The slope, being inclined almost seventy degrees, the path became impracticable.
But if the engineer and the boy were obliged to give up thoughts of following a circular direction, in return an opportunity was given for ascending the cone.
In fact, before them opened a deep hollow.

It was the rugged mouth of the crater, by which the eruptive liquid matter had escaped at the periods when the volcano was still in activity.

Hardened lava and crusted scoria formed a sort of natural staircase of large steps, which would greatly facilitate the ascent to the summit of the mountain.
Harding took all this in at a glance, and without hesitating, followed by the lad, he entered the enormous chasm in the midst of an increasing obscurity.
There was still a height of a thousand feet to overcome.

Would the interior acclivities of the crater be practicable?
It would soon be seen.

The persevering engineer resolved to continue his ascent until he was stopped.