows that the test results are always the same. But if the samples were contaminated then the results are still flawed. They’ll just be flawed in the same way each time we check.
5. This doesn’t affect anything one way or another. Assuming there are sites elsewhere in the world that are just as old, carbon dating should be accurate.

Recap: The question begins with “Samples from the floor of a rock shelter in Pennsylvania”. It is a Weaken question. Learn how to master LSAT Weaken questions on the LSAT Logical Reasoning question types page.