In Part Three Giselle will emerge as an allegory for fairy-tale utopian socialism, hiding from the light (reality), while Linda will stand for lawful society, i.e., the disillusioning reality that men must be restrained for the sake of order. As such, the fearless Linda will become the symbolic keeper of the light (in the lighthouse).

Almost lost in these prefiguring glimpses is Teresa, caught in the sunlight and succumbing not to prayer, as Giorgio thinks, but to despair -- she is literally putting her face in her hands. Note that her own symbolic sunshade (the dark shawl) has fallen aside.
The trials of Teresa in this extended scene, which included her terrified faint in the previous chapter, are artfully masked by Conrad, and we don't learn their full significance until later in the novel. In hindsight, we can guess that Teresa's despair is caused by her pain, her sense of impending death, and her knowledge that she has not yet secured Nostromo as husband/protector for her girls, which is her overriding motivation.