Although this style of bonnet was popular from the late 1840's to the late 1850's, this one dates to the mid/late 1850's. It retains the long low sides of 1840's bonnets and doesn't come as far forward on the head as earlier bonnets did. 1860's bonnets would rest even further back on the head. Note the runched silk satin which is a characteristic of Drawn bonnets and the neck ruffle at the back of the bonnet, which would disappear in the mid 1860's. It was considered "fast" to show your neck while wearing a bonnet, hence the ruffle at the back for modesty. When neck ruffles went out of fashion, many of the older ladies were shocked at the young generation showing their necks!
(Image courtesy of Mrs. E Huggins)