Date: c. 1858
Dress: The dress is made from heavy watered silk, with three rows of gathered ruffles at the hem. The ruffles may be of the same fabric or a coordinating fabric; from their more wrinkled texture, my guess is that they are a taffeta. The ruffles are finished with a large (about 3/4″ wide) scallop, which was a popular edging at the time.
The woman wears a wide (about 3″) flat collar with some texture, indicating that it is lace or embroidered, and a slightly ruffled edge. Note how she has placed the collar on top of her wrap. She also wears full undersleeves, though details are not visible. The dark spot below where her collar meets in front may be a brooch, but because it is much lower than most brooches, I think it is probably a button. It may also be the hanging portion of a brooch.
Outerwear: She wears a mantle, which is in a style that was popular throughout the 1850s and into the 1860s. (In fact, it survived in some form for most of the Victorian era). This style mantle is characterized by opening in front, being slightly fitted over the shoulders, shorter in length over the arms, long tabs in the front, and a full back designed to flow over the full skirt. The back of the mantle could range from just below waist-length to almost the knees.
This mantle also has a ruffled edges extending from the very elongated shoulder-line to her waist, where she holds it in place. The fabric of the mantle is difficult to discern. Some texture on her left arm appears to be lace (likely Chantilly lace, which was wildly popular). Although most chantilly lace was worn as a simple triangular shawl, there are plenty of extant examples of cut-and-sewn garments from the lace to make it possible that this is one.
Hair: Her hair center-parted and arranged in several loose, curl-like rolls towards her face. Her hair is at its widest between her temples and chin, which was the fashionable shape throughout the 1850s. She is wearing some sort of headdress, with chenille or tassels of some other material hanging stylishly at the side.