My teacher asked for me to find two quotes that demonstrate the presence of a frame narrative in Heart of Darkness by Joseph Conrad. Is the passage that begins as follows an example of the frame narrative? Between us there was, as I have already said somewhere, the bond of the sea. Besides holding our hearts together long periods of separation, it had the effect of making us tolerant of each others yarns—and even convictions. The Lawyer—the best of old fellows—had because of his many years and many virtues... If this is a good example of the frame narrative, then I only need help finding one more quote that demonstrates the frame narrative. If this isn't a good example, please show me two quotes that are and explain why the one I selected is not.

Heart of Darkness is a 1899 novella written by famed Polish-British author Joseph Conrad, which was originally published as a three-part serial story. The novella has a very unique narrative structure, as Conrad employs a frame narrative (a story within a story) and presents an unnamed narrator who is both a listener to and a narrator of Charlie Marlow's story. Instead of telling the story entirely from the perspective of Marlow, Conrad includes a frame narrator who contrasts Marlow's views and opinions so that he can indirectly tell the readers that what he has learned about England and Africa so far is false and untrue. Thus, it comes as no surprise that by the end of the novella, the narrator's attitude toward the past is completely changed.
Marlow tells the main story of his experiences sailing down the Congo River in Africa, in first-person singular, while the unnamed narrator frames it by telling the story in first-person plural and describing the sailors' journey on the Nellie—a yacht sailing down the Thames River in England. The unnamed narrator's purpose is to connect Marlow's story to the readers:

Between us there was, as I have already said somewhere, the bond of the sea. Besides holding our hearts together long periods of separation, it had the effect of making us tolerant of each others yarns—and even convictions. The Lawyer—the best of old fellows—had because of his many years and many virtues.

Thus, the passage you've cited is, in fact, an apt example of a frame narrative, as the unnamed narrator practically sets the scene. However, I would like to give you a slightly better example, which might showcase the transition from the frame story to the central story:

His remark did not seem at all surprising. It was just like Marlow. It was accepted in silence. No one took the trouble to grunt even; and presently he said, very slow—


“I was thinking of very old times, when the Romans first came here, nineteen hundred years ago—the other day."

Here, the point of view directly shifts from the unnamed narrator to Marlow. We tend to see the unnamed narrator as more reliable and more trustworthy than Marlow, as he is a part of the audience as well, and he casually observes and comments from the side. The point of the frame narrative, in this case, is for the readers to connect to the unnamed narrator in order to better understand the author's parallels between the two rivers, Thames and Congo; the two continents, Europe and Africa; and even the two peoples—black and white.