1. Yes. I'd say that that's exactly the period we're talking about. Not the beginning of the century, but a few decades in. 2. No. It refers to the period which ENDS in 1900. If you say that something had disappeared by 1900, it means that the disappearance was going on during the 1890s. By the time 1900 came around, the disappearance was complete.

Yes Correct for both sentences. But first sentences is a little bit out. Different people would interpret the times a little differently. First sentence after 1850 and closer to 1875-1885 Second sentence after 1850 :: -> 1890-1899 or a slightly earlier start date, but before 1900 end date.