The top is conical and rusty and can best be seen in the last photo. Thanks, the_living_end, for identifying its use--I had always wondered! I don't have access to the local Sanborn Fire Insurance maps right now, but the copies I do have show another mini-golf course just up the street on the same side, between 63rd and North Sts. (in the Sanborn coverage of Oakland, Vol. 3, probably sheet 258, 1911-Oct. 1951).
Photobucket is really good for free image hosting.
What's especially ironic about this decayed site is that the prices for small houses on the residential streets nearby have risen to levels that seem absolutely ludicrous (although still cheaper than San Francisco, across the bay). I have a collection of photos from my visit back to Maine last year that I'm planning on posting in rural_ruin, but I hadn't really considered much of this area to be a really good site for finding urban decay--there are many, many renovation and construction projects happening all over.