Thought I would bump this with a brief review, having viewed the exhibit today. Most of the photographs are dated from the early 1870's, the albumen prints are about 8x10 in size. There was a photograph of a Spanish inscription dating from the 1600's on "Inscription Rock" which I had never seen before. (I'm familiar with the one from 1526, where a ruler is held beneath the inscription to give an idea of scale.) The skies in all photos are washed out because of the material they used (this is explained in Szarkowski's little essay about O'Sullivan in "Looking At Photographs"), so the compositional problems were correspondingly greater. Bell has at least one composition up at canyon walls that caught my attention - pay attention to who made the photograph!
Well worth doing if you've ever wanted to see O'Sullivan prints....