The 1219 eruption of Sneffels is the geological event that both ended the mountain’s volcanic activity and left behind the passage through which the expedition descends. Its lava, hardened into a shining tunnel, becomes a visible record of the mountain’s past and the explorers’ first guide into a subterranean world where geological time appears to unfold beneath their feet.

Background and Extinction

When Henry questions whether Sneffels might be extinct, Professor Hardwigg explains that no eruption has occurred since 1219 and describes the mountain as having ceased to be a volcano. The claim matters practically: the expedition is entering a route through a mountain believed no longer to erupt, rather than approaching an active crater. The date gives Sneffels’ apparent inactivity a historical measure, while the journey will test how securely that past can be separated from present danger.

The Lava Passage

The explorers encounter the eruption’s material legacy at the entrance to their descent. Lava that flowed from the mountain’s depths in 1219 forced a passage through the tunnel and coated its interior with a thick, brilliant layer. Under the electric light, the rock gleams; the old flow is not merely evidence to be inspected but the surface of the route itself.

That route is steep enough to make descent hazardous. Cracks and irregularities in the ground serve as footholds, while the heavy luggage is lowered ahead on a long cord. The lava’s forms change along the passage: what provides steps underfoot becomes stalactites overhead, and porous rock swells into rounded blisters. Quartz crystals and drops of natural glass hang from the roof, catching the light as the party passes. The eruption thus furnishes both the practical obstacles of the descent and its startling visual splendor.

A Passage Through Geological Time

As the party moves beyond the lava-coated section, the appearance of the rock changes. Shining volcanic surfaces give way to living rock and strata the narrator identifies with the Silurian period. Farther on, Professor Hardwigg and Henry observe fossils and marbles bearing traces of earlier life; Henry interprets the succession as a visible ascent through geological formations and animal history. The eruption’s tunnel is therefore a threshold: it leads from the mountain’s comparatively recent volcanic past into strata that the explorers read as records of far older worlds.

The contrast between geological evidence and scientific expectation recurs during the expedition. Professor Hardwigg asks Henry to check the thermometer; its reading is 27.6, far below the temperature predicted by the theory of increasing heat with depth. He invokes Humphry Davy’s account of the Earth’s interior to explain why he trusted the measurement. This dispute does not establish anything about the 1219 eruption itself, but it echoes the larger tension between what the explorers expect from geological theory and what they encounter underground.

Retrospective Significance

The eruption is recalled rather than witnessed. Its date and consequences are supplied through explanation and through the physical signs left in the tunnel, letting the narrative turn a past event into a present setting. The explorers’ notebook, begun on Monday, June 29, records their preparations at the point of departure; the enduring lava, by contrast, carries them into a history measured in layers and fossils.

That deeper chronology becomes particularly consequential when the expedition reaches a vast fossil-bearing plain and finds a recognizable human skull. The narrator sets the discovery against contemporary disputes over the antiquity of humanity: the Moulin-Quignon jawbone, found in 1863 fourteen feet below the soil near Abbeville, had intensified arguments over whether humans lived in the Quaternary period. The account also describes claims that worked fossil bones from Tertiary deposits pushed human presence further into the past, even before the mastodon. These discoveries do not follow from the eruption, but they extend the geological perspective opened by the descent: the mountain’s lava marks one dated event, while the subterranean strata invite the explorers to confront time on a far larger scale.