I think he pushed the pace unwittingly, under the prick of those emotions which otherwise were in such excellent control.

I can see him now, continually waiting for us on the brow of some glistening ice-slope, leaning on his alpenstock and looking back, jet-black by contrast between the blinding hues of ice and sky.
But once he waited on the brink of some unfathomable crevasse, and then we all three cowered together and peeped down; the sides were green and smooth and sinister, like a crack in the sea, but so close together that one could not have fallen out of sight; yet when Bob loosened a lump of ice and kicked it in we heard it clattering from wall to wall in prolonged diminuendo before the faint splash just reached our ears.

Mrs.
Lascelles shuddered, and threw out a hand to prevent me from peering farther over.

The gesture was obviously impersonal and instinctive, as an older eye would have seen, but Bob's was smouldering when mine met it next, and in the ensuing advance he left us farther behind than ever.
But on the rock where we had our lunch he was once more himself, bright and boyish, careless and assured.

So he continued till the end of that chapter.