Answer
The pendulum clock will run too slowly when it is high in the mountains.
Work Step by Step
Some internet research about pendulums, or referring to Footnote 2 on page 358, shows that the period $T = 2 \pi \sqrt{\frac{L}{g}}$.
The period of a pendulum depends on the acceleration due to gravity. High in the mountains where the gravitational acceleration g due to the Earth is lower than it is at sea level, a pendulum will oscillate more slowly, with a longer period.
The clock will run a tad slowly.
