"Volcanoes, as you know, are all -- and this can be definitely
asserted -- located at the edge of the sea or near it -- Vesuvius,
Etna, Hecla, Chimborazo -- in the New World as well as the Old.
The natural conclusion to be drawn from this is that they must
be in underground communication with the oceans. Water filters
into them, quickly or slowly, depending on the composition of
the soil. It reaches the interior fire, where it's heated and turns
to steam. When this steam, trapped in the bowels of the earth,
attains a high pressure, it creates an internal upheaval and tries
to escape to the outside, dragging ashes, slag, and rocks out
through the chimney, surrounded by swirling clouds of smoke
and flame. That, without any doubt, it the cause of eruptions
and probably of earthquakes, too . . ."
All eyes were on the engineer at that moment. The explanation
he had just given of volcanic phenomena was certainly
an accurate one.
I don't know if this faux-science was something that's been put forth as a legitimate theory of vulcanology or if it's just an idea Verne had come up with and was throwing out there. I suspect the latter. I'm pretty sure the realization that Yellowstone is a caldera of a supervolcano came after Verne's time, so we can't fault him there, but there are many volcanos that are a long way from the sea. Maybe it depends on how generously we define "near" the sea to be.
Also, I have to say that after all the build-up I was a little disappointed to find that the mountain of the title was 800 to 1,000 feet tall.