@Justin Xu,
The roar of Niagara has been notoriously utilized as a cover to unlimited osculation, and Adolphus looks up at the sky-cleaving peak of Mont Blanc only to look down at Angelina's countenance with a more vivid appreciation of its superior attractions.
OK, here we go . . .
The falls of the Niagara River, commonly called the Niagara Falls make a very loud noise, a roar. The author is saying that that loud noise, that roar, is notoriously (everybody knows about it) used as a cover, a distraction to other people, while two people kiss. Bellamy lived in the 19th century, when people did not commonly kiss one another in public. The word osculation means kissing, so he can't even bring himself to say kissing, which is typical of that age. He's complaining about "spooning," which is a 19th century term for people engaged in kissing, carressing, fondling one another, and he disapproves. He is also complaining that they use "Nature" as an excuse or a distraction. The word Nature (with the "n" capitalized) in 19th century literature refers to an obsession people in western civilization had with the natural settings of the world, often described as sublime, and a cause for any amount of foolish and essentially meaningless writing--poetry, songs, travel books, endlessly boring personal narratives.
Then he gets down to the meat of the action--or at least as much as we can expect from him. Mont Blanc is a famous peak in the Alps, and with the obsession people in the 19th century had about "Nature," it was a popular destination. You're supposed to go there and stare at the mountain, and be deeply, emotionally moved by the experience. Adolphus looks at the mountaing, the looks at Angelina, and decides he'd much rather look at Angelina.
Good thinking, Adolphus.