Definitely wood chips. At that point in the 1950s (1958), Georgia Pacific was shipping wood chips from the mill at Toledo, Oregon. GP purchased the Toledo mill from C.D. Johnson in 1956. I just spotted a picture of the first outbound load of wood chips from Toledo in a 1957 photo in "Forest Rails, Georgia-Pacific's Railroads." That picture has SP 350477, a G-50-20-A, a composite GS gon with early chip extensions---the ones with an inboard-slanted top. At this point, the Toledo mill, a former US Army Spruce Division mill sold in the 1920s to CD Johnson was strictly a lumber mill.
Sometime later (date TBD), a pulp and paper mill was added at Toledo--well past the era of our Steam Era list. That must have been in the 1960's, as I well remember long strings of wood chip gondolas rolling through Corvallis headed out TO Toledo (not from), making for heavy trains to and from Toledo on the Oregon Coast. That is beyond our list era. Actually, even the subject photo (1958) and my "first wood chip shipment" photo (1957) are beyond steam, which died on Western Oregon lines of the SP in late 1955.