Well, as there isn't much of the lady's clothing showing, and the major dating clues of the surround and the back not being available, I'll make a guess.
From the little bit of waistline showing, I'd say it's an 1890s style bodice and skirt. The bonnet looks quite a bit earlier, but some older ladies didn't keep up with fashion trends. The strings or ties of the bonnet look very much like crape, which was a material associated only with mourning dress, and was pretty much mandatory. However her cape is quite highly decorated and although we can't be sure what colour it is, if it's black she is not going to be in one of the early stages of mourning.
Also the brooch on the cape looks like it doesn't have any bright shiny bits on it, so it might be "approved" mourning style jewellry as well.
So, although I'm just guessing, I would say 1903/4 or so is plausible.
Cheers,
China
