These sites say St. Helens was active 1831-1857. Mary C.

From USGS/CVO site for Mt. St. Helens volcano:
http://vulcan.wr.usgs.gov/Volcanoes/MSH/NatMonument/PointsInterest/msh_volcano.html
> The volcano was particularly restless in the mid-19th century, when it was
> intermittently active for at least a 26-year span from 1831 to 1857.
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And from: http://vulcan.wr.usgs.gov/Volcanoes/MSH/description_msh.html
> ... Contemporary sketches and paintings by Paul Kane suggest the mountain was probably
> erupting at a point halfway down the north slope before or during 1847. The vent was apparently
> the Goat Rocks dome, which was removed by the 1980 eruption. On the basis of these and other
> observations, scientists think eruptive activity may have continued intermittently until 1857.