I've just finished reading 'La Mystère de la Chambre Jaune' by Gaston Leroux, and I noticed something rather interesting. I could tell from the flavour of the prose, before I reached any obvious signs such as description of transport etc, that it was written in the late 19th C. It was the first book of that era that I'd read in French, but of course I've read much from that period in English. What I'm not sure of is: what features of the prose carry across so well that knowing 19th C style in English makes it possible to identify 19th C style in French.
It seemed to me the similarities were in clause structure, and slightly florid descriptions. Complex sentences with multiple clauses in non-obvious order, and phrases with two or more adjectives to describe the same object seem, off the top of my head, to be characteristic in both languages.