It was founded upon the sense of personal responsibility.

She alone knew the secret mission upon which Toyner had left; she alone knew of the glimpse of her father which she had caught the night before, and she doubted now whether she had seen a spirit or visible man.

What had happened in the dark hour in which Toyner and Markham had met, and which of them had brought back the boat?
The misery of these questions grew to be greater than she could endure; but to confide her distress to any one was impossible.

To do so might not only be to put her father's enemies upon his track, but it would be to confess Bart's unfaithfulness to his public duty; and in that curious revolution of feeling which so frequently comes about in hearts where it is least expected, Ann felt the latter would be the more intolerable woe of the two.
Then came another of those strange unearthly sunsets.

Ann's mind was made up.