She is wearing mourning dress. My guess is that it is later 19th century because of the lower part of her garb.
Around 1860-70 such dresses tended to be quite voluminous, while this one seems much slimmer.
The hat appears to be some kind of bonnet with a construction on top. It is a bit odd looking as it is captured in this photo.
My guess is about 1890, but this really is a bit of a guess. The trend later is for large brimmed hats rather than bonnets, but I am not sure that there is a definitive cut off period for mourning dress styles.
Below is some pictures for you to make your own guess from.http://pinterest.com/saskiakuus/mourning-dress/