Thumb back through the pages of Artforum thirty years ago. “Robert Smithson,” declared John Coplans in the first sentence of “The ‘Amarillo Ramp,’” “was a problem from the beginning.” An odd enough pronouncement, given its quasi-necrological context: Published in 1974, the essay was the first that the magazine devoted entirely to the artist since his death the year before. Of course Coplans (then a little over two years into his controversial stint as editor) was quick to explain his claim. One problem was formal. Smithson’s sculpture was eccentric, its spiraling antigeometries dissident answers